Emerging infectious diseases of crop plants in developing countries: Impact
on agriculture and socio-economic consequences
Article · June 2010 DOI: 10.1007/s12571-010-0062-7
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REVIEW
Emerging infectious diseases of crop plants in developing
countries: impact on agriculture and socio-economic
consequences
Maurizio Vurro&Barbara Bonciani&Giovanni Vannacci
Received: 10 February 2010 / Accepted: 29 March 2010 / Published online: 16 April 2010 #Springer Science+Business Media B.V. & International Society for Plant Pathology 2010
Abstract Emerging infectious diseases (EIDs) caused by
plant pathogens can develop into unexpected and very serious epidemics, owing to the influence of various characteristics of the pathogen, host and environment. Devastating epidemics, having social implications by increasing the rate of urbani-zation, occurred in the past in Europe, and many other EIDs still occur with high frequency in developing countries. Although the ability to diagnose diseases and the technolo-gies available for their control are far greater than in the past, EIDs are still able to cause tremendous crop losses, the economic and social impact of which, in developing countries, is often underestimated. In the present article, four of the most important EIDs in developing countries are considered from the standpoint of their origin, characteristics, symptoms, mode of spread, possible control strategies, economic impact and the socio-economic consequences of their dissemination. They are Cassava Mosaic Virus Disease,
capable of reducing yields by 80–90% and causing the suspension of cassava cultivation in many areas of East
Africa; Striga hermonthica, a parasitic weed affecting
cereals in an area of at least 5 million hectares in
Sub-Saharan Africa; Xanthomonas Wilt of Banana, a bacterial
disease that caused around 50% yield losses at the beginning of 21st century in Uganda and is threatening the food security of about 70 million people owing to its impact on an important staple crop; and race Ug99 of the rust fungus
Puccinia graminis f. sp. tritici, which is having a tremen-dous impact on wheat in Uganda, and is also threatening most of the wheat-growing countries of the world.
Keywords Emerging infectious diseases . Cassava Mosaic
Virus Disease . Striga hermonthica . Banana Xanthomonas Wilt . Wheat Rust Ug99
Introduction
“Emerging infectious diseases”(EIDs) are those caused by
pathogens which, for a number of different reasons, develop into epidemics that may be both unexpected and devastating. Some of the best known epidemics appeared during the 19th century and coincided with the advent of more intensive agriculture and reduction in the duration of sea voyages. The latter allowed increased international trading and, as a corollary, the introduction into new areas of foreign species of plants and parasites, resulting in more frequent upsetting of agro-environmental balances.
Sometimes EIDs developed into pandemics over whole nations and even continents, causing famine and favouring human diseases, socio-economic disasters and technical crises for the management of whole agricultural
communi-ties. For example, Phytophthora infestans, the Oomycete
Authors have contributed equally to the preparation of the manuscript. Dr. Bonciani prepared the paragraphs relating to the economic and social impact of the case-studies considered.
M. Vurro (*)
Istituto di Scienze delle Produzioni Alimentari, Consiglio Nazionale delle Ricerche,
Via G. Amendola 122/O, 70126 Bari, Italy
e-mail: maurizio.vurro@ispa.cnr.it
B. Bonciani
Dipartimento di Scienze Politiche e Sociali, Università di Pisa, Via Santa Maria 46,
56100 Pisa, Italy
G. Vannacci
Dipartimento di Coltivazione e Difesa delle Specie Legnose “G. Scaramuzzi”, Università di Pisa,
Via del Borghetto 80, 56100 Pisa, Italy
agent of potato late blight, was the primary cause of the great Irish famine of the nineteenth century. The pathogen originated in the Andes and was observed in North America
in 1843 (Gomez-Alpizar et al.2007). Due to intense trade it
reached Europe two years later (Fry et al. 1992). The
disease caused significant yield losses, which were partic-ularly catastrophic in Ireland owing to the wetness of the climate and the almost total dependence of a large
proportion of the population on potato (Large 1940). Out
of a population of 8 million approximately one million died of starvation and 1.5 million emigrated, of which about a
quarter died in transit (Klinkovski 1970). Thus, the first
massive migration of modern history was caused by a plant disease.
The most recent of the great famines occurred in East Bengal in 1943, where the failure of the rice crop caused the starvation of an estimated 2–3 million people. The aetiology of the disease is disputed but many attribute it to
the fungal pathogen, Cochliobolus miyabeanus, the
dis-semination of which was favoured by the environmental
conditions pertaining at the time (Padmanaban 1973).
Others suggest that high levels of iron or aluminium or an outbreak of the brown planthopper were the cause of the
problem (Strange2003).
The great Southern Corn Leaf Blight epidemic was
caused by a variant strain of the fungus, Cochliobolus
heterostrophus, named race T, which was specifically virulent for maize containing a cytoplasmically inherited
gene for male sterility (Tcms). Because of the advantage
conferred by the gene in breeding this self-fertile crop, it had been incorporated into about 85% of the American crop by 1970. As a result and aided by favourable climatic conditions a pandemic developed with the epicenter in the
“corn belt” causing enormous damage in 1970–71. The
pandemic was halted by the withdrawal of susceptible varieties and the establishment of new hybrids. Southern Corn Leaf Blight is infamous for having shocked the world feed market and for having set a record in terms of economic losses produced on a single agricultural crop in
a single season (Scheffer1997). Subsequent to the disaster,
the reason for the specificity and high virulence of C.
heterostrophusrace T forTcmsmaize was determined to be the production of a so-called host-selective toxin by the fungus.
Land use by English settlers and population growth had led, in Ceylon (now Sri Lanka), to an enormous expansion of coffee cultivation during the first 50 years of the nineteenth century. In 1868, prosperity from the crop had
reached a maximum but then Hemileia vastatrix, a rust
fungus, was found which was likely to have spread from Ethiopia, the centre of origin of both the plant and its rust. Initially damage was thought to be light but may have been underestimated by the British planters who could
compen-sate for lower production with increased prices. But the disease spread to all the plantations and production losses quickly became economically unsustainable. By 1905 the area planted to coffee in Ceylon had shrunk from 275,000
acres in 1878 to around 3,500 in 1905 (Mills 1964).
Because of the pandemic, coffee had to be replaced, luckily with success, by tea.
Thanks to technological advances in, for example, diagnostics, agronomic practices and the use of specific disease management strategies, the risk of epidemics occurring with catastrophic consequences has been sharply reduced in developed countries compared to developing
countries (Waage et al.2009). Unluckily, the stability of the
agricultural systems reached with great difficulty is very often upset by the sudden appearance of novel parasites and pathogens, as well as by environmental and technological alterations in management practices. If we consider the many factors relating to the pathogen, the host or the environment that can affect a disease, it is easy to
understand that the “emergence” of a disease is the
coincidence of a number of unfortunate events. Further, if we also include consideration of socio-economic conse-quences in the evaluation of the seriousness of a plant disease, we will find that some devastating epidemics, reported only in history books for Europe, are still occurring very frequently in many developing countries.
