Acknowledgements
3.5. Brief History of CWD in Uganda
An account of the spread of the disease from the time it was first reported and the actions that followed are provided here and are meant to provide les- sons learnt for the whole coffee sector and should allow for a better response to future epidemics.
In 1992, information was received from John Schluter, a businessman on a visit to the Democratic Republic of Congo (DRC), of a devastating robusta coffee disease in the Beni and Isiro areas of DRC and warned of its conse- quences if allowed to cross into Uganda. To find out as much information as possible on the new disease, its symptoms, causal organism, epidemiology and control, the senior author turned to literature to be able to recognize the disease and what to do about it should it appear. Recommendations for con- trol of the disease were obtained from literature (Wrigley, 1988; Coste, 1992).
Uprooting and burning of infected plants at the earliest symptoms and other sanitary measures were intended to destroy sources of infection/inoculum to reduce the rate of spread within the field and outside the field. Infected plant parts carried by people as firewood was a mechanism for disease spread as were agencies such as wind or rain. Spread of the disease through these agencies ensured dissemination far from the original affected farms.
Ironically, in September 1993, wilting and death of a few robusta coffee trees were observed in a 2.8-ha experimental plot at CORI, Kituza, in Mu- kono district of central Uganda. The plant pathologist (main author) recov- eredF. xylarioides, the anamorphic form of Gibberella xylarioides, from infected plant parts (stems and roots). Identification of the pathogen was based on cultural and spore morphological characters as described by Booth (1971) and Gerlach and Nirenberg (1982). Reproduction of symptoms on seedlings was done by transplanting 6-month-old seedlings/clones in sterile soil in- fected with spores of the pathogen and maintaining the inoculated plants in the screen house until symptoms appeared. Re-isolation of the pathogen from infected seedlings confirmed the pathogenicity of the fungus causing the wilt.
The recommended standard control method for CWD is uprooting and burning of the diseased plants to eliminate potential inoculum sources for further infection and was maintained in this particular plot from 1993 to 2000. At first, the method appeared effective. However, with time, the method proved unsustainable due to the high cost of labour for uprooting the diseased plants. It appeared that inoculum sources were not only from
within the same field but also from external sources. It transpired that coffee trees in the adjacent forest were also infected by wilt, and it was suspected that this was the source of inoculum. As long as other sources of inoculum exist, from outside the targeted field, infection continued. Farmers should therefore work in conjunction with their neighbours to minimize the transfer of inoculum from one farm to another.
Plants can be affected at any stage of development, from seedlings to adults. The first symptom exhibited from the top of infected plants is a flac- cidity of the leaves on one side of the affected stem. Chlorosis may or may not occur. Wilting, rapid defoliation and dieback soon follow (Plate 3). Dur- ing the rainy season, leaf wilting may not be readily observed, and the only indication of wilt is rapid defoliation (noticed as carpet of leaves at the base of trees) and dieback, which is the progressive death of branches or shoots beginning at the tips leaving behind bare dry twigs. Dieback at the tips of the main stem spreads down to all primary branches resulting in death of stem.
Dark brown necrosis occurs on stems of young suckers and branches, which leads to death of those parts. Leaf veins show dark brown necrosis. On a multistemmed coffee plant, stems die in sequence, one by one until the whole bush dries up completely. Coffee berries on the affected tree ripen prema- turely (turn red) and dry up but remain attached to the primary branches.
CWD is confirmed by the presence of blue–black streaks or bands in the wood when a bark of the affected stem is peeled off. This distinguishes CWD from all other wilting, which could be due to root mealybugs A. mel- lea and other fusaria such as Fusarium oxysporum,Fusarium solani, Fusarium lateritium and Fusarium decemcelulare. At the advanced stages, cracks may oc- cur on the stem from around the collar region and up along the stem. Within these cracks in the bark are embedded brown to black perithecia, which bear numerous ascospores. Ascospores are the sexual or teleomorph phase of the fungus. These are spread by wind or washed by rain into soil to become soil inoculum, then spread by runoff to other plants in the vicinity.
Infected plants do not recover even after pruning and/or stumping.
When pruned or stumped, any suckers that may sprout develop dark brown necrosis and eventually wilt, die and dry up.
The affected and dried up plants remain firmly rooted in the ground, unlike those affected by fungi such as Armillaria or root mealybugs, which easily topple over when pushed.
Following the outbreak in Mukono, recommendations were made based on literature (Coste, 1992) as an emergency measure in the absence of any other information and were not then based on research findings done in Uganda at the time.
Thus, regular field inspection to detect early symptoms of the disease was recommended to farmers. Any plants confirmed as infected were to be uprooted and burnt on the spot. Farmers were advised to avoid using coffee husks as mulch in their coffee fields because these were strongly suspected to be contaminated with the pathogen. Some districts, e.g. Masaka, passed a by-law prohibiting milling of coffee from outside the district. Restriction on the movement of infected plant materials, coffee husks and soil was also
recommended. The berries on infected plants were to be separately picked from coffee on healthy plants. Farmers were also advised to plant coffee on new land, preferably some distance from the infected field. Replanting in the affected fields was recommended not be done immediately after uprooting but to delay planting for at least 1 year and preferably longer; other crops can be grown. Tools were to be cleaned by flaming or use of disinfectant to avoid passing infection from an infected plant to another. When pruning or handling plants, always start with healthy-looking plants first and sterilize tools after each plant. Wounding the stems during cultivation (such as weed- ing) of the coffee should be avoided because wounds are likely sites for entry of the pathogen into the plants. These recommendations are now backed by research findings that have been conducted in the past 4 to 5 years during the project period, and therefore, the above management measures remain.
The first report of wilt outside Mukono district was received in Octo- ber 1993 from Bundibugyo district in south-western Uganda bordering the DRC concerning the presence of a destructive wilt disease of robusta coffee.