As with pathogens of humans and domestic or wild animals, the emergence or re-emergence of phytopathogen-ic agents is very often due to man’s activities, such as their introduction into novel areas as a consequence of mass tourism, global trade, farming changes and environmental changes. Although only a fraction of a pathogen commu-nity is introduced together with a newly-introduced plant species, this seems to be the most important cause of expansion of an emerging disease to a new area. Moreover, if the pathogen responsible has a wider host range than the plant species introduced it may infect indigenous plant species, which may be particularly vulnerable as they will not have co-evolved with the pathogen.
Although introductions of alien pathogens may occur owing to the trading of vegetables, germplasm, grafts or whole living plants, introductions via international seed trading is a particularly important vehicle for pathogen introduction and dissemination. For example, it has been estimated that at least 2,400 different plant pathogens were
contained in the seeds of 380 plant genera (McGee 1997),
and that up to one third of the plant pathogenic viruses are transmissible through seeds to at least one of their hosts
(Stace-Smith and Hamilton 1988).
environmental conditions or the genotypes of the potential host plants. In particular, in the case of pathogens which are transmitted by vectors, it is the introduction of the vector
into a new area that may be the “real” cause of the
occurrence.
Lacking the elements favouring their further dissemina-tion, some pathogens may remain restricted to their area of introduction, making very limited impact. For example, Citrus Tristeza Virus (CTV) was probably introduced into South America in the 1920s but only became economically devastating in the 1950s owing to the introduction from
Asia of a very efficient vector, the aphid Toxoptera
citricidus. Since then, in Brazil, more than 6 million citrus
trees have been destroyed (Bar-Joseph et al.1979). Pierce’s
Disease (PD) of grapevine, caused by the bacteriumXylella
fastidiosa, was reported in California as being not serious for more than a century but in 1997 a new vector,
Graphocephala atropunctata, was introduced into Califor-nia. This allowed the rapid development of the disease in the vineyards, with damage estimated in 1999 at 6 million
dollars (Anderson et al.2004). In some cases the reciprocal
situation can occur, i.e. plants species introduced in novel areas are affected by endemic pathogens. For instance, cassava mosaic viruses are not known in South America, the centre of origin of cassava, but as will be described below, they have caused havoc in the crop in Africa to which the plant was exported in the 16th century (Strange 2003).
Climatic changes have very often been connected with the appearance of epidemics in humans and animals, but very little is known about their effects on plant EIDs
(Garrett et al.2006). Changes in the incidence and severity
of plant diseases are likely to occur and these will vary according to the particular pathosystem. In addition, incidence and severity will also be influenced by other factors, such as the use of transgenic plants, the availability of new chemicals and changes in land management.
Changes in farming systems have determined the appearance of a number of EIDs both of crop and wild species. In some developing countries, the lowering of the value of traditional crops and the higher demand for non-traditional crops has caused an increased cultivation of the latter. The introduction of highly productive selected monocultures has reduced genetic variability, increasing the risk of exposure to pathogens. For example, in 1970, outbreaks of Southern corn leaf blight and yellow corn leaf blight destroyed 17% of all US maize crops, 85% of which were of the same variety, susceptible to these diseases
(Pring and Lonsdale1989).
In the next sections, four EIDs of great importance to developing countries, in particular to Sub-Saharan Africa (SSA) will be considered as models, chosen for their different origins, agents and means of dissemination.
Species of the genus Striga, although often referred to as
parasitic weeds, are actually pathogens and are therefore considered. Although there is an extensive bibliography available regarding the biology, symptoms, distribution and crop losses of some pathogens, data on their economic and social impact are scarce. As a result, several of the estimates of impact on crop production losses due to disease cited in this paper are at best based on assumptions rather than certifiable data. This is much more evident for pathogens affecting crops which are neither widely grown nor exported and are therefore of interest only to local populations. Here the lack of a network of technical assistance for monitoring, surveying and controlling disease means that such information is far from complete and this therefore represents a considerable handicap for crop protection. These problems are particularly severe in developing countries where when a disease is reported as
“new”, it is often already widely distributed in the
environment without any control, increasing the risk of pandemics.
Cassava Mosaic Virus Disease
The plant host
Cassava (Manihot esculenta Crantz) (Fig. 1) is a shrubby
perennial plant belonging to the Euphorbiaceae family. It has been cultivated in South America and particularly in the Amazon basin for millennia for its starchy roots, but it was only introduced into Africa by the Portuguese in the sixteenth century. Thereafter, its drought tolerance and ability to produce yields on even marginal lands was appreciated. As a result, its cultivation spread slowly across Africa, largely by way of the river trade in Central and
Western Africa (Legg and Thresh 2000). There was a
considerable expansion of the area devoted to the crop during the colonial period, reaching its present distribution
during 1920–1930 as a consequence of the authorities
encouraging its cultivation as a food reserve for periods of famine and drought. Moreover, cassava (as well as banana, considered in another section of the article) is ideally suited to SSA, because it requires minimal field management. Cassava now constitutes a major source of food in SSA, and a significant source of revenue from the sale of the fresh or processed crop. The status of cassava cultivation today is changing from subsistence farming to an industrialized system designed to process cassava into a diverse spectrum of products, including starch, sago grains, flour, chips, animal feed and, potentially, biofuel, all derived from a crop
that has the ability to grow in poor soils (Thresh2006).
Origin, distribution and impact
Cassava Mosaic Virus causes the most important disease of cassava in Africa. Symptoms consist of mosaic yellow or yellow-green chlorosis, leaf deformation and stunting
(Fig. 2). The disease was first reported in Tanzania
(Warburg 1894) and it was assumed to be caused by a
virus, as the pathogen could be transmitted mechanically and was not visible. Only in more recent times has the exact aetiology been determined, with the agent of the disease
being identified as a geminivirus (Bock and Woods1983).
There are some reports of its spread in the early decades of
the last century, but none that were“alarming”as regards
either disease severity or rate of spread. However, there
were some sporadic outbreaks during the period 1920–1940
that led to the initiation of some crop protection pro-grammes, especially the introduction of resistant varieties.
Nevertheless, until the mid-1980s, Cassava Mosaic Disease (CMD) was considered to be just one of several
diseases affecting cassava (Otim-Nape1987). The situation
changed suddenly in 1988 when a serious outbreak was reported in northern Uganda. Owing to social and political insecurity and instability at that time, following the flight of the dictator, Idi Amin, 10 years previously, it was not possible to obtain accurate analysis of the situation. Based on a series of observations, it was hypothesized that higher temperatures and lower humidity in that area of the country
had favoured the spread ofBemisia tabaci, a polyphagous
white fly insect and vector of the virus, thus indirectly
favouring the spread of the virus itself (Otim-Nape 1993).
However, this hypothesis soon became untenable because, in subsequent years, the disease spread southward to areas
that were more humid and colder at the rate of 20–30 km
per year (Otim-Nape et al. 1997; Legg and Ogwal 1998).
Also, symptoms were more severe at the disease front, whereas vector populations were more numerous where the disease was already present.
The effects of virus disease on the farming communities in Uganda became evident in the early 1990s. The initial impact was greatest in the north-eastern areas of the country, particularly because of the cultivar grown, Ebwa-nateraka, which later proved to be the most susceptible to the virus. Here, cassava production between 1990 and 1993 was reduced by 80 to 90% and many farmers suspended its
cultivation (Thresh and Otim-Nape 1994). In 1993, the
failure of the crops of maize, beans and other food crops, owing to drought, compounded the lack of cassava as a food reserve, leading to widespread food shortages and
famine-related deaths (Thresh and Otim-Nape 1994). A
common reaction to this situation was the cultivation of other crops, mainly sweet potatoes. The impact of the epidemic in central and western regions of Uganda was less acute, mainly due to the use of a greater range of varieties, and therefore to the presence of certain varieties more tolerant to the disease. But the effects were still extremely serious. Several attempts have been made to quantify the losses due to the virus, the most reliable estimate being around 600 thousand tonnes per year valued at 60 million
dollars (Otim-Nape et al. 1997,1998).
When the impact of the epidemic became very clear, the real causes that had led to the outbreak of a disease that had hitherto been relatively innocuous were investigated. Serological and molecular techniques demonstrated the existence of virus variants which differed in virulence. Two are prevalent in Africa, African cassava mosaic virus (ACMV), and East African cassava mosaic virus (EACMV)
(Swanson and Harrison 1994). However, in Uganda, a
variant, which appeared to be a recombinant hybrid of EACMV and ACMV has been detected and is referred to
either as the Uganda variant (UgV) (Zhou et al.1997) or as
Fig. 2 Cassava plant with the typical symptoms of cassava mosaic
a distinctive strain of EACMV (EACMV-Ug) (Deng et al.
1997). The enhanced severity of the disease was related
both to higher virus titres in cassava plants infected with UgV and the widespread occurrence of plants with mixed ACMV/UgV infections in which symptoms were most
severe (Harrison et al.1997). The whitefly vector, although
polyphagous, was much more prolific on infected plants. Also, in seeking less crowded areas, whiteflies enhanced the rate of dissemination of the virus. Moreover, the decline in cassava cultivation increased the capability of whiteflies to spread the disease to non-infected areas (Legg and
Thresh2000).
The severe CMD epidemic subsequently expanded rapidly to Kenya, affecting in a few years (from 1995 to 1998) virtually all the areas of cassava cultivation (Legg et
al.1999). Field observations estimated yield reductions of
approximately 140 thousand tons per annum in those areas. The disease subsequently spread to the Sudan and Congo but, probably because of political instability in these regions, an accurate assessment of the magnitude of the pandemic was not possible, although it seems very serious
in these areas (Legg1999).
Social and economic aspects
Cassava is a main staple food in tropical areas and its production is extremely important in the poor central, eastern and southern African region for the role that it plays with other food crops in the local diet. Its adaptability to different environments as well as its tolerance to long periods of drought make it one of the most important staple foods in many parts of the world where soil stresses and human conflicts constrain production.
CMD is one of the most serious and widespread diseases throughout cassava growing areas in those African regions. It has threatened food security of millions of people through its impact on cassava production. According to FAO, in Africa cassava is the primary source of food for an estimated 70 million people, contributing over 500 kcal
per day per person (FAO2009a). Cassava has been defined
as ‘the crop of the poor’. Its contribution to rural
development and poverty alleviation of marginal
popula-tions is an important reality (Howeler et al.2001).
In many Asian and African countries, cassava is the real catalyst for development of rural areas, its production representing the main source of income for the poorest rural households. In those areas in which food security con-ditions remain alarming, further development of the cassava production sector and of disease control and prevention could be very important contributions to the achievement of
poverty reduction (FAO2009a).
CMD is the most important disease of cassava in Africa,
Sri Lanka and southern India (Otim-Nape & Thresh2006).
Currently the presence of the disease has been registered in many countries of SSA, such as Angola, Burundi, Central African Republic, Democratic Republic of the Congo, Gabon, Kenya, Malawi, Mozambique, Rwanda, Southern Sudan, Tanzania, Uganda, Zambia and Zimbabwe.
Accord-ing to FAO (2009a), CDM threatens the whole cassava
production system in the Great Lakes regions. In those areas the disease has reduced cassava yields of affected farms by up to 80 percent. The highest levels of the disease have been reported in Burundi, Malawi, North and Central Uganda, North Zambia, Tanzania and the central areas of Kenya. In the Democratic Republic of the Congo, it is estimated that the disease can cause losses as high as 90% (FAO 2009a).
There are several reasons for cassava’s importance for
food security in African countries. In order to understand the socio-economic impact of cassava mosaic disease on the continent we have to consider that 50 percent of the current world production of cassava takes place there. Cassava is extremely important for the poorest smallholder farmers cropping on marginal and sub-marginal lands. First of all, it provides them with their main source of income. Secondly, it contributes to their living standard as cassava is the source of simple food products which are cheaper and more accessible than those of rice, wheat and maize
(Nweke and Ezumah 1988). Cassava plays a major role in
the alleviation of famine because of its efficient production of food energy and year-round availability. The crop is tolerant to drought and does not require the use of fertilizer or purchased seeds that are very expensive and not
accessible to poor farmers (Nweke 1995). Another reason
for cassava’s importance is that it can remain in the soil untended for up to 2 years without losing its nutritional properties, an important property in the event of temporary
civil strife (Nweke et al.2002).
The current widespread existence of CMD on the African continent is alarming the whole international community (developed countries, research and humanitar-ian organizations) because of the precariousness of live-lihoods there owing to the high incidence of civil unrest and the reduced capability to face natural crises. These have resulted in millions of refugees crossing borders, putting a
strain on the recipients’food supply and also contributing
to the spread of the disease by the transport of infected vegetative material.
considerable contribution to the achievement of sustainable food security and poverty alleviation.
There have been interesting political interventions to buffer the impact of the disease indirectly. In Nigeria great strides have been made, as a result of a Government initiative, through collaboration with the International Institute of Tropical Agriculture (IITA), to penalize impor-tations of wheat flour and promote the of use of cassava flour. All products using wheat flour now contain a defined percentage of cassava flour, hence providing a market for local producers (personal communication). This is impor-tant considering that the population of Nigeria amounts to 130 million (20% of all Africans). This has buffered Nigeria’s food security from fluctuations in global wheat prices. Further, huge strides have been taken to develop processing technologies to turn cassava into chips for human and animal feed and, perhaps more novel, into industrial starch for the textile and food industries (it has even been used in oil drilling activities). This example provides a positive indication of what is possible if the political will is present to create policies and an associated enabling environment for the exploitation of cassava varieties selected for market orientations accompanied by appropriate processing technologies. This approach also reduces the current constraint of massive post harvest losses due to the sale of perishable goods and transport on appalling roads by poorly maintained vehicles.