The Uganda Coffee Development Authority (UCDA) sponsored a multidis- ciplinary team of researchers composed of a pathologist, an entomologist, an agronomist and an economist to travel to Bundibugyo and identify the prob- lem. The main author was part of the expedition. The affected robusta ar- eas were Bwamba, Bubukwanga and Bubandi counties. In this district, both robusta and arabica are cultivated. Arabica coffee occupies higher grounds in Nyankonja, Busaru and parts of Bubukwanga counties. The disease was reported only on robusta coffee. During the survey, two farms in each of the three subcounties were visited, and samples were collected. All the fields vis- ited had some dead plants and partially diseased trees. Symptoms of the dis- ease were not typical of the wilt disease caused by F. xylarioides as described in literature (Coste, 1992) and observed at Kituza hardly a month earlier. The dead and partially diseased plants had dieback, some leaf fall occurred but a lot of dry leaves remained attached to the dead branches. When bark was scraped off from the stem of the affected plants, there was no blue–black streak characteristic of F. xylarioides, instead only brown to dark brown disin- tegrating/rotting tissues were observed. A whitish powder covered most of the dead and blackened suckers and branches in many trees. From the sam- ples collected, the fungi recovered in the laboratory were Fusarium stilboides, F. lateritium, F. solani and F. oxysporum, which can also cause wilting and death of plants under stress. In addition, there was a high incidence of stem borer attacks leading to wilting and death of trees in the same fields. During this first visit, the presence of CWD was somehow missed probably due to the high incidence of other fungi and stem borers. It was later that these other fungi became less frequent, and F. xylarioides became most dominant from specimens received from this area (Hakiza, 1998, unpublished report).
More reports and samples were received in 1994 from Kanungu district (formerly part of Rukungiri) in western Uganda, and the CWD pathogen was consistently recovered from samples of roots, stems and branches/pri- maries. Specimens from the Bundibugyo district, received about the same time, also clearly revealed the presence of the pathogen.
The worsening situation prompted UCDA in 1996 to organize and fund a joint survey conducted by a multidisciplinary team from the Ministry of Agriculture Animal Industry and Fisheries (MAAIF), Coffee Research Cen- tre (COREC) and UCDA to establish the distribution/extent and damage caused by the disease in ten major coffee-growing districts of Uganda. It revealed the presence of the disease in ten districts: Mukono, Mubende, Mpigi, Ntungamo, Kasese, Kabarole, Rukungiri, Bundibugyo, Kiboga and Masaka.
In the same year, UCDA invited Dr Julie Flood of CABI to assess the situ- ation, as she had just conducted a survey in Eastern DRC (Isiro area) on the same problem (Flood, 1996), and she made an assessment in January 1997.
Her findings confirmed the presence of CWD as had been reported by the na- tional scientists. She recommended regional collaboration to enable Uganda to benefit from the past experiences of African countries that had previously successfully managed outbreaks of CWD in earlier decades.
A regional workshop on CWD was held in the International Conference Centre, Kampala, from July 28 to 30, 1997. Representatives from the Interna- tional Coffee Organization (ICO), CABI and CIRAD attended the meeting, as well as participants from African countries. The main outcome of the meet- ing was the initiation of a regional research project proposal (Regional Coffee Wilt Programme) to contain the situation. Uganda and DRC, which were the worst affected countries, were to play leading roles in research and develop- ment on CWD. This was a major landmark in the struggle against CWD in the region. The project was finally initiated in 2000.
However, the incidence of the disease had continued to increase through 1995–2000, and samples and reports were received from other parts of Mukono district, Kiboga, Mpigi, Jinja, Iganga and Mayuge. In 2000, reports and samples were received for robusta coffee from the West Nile region where both coffee species are grown; the pathogen was reisolated. By the end of 2000, all robusta coffee districts in Uganda had been affected. Consequently, during the inter- vening time between the research proposal (Regional Coffee Wilt Programme) being written and obtaining the funding, the National Agricultural Research Organisation (NARO) of Uganda became so concerned that they convened a technical workshop on CWD in 1999 to examine research progress and define action points to accelerate research activities toward finding a solution. Fol- lowing this meeting, a task force composed of plant pathologists, plant breed- ers and socio-economists from NARO and Makerere University was formed to formulate a research programme on CWD and to decide on its implementation.
A comprehensive research programme was drawn up for a period of 5 years together with its budget. CORI is continuing to implement the programme.
Many reports of wilt on arabica coffee and samples received at different times from all the arabica areas in the eastern, western and north-western/
West Nile region of Uganda revealed other Fusaria but not G. xylarioides.
Many follow-up visits to those areas have also been made to investigate and verify the reports. In all cases, G. xylarioides has not been recovered from samples or observed in any of the field visits made. The fungi that have been consistently recovered from samples were F. solani and F. oxysporum and
were confirmed as the cause of localised wilting and death of arabica coffee in those districts (Arua, Nebbi, Bushenyi, Kasese, Ibanda, Rukungiri, Mbale, Sironko, Bududa and Manafa districts). Root rot caused by A. mellea Vahl has also caused concern and has been mistaken for CWD. Insect attacks, the most common of these encountered in the arabica areas, that induce wilt symp- toms that can be confused with CWD are coffee stem borers (B. sierricola), coffee root mealybugs (Planococcus ireneus) and twig borers. In essence, to date, CWD infects only robusta coffee, even in districts where the two crops are grown side by side.
The absence of the disease in arabica coffee in Uganda shows that the pathogen is currently host specific. It is not known if with time the pathogen will change and infect arabica as well, particularly in areas where the two species are grown side by side. G. xylarioides remains a potential threat to arabica coffee in Uganda.