Striga hermonthica
Origin, distribution and impact
Striga hermonthica (Family Scrophulariaceae) is a
hemi-parasitic weed, currently the main biotic “problem” for
cereal crops in the SSA region (Fig. 3). It is very
widespread, infesting land in western, central and eastern SSA, from Gambia in the west, to Ethiopia, Kenya and
Tanzania in the east (Parker 2009), and Sahel in West
Africa, including northern areas of Cameroon; besides S.
hermonthica, economically the most important, there are
three other main species of Striga in SSA which are
agriculturally important:S. asiaticais economically
signif-icant in eastern and southern regions;S. forbesiiis limited
to certain areas of Zimbabwe;S. gesnerioides, is found in
areas of Nigeria and Tanzania. Sorghum, millet and maize
are particularly susceptible toS. hermonthica, whereas all
grasses are attacked byS. asiaticaandS. forbesii. Legumes
are attacked by S. gesnerioides, with cowpea (Vicia
unguiculata) being very susceptible.
S. hermonthicais native of the tropical grasslands of the
“old world”and reached its highest biodiversity in regions
where it co-evolved with cereals, especially sorghum, millet
and rice. It then spread widely and became a scourge for the production of cereals (including maize) in areas where fertility is low and water availability is limited or erratic. Maize was introduced into Africa many years ago, replac-ing sorghum and millet species more tolerant and better adapted to the scarce water resources available. The reasons for this introduction are many, such as increased produc-tivity compared to sorghum, at least in favourable years, consumer preference due to greater palatability, and a covered panicle reducing the risk of predation by the weaver bird, whose flocks of millions can destroy entire
plantations of sorghum (Doggett1988).
S. hermonthicaseeds germinate only in the presence of the host plant, owing to the release of germination stimulants by the roots of hosts. The germ tube grows towards the root, attaches to it by a haustorium, and begins to steal nutrients and water from the host. The small seeds survive for many years in the soil, and therefore crop rotations have little effect on control once a certain threshold of the seed bank has been reached. When
infestation is heavy, the plant seems also to be“poisoned”
during the underground phase, making the injury more
serious than the “mere” withdrawal of nutrients. It is not
clear whether the compounds responsible are directly
produced byS. hermonthica, or are the result of metabolism
by the crop. Plants that initially appear healthy suddenly
turn yellow and wither, as if they had been bewitched—
giving the parasite its common name“witchweed”. After a
phase of underground growth, when the parasite accumu-lates nutrients, it emerges from the soil, where its own photosynthetic activity allows it to complete its life cycle.
Fig. 3 Maize field destroyed by infestation ofStriga hermonthicain
An estimate in 1991 showed that there were at least 5 million hectares infested in six countries of Central-West Africa with an average loss in production of 12%
(Sauerborn1991). In northern Ghana, the estimated losses
of the sorghum and millet crops reached an average of 20%. Overall losses in economic terms were estimated at more than $300 million although, considering the incompleteness of information, this may be an underestimate by a factor as high as ten. Recent estimates report a rapidly deteriorating situation with an increase in the total area infested to almost 50 million hectares. Nigeria is the country worst affected, with over 8 million hectares infested. According to these estimates, the infested area in Ghana would be between 12 and 27%, while in Central and Eastern Africa more than 6
million hectares of maize are infested byStriga. A report on
the distribution of S. hermonthicain 25 African countries
estimated that infestation of maize fields varied from 20 to 30% of the total in Togo, Mali and Nigeria and up to 65%
in Benin (De Groote et al. 2008). In the province of
Nyanza, in Kenya, clean fields planted with maize yielded about 1.5 t/ha, whereas the yield was about 750 kg/ha in moderately infested fields and only about 300 kg/ha in
severely infested fields (Manyong et al.2007).
Management
Generally speaking, one of the most widely used conven-tional solutions for the management of weeds is the use of herbicides. There are systemic herbicides that, when sprayed on leaves, are absorbed and move through the vascular system until they reach the roots where they may come into contact with parasitic plants, controlling them. Initially herbicides were applied at low doses, so as not to affect the crops, but without positive results. Then the use of hybrids of crops resistant to herbicides was attempted
(Joel et al. 1995). This strategy, although extremely
valuable, would still require supply and adoption of commercial seeds and machines for treatments, which would be too expensive for most African farmers. Recently, the use of seeds pre-treated with herbicides has been proposed. The herbicide spreads systemically in the plant
after germination, protecting it from attack byStriga. The
advantages of this approach are that it does not require machinery or technical knowledge and the consumption of herbicide is much lower, and therefore more
environmen-tally compatible (Gressel 2008). Herbicide seed coating
could also be combined with biocontrol agent coating, to improve the efficacy and reduce the risks of resistance occurring due to selection pressure. A study on the acceptability of this technology in which treated seed was distributed has recently been done with the help of
nongovernmental organizations (De Groote et al. 2008).
The success was resounding, with the result that companies
providing the treated seed were not able to meet the demand in the following year. Even greater benefits were reaped by supplying farmers with bags containing fertilizer along with the treated seed. Another advantage of this technology, in addition to its affordability, is that the cultivation of legume intercropping is not precluded, as would be the case with traditional herbicide treatments
(Kanampiu et al.2003).
Researchers have tried for decades to find resistance genes
in maize effective againstS. hermonthicabut as the centres
of origins of the two species are on different continents, the Americas and Africa, respectively, they have not co-evolved and it is therefore probable that maize has no intrinsic
resistance (Gressel and Valverde 2009). Only recently, an
indigenous speciesZea diploperennis, similar to maize, has
been found to have a modest level of resistance (Amusan et
al.2008). In the case of sorghum the situation is different as
its centre of origin and diversity, like that of Striga, is in
Africa (Ethiopia) and consequently there are likely to be resistance genes in wild populations. Recently, significant progress was made as plants were found and characterized that produce small quantities of stimulants that hamper the development of the haustorium or block the penetration of the germ tube. The combination of a few of these factors
allowed sorghum to have normal yields, even if someStriga
plants grew and set seed. Breeding these traits into local cultivars has been facilitated by the use of genetic markers, resulting in sorghum lines with high resistance (Ejeta et al.
2007; Gressel and Valverde2009).
Often African farmers have intercropped legumes with maize, both as a nitrogen source for the crop and to ensure a food supply in the event of total loss of the
maize crop due to drought or Striga. Legumes are
generally not antagonistic to S. hermonthica but recent
work has shown that the legume shrub, Desmodium
uncinatum, originally investigated as a catch crop for the
maize borer, Sesamia cretica, had an excellent effect on
the control of Striga. Unfortunately, owing to its lack of
adaptability it has the disadvantage of only being able to grow in restricted areas and can only be used, if freshly
harvested, as animal feed (Khan et al. 2007). Studies are
underway to identify the factors responsible for control-ling Striga in order to identify other crops with similar properties which are better suited to the different cultural needs and environmental characteristics of African regions
(Hooper et al.2009).
Interesting results were achieved with the use of isolates of Fusariumas mycoherbicides specific toStriga,
particu-larly strains of F. oxysporum (Ciotola et al. 1995; Elzein
and Kroschel 2004). The conidia of Fusarium can be
industries and programmes were initiated in this direction. However, as the culture of the microorganism is not easily accomplished, recent trends favour the organization of a centralized unit for the production and distribution of the material, which may provide higher quality and reliability
of the microbial product (Venne et al.2009).
Social and economic aspects
In the colonial period, the spread of Striga (and of S.
hermonthica in particular) had been limited for several reasons: in the fertile soil symptoms of the host were not
serious, local labour was used to removeStriga plants and
prevent the production of seeds, and crop rotations helped to reduce its impact and spread. After the colonial period, the situation progressively worsened. National governments wanted cheap grain for the inhabitants of cities, and therefore prices were kept low by decree or by importing grain donated or discarded by Western countries. Fertilizers
have never been used traditionally, and Striga plants
compete better in low-fertility soil (Ransom et al. 2007).
Once the seed bank of the parasite reaches a critical level the situation becomes hopeless: restoring soil fertility is ineffective because both the parasite and the crop take advantage; weeding by hand-pulling becomes impracticable
(Ransom et al.2007).
In the developing countries in general, and especially those in Africa, control of weeds is delegated to women, who are subjugated to a life in the fields, spending as much as 80% of their available time there performing
this manual practice (Akobundu 1991). This is why
women often prefer to accept other jobs, where these are available, even if they are poorly paid, rather than working in the fields. Weeds are a major reason why land becomes unproductive and when this occurs, men leave manage-ment of the land in the hands of women, children or the elderly. The abandonment of land and the proliferation of weeds lead to a further worsening of the situation. Men move to cities in search of work, with the consequence of the spread of sexually transmitted diseases. Men and women living with HIV are debilitated, and thus have even less ability to manage the lands, and the situation becomes worse still when their children are orphaned. There is thus a vicious circle: no fertilizer to limit the initial spread of
Striga, fewer manual workers to remove the parasitic plants because the men leave the farms for the cities, spreading diseases such as HIV-AIDS and malaria, less farm work owing to disease and consequently increasingly
larger areas becoming more severely infested withStriga
(Ejeta 2007; Parker 2009). This situation is further
complicated by the fact that the level of damage is unpredictable, some years being worse than others. Thus, farmers cannot produce more than 80% of the minimum
caloric needs of families, well below the level of survival
(Gressel and Valverde2009).
Banana Xanthomonas Wilt (BXW)
The plant host
Bananas and plantains (Musa spp.) are the fourth most
important staple food in the world, after rice, wheat and maize. The annual world production is estimated at 100 million tonnes, of which only 10% enter the commercial circuit, demonstrating how this culture is more important
locally than for export (FAOSTAT 2006). Approximately
one third of world production is concentrated in the SSA regions, where it provides about 25% of the food to over 70 million people. Banana is a common feature of the agricultural and cultural landscape in Africa, despite being introduced from Asia only several hundred years ago. Moreover banana is a perennial, providing continuous ground cover and preventing soil erosion that would otherwise occur if annuals were cultivated, especially those that are uprooted for their roots and tubers.
Fruits are used at different stages of ripening and for different purposes. For this reason they are named: dessert, plantain, cooking and juicing bananas. Dessert is a snack and in some countries exported (Ghana, Ivory Coast, Cameroon, Kenya, Malawi, Zambia and under irrigation in Somalia). Plantain (West and central Africa) is a staple and commands a higher premium as it takes longer to produce than cooking bananas. Cooking tends to be grown in the Great Lakes region and is a key staple. Juicing is also grown in the Great Lakes region and is processed into alcohol and used for income generation.
The eastern regions (Burundi, Kenya, Rwanda, Tanzania and Uganda) are the major producers and consumers of bananas in Africa. Uganda is the second largest producer in
the world after India (FAOSTAT2004). Bananas have huge
economic and social importance in the Great Lakes area as they represent both a source of food security and of profit
(Edmeades et al. 2007). In countries such as Uganda and
Burundi, they provide more than 30% of daily caloric needs, reaching even 60% in some areas. Also, for some agricultural areas they are the main export crop and therefore constitute a very important source of income
(Abele et al. 2007; Okech et al. 2004). Annual banana
consumption is about 190 Kg per person in Uganda, 140 Kg in Rwanda, 90 Kg in Kenya and 20 Kg in Tanzania
(FAOSTAT 2007). Bananas constitute an important source
of income for approximately 30% of the farmers who generally sell from 25% to 50% of their yield, especially in
the west of Uganda (Okech et al. 2004). More than seven
not surprising that they use the term“matooke”to indicate
both “food” in general as well as “boiling banana” in
particular.
Origin, distribution and impact
Among the many threats to banana plantations, such as reduced soil fertility, insects or phytopathogenic agents, the
disease caused by the bacteriumXanthomonas campestris
pv. musacearum, known as Banana Xanthomonas Wilt (BXW) is one of the most important emerging risks. This disease was initially reported in Ethiopia about 40 years ago onEnsetesp. (Yirgou and Bradbury1968), a genus closely
related to Musa. It was reported in Uganda in 2001 on
banana and from there it has spread rapidly to all regions of Africa where the crop is grown. No varieties have complete genetic resistance but they differ in degree of susceptibility
(Tripathi et al.2008). The cultivar Pisang Awak, originating
in Malaysia, is the most susceptible (Tushemereirwe et al.
2006). Symptoms consist of a progressive yellowing and
withering of leaves, and rapid and premature ripening of
fruits (Fig.4), which suffer internal brown staining (Fig.5),
leaf necrosis (Fig. 6), and rotting of male flowers, rachis
and the bunch. Finally, the plants wither and rot. Symptom appearance is rapid, becoming evident as early as 3 or 4 weeks after infection. This, however, depends on the type of infection, the environmental conditions, the state of the plant and the cultivar. Infection can occur: in inflorescences, when the bacterium is carried by insect vectors (stingless
bees, fruit flies) (Tinzaara et al. 2006); by mechanical
transmission due to the use of infected tools; in the roots when the soil is contaminated by infected plant debris
(Mwangi and Bandyopadhyay 2006; Tripathi et al. 2008);
and by raindrops containing the bacterium. It can also be disseminated by planting infected propagating material.
The disease has a devastating impact because it develops very quickly, giving rise to severe symptoms, leading to the death of entire plants, including those used for propagation
(Tripathi et al. 2007). Moreover, infested fields cannot be
replanted with banana for at least for 6 months, owing to the persistence of the pathogen in the soil. Once the pathogen has initiated infection, damage limitation is extremely difficult and the disease is impossible to cure
(Eden-Green 2004). Since 2001 the disease has spread in
some areas in an impressive manner, causing yield losses of
Fig. 4 Early ripening and rotting bunch caused by Banana
Xantho-monasWilt. Courtesy of Dr. Fen Beed, IITA, Kampala, Uganda
Fig. 5 Typical symptoms on fruit transects showing brown staining,
caused by Banana Xanthomonas Wilt. Courtesy of Dr. Fen Beed, IITA, Kampala, Uganda
Fig. 6 Foliar symptoms caused by BacterialXanthomonasWilt on
up to 60%. This has led the government of Uganda, for example, to set up a task force for the eradication of the disease by cutting down and destroying by fire diseased plantations, removal of male buds to prevent infestation from insects and stopping traders from coming to farms to harvest bunches unless tools are sterilized (Tushemereirwe
et al. 2006). Although these interventions have led to a
reduction in the incidence of the disease, there has been little support for them because of the high costs and the difficulty in convincing farmers of their necessity.
It has been estimated that, if not controlled, the pathogen can increase the area infected at a rate of 8% per year
(Kayobyo et al.2005). The damage caused by the disease
each year is estimated at 2 billion dollars, and at least 8 billions if projected over a period of 10 years. A recent study estimated 53% yield losses in banana production in Uganda in 10 years. Production losses caused by the disease threaten the food security of about 100 million people and the income of millions of farmers in the Great Lakes region of Central and Eastern Africa (Tripathi et al. 2009).
Management
The scenario described in the previous paragraphs has some important consequences for disease management. Usually, disease control measures are based on an economic threshold and are put into practice when the losses outweigh the costs of disease management (Peterson and
Hunt 2003). In this regard, management of bacterial
diseases presents several problems, such as mild symptoms in the early stages of an epidemic, and thus reducing the propensity of farmers to take action as the losses resulting from the destruction of plantations would, in the short term, outweigh the benefits. This, allied with the rapid onset of severe symptoms and spread of the pathogen mean that farmers only begin to take action when it is already too late
(Biruma et al.2007).
The management of tropical diseases in perennial crops such as bananas and plantains is a continual challenge. Management measures include a combination of: preventa-tive interventions to reduce disease establishment; curapreventa-tive control through destruction of infected plants where the disease is already present; and, rehabilitation of areas that were previously infected. Information campaigns, technical assistance and financial input from national governments supported by international and transnational organizations are crucial in these circumstances. From this point of view, the results in different countries have been different. In countries such as Uganda and Tanzania, with active political leadership, disease reduction of more than 90% was achieved. In other countries with different social and political situations such as the Democratic Republic of the
Congo, the spread of the disease has, on the contrary,
almost quadrupled (Mwangi et al. 2008).
Thanks to a suite of international donors, a Task Force was set up over the period from 2005 to 2008, which has helped the poorest countries to mitigate the effects of the disease in terms of both social security and food security, so that the reduction of banana production in countries such as Burundi, Kenya and Tanzania has had a less devastating social impact. This Task Force implemented differing interventions according to severity of the disease in order to: reduce the spread in areas where it was not yet widespread; provide opportunities for cultivating alternative crops in areas where it had had devastating effects; and, prepare for a gradual replacement of bananas with other crops, where the situation was gradually worsening. Properly informed and trained, the majority of small farmers have shown willingness to replace
banana cultivation with annual crops resistant to
Xantho-monas, such as beans, cassava, maize and potato, which are viable alternatives for cultivation and consumption
(Tushemereirwe 2001).
The use of certain farming practices and cultivation operations can reduce the spread of the disease. For example, timely removal of male buds prevents dissemina-tion of the disease by insect vectors, which in some areas is the most important means of spread. However, this practice has found little application in some areas as farmers, owing to ignorance, refuse to adopt it, considering it to be
detrimental to the quality of the crop (Kagezi et al. 2006).
Once the disease is established, there is no remedy other than to remove and destroy all plants and debris, in which the pathogen can survive for a considerable time. If this is done carefully, the crop can be reintroduced after a fallow
period of several months (Turyagyenda et al. 2007). The
plants are propagated by detaching the many suckers that are formed, and replanting them. Selection of planting material from fields known to be free of BXW can do much to prevent the re-establishment of the disease. Diagnostic methods are also valuable such as the use of semi-selective media or PCR-based diagnostics, preferably controlled
through a robust regulatory system (Mwangi et al. 2007;
Tripathi et al. 2007). A current constraint to the
re-establishment of banana is the lack of systems to supply micro-propagated (tissue culture) or macro-propagated (suckers) planting material, or certification systems to ensure material is disease free.
charac-teristics that make some varieties less exposed to the risk of
infection (Mwangi et al.2006).
The use of resistant varieties would be extremely desirable and economically viable. Germplasm of some local varieties has resistance characteristics (Tripathi et al.
2008) but these have no value as food so inclusion in
breeding programs may not yield commercially viable germplasm. Also the genetic diversity of banana in Africa is very narrow. It would be necessary to include germplasm from elsewhere in the world for an effective breeding program.
Other possibilities under study are based on genetic engineering and the transfer of resistance genes that induce the hypersensitive response through the use of embryogenic cell suspensions and the development of meristematic tissue
culture (Ganapathi et al.2001; Hernandez et al.1999; Wei
and Beer1996). Of particular interest are the ferredoxinlike
amphipathic protein (pflp) and hypersensitive response
assisting protein (hrap), isolated from sweet pepper
(Capsicum annuum), which can intensify the hypersensitive response. The International Institute of Tropical Agriculture (IITA) in collaboration with the National Agricultural Research Organization (NARO) in Uganda has generated several transformed lines of banana cultivars and these are currently being evaluated for disease resistance under
laboratory conditions (Tripathi et al.2009).
Social and economic aspects
In countries where the disease is present, BXW causes a drastic reduction in banana production, with consequent economic and social problems, which differ from country to country. Today, cultivation of Cavendish bananas is particularly risky because of the simultaneous presence of BXW and Banana Bunchy Top Virus (BBTV). These pathogens can seriously compromise the economies of
some countries (FAO2009b). The most alarming economic
and social situations have arisen from the spread of BXW in the countries of central and west Africa, seriously affecting the livelihoods and food security of the people there.
In Ethiopia and in the countries of the Great Lakes region (Kenya, Rwanda, Tanzania and Uganda) losses of production are thought to be serious but are difficult to
quantify (FAO 2009b). In Uganda, the disease was
registered in a total of 39 districts in 2008 and has spread from the centre of the country, where the subsistence economies are largely dependant on the production of bananas, to the west, where banana cultivation is intensive and largely dedicated to local markets. The greatest number of registrations was from the central areas of the country, where exotic, susceptible varieties such as Pisang Awak are mainly cultivated. In other areas of intensive production, such as the southwest, the disease has had less impact as, in
these areas,“cooking bananas”, which are less susceptible
are the principal cultivars grown (Tushemereirwe and
Opolot 2005). In the period between 2001 and 2004, crop
losses were estimated to be between 30 and 50% (Karamura
et al. 2006). These losses have dramatically affected
farmers’ families as 60% of their income is derived from
the banana crop. As a result, many families have abandoned the cultivation of bananas. It is estimated that over a period
of 10–15 years the cumulative loss in yield may exceed 5
billion dollars, with annual losses of food and income for each farmer of over $200, enormous sums for such
economies (Kalyebara et al.2006).
Banana production has almost halved over the last 10 years because of lack of nutrients, water and disease management, but the population has nearly doubled. This has led to a substantial increase in demand over production with consequent price rises, prices sometimes quadrupling
in a few years (FAOSTAT2007). The farmers initially tried
to compensate for crop losses by raising prices on local
markets (FOODNET2006). However, the losses became so
high that this approach became unsustainable. As a result local consumers with the low incomes that are prevalent in weak economies like those of most central African countries, could no longer purchase their main source of nutrition, giving rise to enormous problems in providing food, social tensions and political instability (Abele and
Pillay 2007; Kayobyo et al. 2005). In the Democratic
Republic of the Congo, banana is one of the staple foods and
is responsible for 90% of farmers’ incomes. Here BXW is
most devastating in the provinces of North Kivu, near the border with Rwanda. The economic activity in these provinces is now in crisis, exacerbated by conflicts and violence. Banana plantations occupy 23% of arable land in Rwanda with an annual yield of around 2.4 million tonnes. Here it is also the staple food and accounts for 60–80% of income (Okecie et al. 2004). The most dramatic consequen-ces of BXW of banana will probably occur in countries where food security depends on consumption of the crop and where there are already high or medium levels of food insecurity.
Wheat rust - Ug99
Origin, distribution and impact
The microorganism responsible for stem rust of wheat is a
microscopic fungus, Puccinia graminis f. sp. tritici. The
pathogen is also known as black rust owing to abundant
production of glossy black teliospores (Fig.7) formed at the
end of the summer. It is considered to be a very serious disease in many wheat-growing areas around the world and
has been known since Roman times (Savastano1890). The
biological cycle and unpredictability of its occurrence and impact.
Indeed, a field apparently healthy during kernel enlarge-ment and maturation may be quickly reduced to a pile of dark stalks and broken shrivelled kernels a few weeks
before harvest (Figs.8and9). Only in the last century have
studies allowed a better understanding of the pathogen and
the recognition of the existence of different races of the pathogen differing in ability to cause disease on different genotypes of the host. Successful breeding programmes for resistance have created resistant cultivars and reduced the harmfulness of the disease but, over time, the pathogen often overcomes the resistance, necessitating further breed-ing. Some major epidemics occurred in the 1940s and 1950s in Australia and the United States (Saari and Prescott
1985), but were then managed. In other areas appearance of
the disease from time to time is a very serious matter. This rust is particularly important in the final growth stages of the wheat plant, on late-sown or late-maturing crops, and at low altitudes. In warm and humid areas such as parts of Africa, the disease can survive from year to year on infected crops and wild grasses. As for all the rusts,
spores of P. graminisare dispersed mainly by air. Most of
the spores move only short distances, contributing to the local development of epidemics. However, low numbers of spores can be transported over long distances and cause
new infections. For example, Watson and de Sousa (1983)
reported transport of spores from South Africa to Australia.
The rust has a complex life cycle (Fig.10), that requires
barberry (Berberis vulgaris) as well as a cereal species. In
the spring and summer, stem rust infections on cereal plants produce urediniospores, which are spread by the wind to nearby cereal plants, where they germinate and infect cereals by penetration through the stomata, the infections producing a new crop of urediniospores in as short a time as a week. As a result, this phase can rapidly increase the severity of attack and spread the infection over a wide area. Towards the end of the growing season, the rust converts to producing teliospores, which remain dormant until the next spring when they produce basidiospores, that cannot infect cereal plants, but are instead carried in the wind, and infect young leaves of common barberry. The basidiospore penetrates the leaf epidermis directly, and the resulting infections produce specialized structures called pycnia,
Fig. 9 Symptoms caused by race Ug99 of stem rust on wheat stems.
Courtesy of Prof. David L. Hansen, University of Minnesota
Fig. 8 Wheat plants field severely affected by race Ug99 of stem rust
in Kenya. In the background healthy resistant plants. Courtesy of Prof. David L. Hansen, University of Minnesota
Fig. 7 Typical“black”symptoms caused by race Ug99 of stem rust
which are the sexual stage of the fungal life cycle. The resulting fertilized hypha forms an aecium which produces aeciospores that are carried by the wind and infect cereals by penetration through stomata. Development of these infections gives rise to urediniospores, completing the lifecycle.
In the case of wheat in tropical areas, the alternate barberry host is no longer considered important from the epidemiological point of view as inoculum survives on
“volunteer”wheat plants, i.e. self-sown plants growing out
of season. The fungus remains in the repeating
uredinio-spore stage on these (Fig. 10). Given a high density of
susceptible hosts over a large area, serious epidemics are likely to ensue.
In recent years, a race named Ug99 after its discovery in Uganda in 1999 has caused serious epidemics in some countries of East Africa and around the Horn of Africa (Ethiopia, Kenya, Sudan, Uganda). In 2001 the epidemic reached Kenya and arrived in Ethiopia only two years later. Today Ug99 has reached countries such as Yemen and Iran and is endangering the whole of Central Asia and the
Caucasus (Mackenzie 2007). These areas of the world
together account for 37% of the world’s wheat production
(FAO 2008a). The ability of Ug99 to overcome the
resistance conferred by many genes effective against other races of the fungus makes Ug99 one of the most dangerous of emerging plant diseases. The International Centre for the
Improvement of Wheat and Maize (CIMMYT 2005) has
estimated that at least two thirds of the wheat grown in India and Pakistan, which amounts to about 20% of world production, are very susceptible to Ug99 and that at least 80% of wheat varieties that are grown in Asia and Africa are potentially exposed to the fungus as its spores can be carried by wind over long distances and across continents.
FAO (2008b) identified countries at immediate risk of
infection as Afghanistan, Eritrea, Iran, Oman and Pakistan. These are followed by the Caucasian and Central Asian countries (Armenia, Azerbaijan, Georgia, Kazakhstan, Kyrgystan, Turkmenistan and Uzbekistan) which are thought to be at high risk of epidemics. Other countries at risk are Egypt, Iraq, Jordan, Syria and Turkey. It is feared that Ug99 may reach the Indo-Gangetic plains of India, and then threaten one of the most crowded areas in the world
(Fig. 11).
Management
Currently about 50 genes have been identified that confer
resistance to different races of stem rust (Sr genes), often
obtained from species related to wheat. However, isolates of the pathogen that can overcome the resistance conferred by them are already widespread, making them no longer useful in breeding for resistance. Luckily there are some genes for which there are no reports of races of the pathogen able to overcome them yet. The tremendous consequences of the spread of Ug99 from Uganda
(Pretorius et al. 2000) are mainly due to the fact that this
race is capable of overcoming resistance conferred by the
geneSr31, which is widely used in almost all varieties. It is
also capable of overcoming resistance conferred by almost all the resistance genes originating from wheat, and also the
Sr38 gene, introduced from Triticum ventricosum into
many wheat varieties grown in Europe and Australia. A very important aspect of the problem is to understand at the global level the potential for further spread of the disease, and to determine which of the world’s germplasm is at risk of infection by Ug99. In regions of East Africa, favourable weather conditions occur and host plants are present throughout the year, which certainly favour the pathogen. However, there is much evidence that it may go well beyond the borders of African and Arab countries and could easily reach South Asia and even East Asia and the United States.
In order to provide continuously up to date information on the current status and potential spread of wheat stem rust
race Ug99, the so-called“RustMapper”has been developed
by CYMMYT (http://www.cimmyt.org/gis/rustmapper/)
showing: the current known sites and status of Ug99 or derivatives; the country summary information on wheat production and susceptibility to Ug99; near-real-time wind trajectories from known sites; and, major wheat production areas in Africa and Asia. Integration of this information presents a near-real-time summary relating to stem rust race Ug99 (or derivatives) using Google Earth for visualization. Some resistance genes effective against Ug99 have been identified which may be introduced into cultivated varieties and which would help to reduce the impact of the disease.
Some of these genes could be transferred quite quickly while others will require much longer breeding pro-grammes. However, there is a race against time to produce cultivars that are resistant to Ug99 in order to prevent the occurrence of widespread epidemics. The identification and introduction of resistant genotypes that are adapted to the environment prevailing in the countries concerned, fol-lowed by a rapid and extensive production and distribution of seeds in these countries, remains the best possible control strategy. For poor farmers of Kenya and Ethiopia, for example, this is the only really affordable management approach. These measures would also reduce the ability of the disease to spread to neighbouring areas and, when combined with the simultaneous introduction of resistant varieties in areas where the disease has not yet arrived, would reduce the losses should the disease reach those areas.
Social and economic aspects
In the event of a pandemic, a large number of families of wheat farmers would be seriously threatened, especially those who have few alternative crops that can be cultivated. In these circumstances, landless workers dependent on agricultural work would also be severely affected and there would be an increase in the abandonment of small farms and increased migration to cities. As already indicated for other diseases, this would have enormous social and
economic consequences at the level of individual nations, and would also be reflected globally.
The Ug99 epidemic has occurred at a historical period when the world’s wheat reserves had reached their lowest level for four decades and when production of bio-fuels are occupying large tracts of land which could be used for food production. In recent years, the European Union and the United States have adopted policies aimed at drastically cutting traditional grain reserves. Forecasts of world wheat production provided recently by the FAO are alarming, particularly in relation to countries outside the OECD area, where there is significant growth in consumption of cereal products. An added concern is that droughts caused reduction in cereal production worldwide by 3.6% in
2005 and 6.9% in 2006 (FAO2008a).
The reduction in cereal world stocks, and wheat in particular, was primarily a consequence of high levels of volatility in the prices of these goods in local and international markets. This was due to different and concurrent causes, e.g. the difficulties of stock titles and securities that turned the commodities (raw material and agricultural products) into safe assets; the acquisition of the titles of commodities by big speculative investors; the increased incidence of bio-fuel crops which reduce land available for food production; the strong rise in the price of energy; the weakness of the US dollar as the SSA countries have invested their reserves in dollars. All these contributed to the phenomenon of agro-inflation (increases of food
Fig. 11 Spread of race Ug99 of
prices) which was reflected in the significant rise in cereal prices. For example, in the period between 2002 and 2007 the price of rice increased by 70%, that of soybean by 90% and that of wheat by 130%, and increased further in the last two years. There were dramatic consequences in countries that are net importers of cereals. Around the world the inflation phenomenon hits the poorest social classes worst as their income is spent mostly on food. The most serious consequences are for the 77 poorest countries which are net importers of food products (Low Income Food Deficit
Countries) (FAO2010). Many of these are already affected
by Ug99 or are at high risk of its introduction.
In many countries already affected by the epidemic and in most of those at immediate or high risk of infection, wheat is a staple food and provides about 40% of daily calorie intake required by individuals. In some countries where the disease is already present there is a food crisis due in large part to the rise in prices of wheat and other cereals. FAO has recently ranked countries which are in a situation of food crisis and those at higher risk: the first group includes countries such as Ethiopia, Eritrea, Kenya and Tajikistan. Among the countries at higher risk is Yemen. These are countries where Ug99 is present or is likely to enter.
The most dramatic economic and social effects are occurring in the Horn of Africa, where the economy largely depends on agriculture. There, about 70% of the population lives in rural areas and owes its survival to the production and consumption of cereals such as maize, sorghum and wheat and also cassava. In Ethiopia and Kenya, countries where the disease is already present, wheat contributes significantly to food security, and the annual consumption per capita over the last decade has progressively increased, reaching values of 30 and 27 kg, respectively. In these countries, however, domestic wheat production is unable to meet the internal demand, and hence they have to import about 16 and 22% of their cereals, respectively.
The small amount of data in the literature concerning the socio-economic effects of the spread of Ug99 highlights the loss of significant production of wheat, leading to new
levels of social vulnerability (Fekadu and Gelmesa2006).
In Kenya it is estimated that wheat losses due to Ug99 in some areas are over 70% of the total production. This has necessitated increased imports of wheat from abroad and thus increased dependence on external supplies. Production losses have also led to higher wheat prices on local markets which affected the urban and rural population with lower incomes. In turn, this has caused an increase in the number of people suffering from hunger, an overall increase in the level of food insecurity and a general loss of social status of farmers who have had to abandon their crops.
To understand the social effects caused by Ug99 in these areas of the world, one must take into account that the loss of production, due to the fungus, amplifies already existing
food shortages, resulting in particular from drought. These two factors are responsible for generating one of the most alarming food crises in the world.
For example, in Kenya the particularly serious shortage of rainfall during the months of March and April 2009 in the South East and along the coast, coupled with disease, resulted in the loss of most crops. As a consequence there
were increased imports of wheat and maize from abroad—
during the period November 2008–June 2009 1.1 million
tonnes of wheat and maize were imported to meet domestic demand. The low water availability in pasture areas of the coastal area and south-eastern areas has dramatically worsened the living conditions of the population, increasing levels of mortality due to hunger, which affected mostly the poorest people. The situation is exacerbated by the concomitant rise in grain prices and other food commod-ities in local markets. In Ethiopia, an estimated 4.9 million people require emergency food aid. Here, the effects caused by the drought were felt particularly in the Oromyia and
Amhara regions (FAO2009a).
Norman Borlaug, whose efforts in breeding high yielding and rust resistant wheat won him the Nobel Peace Prize in 1970 denounced the delay of the international
community in taking Ug99 seriously (Mackenzie 2007).
The achievements of the green revolution had encouraged an attitude of complacency and neglect of agriculture (Ejeta
2009). This had translated into the dismantling of training
courses for local people and programmes of research on resistance of wheat to rust. The lack of seriousness with which this emerging plant disease was faced has threatened the survival of entire communities in the Horn of Africa and delayed scientific efforts aimed at seeking solutions capable of halting the progress of the disease.
The risk of epidemics is also linked to the limited capacity of farmers in these poor areas of the world to manage disease. In Africa, the increased dependence of many countries on emergency food aid has gone hand in hand with a substantial reduction of national government investment in agriculture, particularly in regard to pro-grammes for education of farmers, research and rural development. This general phenomenon has affected all the countries that have been invaded today by Ug99 or are
at high risk of invasion (FAO2008b).
Discussion
It is well known that the risks of introduction / invasion of plant pathogens, whether into monocultures, horticultural crops or natural communities, are increasing because of globalization, increasing human mobility, climate change, and evolution and adaptation of pathogens or their vectors
