Acknowledgements
3.8. Factors Affecting Severity or Incidence of CWD in Uganda
There are probably many factors, some known and many still largely un- known, that influence the incidence and severity of CWD. Some of the fac- tors are examined here.
3.8.1. Lack of information
One of the factors that could have accelerated the spread of the disease in Uganda was lack of information/knowledge on all aspects of the disease.
Research, extension and the Ministry of Agriculture had no useful informa- tion to give to farmers to slow down or stop the epidemic. In addition, it was widely believed that robusta coffee was resistant to most pests and diseases.
This state of uncertainty gave time for the disease to continue to spread to all districts within a relatively short time.
3.8.2. Varietal susceptibility to CWD
Most robusta plantings in Uganda originated from seeds selected by farmers from their own coffee fields. Farmers preferred their own seeds/seedlings, although there are recommended clones or clonal seeds available that have better yields, resistance to some diseases and good bean and cup qualities.
The dependence on self-saved seeds/seedlings was largely because they in- cur no cost in obtaining them whereas the recommended clones/varieties were very expensive. The cost of recommended robusta cuttings was 500 Uganda shillings for each single clone, and seedlings cost 300 Uganda shil- lings each. Clonal seeds from the research station cost 2000 per kilogram. In addition to cost, the source of good planting materials may be far removed from the farmers, so further costs in transportation are incurred; all these costs encourage farmers to use their own materials. This seedling coffee was very susceptible to CWD and encouraged the spread of the disease. Although CWD can also infect the recommended clones, partial resistance is evident as compared to seedlings.
3.8.3. Environmental factors
At the time of the onset of the epidemic, rainfall was observed to be higher than normal for a prolonged period of time. Favourable environmental con- ditions, in the presence of susceptible germplasm and an aggressive patho- gen, led to the disease spreading rapidly throughout the country. Two rainy seasons normally occur in Uganda. The first rainy season (March–June) is normally heavier than the second rains (September–November). Recent field observation trials at Kituza revealed an increase of CWD and disease prog- ress in infected plants during the rainy season and a drop in number of new infections during the dry season. This can be attributed to increased plant activities during the wet season, which enhances fungal infection and coloni- zation of the host as well as rain being involved in the spread of the disease.
In addition to climatic factors, soil type seems to have an influence in that farms on predominantly sandy soils had more wilt-infected coffee trees than farms on predominantly loam, sandy–clay or clay soil types.
3.8.4. Lack of preparedness and resources
Generally, the response to the epidemic was slow for various reasons. Because it was new, no one could predict the magnitude or gravity of the problem.
Lack of resources and lack of preparedness to mitigate emergency situations such as this could not allow a quick response. Despite recommendations for uprooting and burning, as well as other sanitary measures, farmers lacked resources to uproot and burn infected plants. The exercise proved too expen- sive and too demanding on the ageing farmers who are the majority. Lack of resources also delayed the start of serious research by the national research
institutions. Research priorities were based on constraints with measurable impact on the farmers and national economy but could not readily accommo- date emerging problems. Lessons learned from these experiences probably caused a more positive action when bacterial wilt of bananas (Xanthomonas musacearum) emerged in Mukono district in 2002 (Banana Bacterial Wilt Task Force Report, 2002, 2003). Mobilization of farmers for uprooting and bury- ing infected banana stools was quickly done and a research agenda defined and funded within the same year the report was received, which was in total contrast to what happened when CWD was first observed.
3.8.5. Human activities
Unknowingly, farmers, farm workers and others spread the disease within the farms and out of the farms to their neighbours and to other districts. Within the farms, farmers and others cut the dry trees for use as firewood at the home- stead. The mode of transport of the dry plants was by dragging them through the coffee field and taking the shortest route to the homestead, to where these were stacked outside the kitchen house. Shortly afterwards, more infected trees were observed along the routes taken through the coffee field. The in- fected stems were also transported on bicycles and through other means of transport to distant places where new infections could have been started.
G. xylarioides is considered a wound pathogen, which can only invade the plant through wounds or damaged tissue (Muller, 1997). In many nor- mal farming practises, such as weeding using hoes or slashers or machetes/
pangas, wounds can be created on roots and stems. A higher disease inci- dence was evident when coffee was hoe weeded as compared with other weed management practises. Pruning and desuckering also create wounds, which provide entry points for the pathogen, and the incidence was greater in coffee where the canopy was unpruned as compared to coffee under can- opy management (pruned). High disease incidence was also observed on farms that did not use fertilizer or manure. Wounds created close to stem base are more likely to result in systemic infection than wounds further up the plant (Hakiza et al., 2004). Under favourable conditions of temperature and moisture, spores of the pathogen deposited on the wounds germinate and penetrate the host resulting in infection, and later visual symptoms of the disease would appear. Shading of the coffee increased the probability of disease incidence, and coffee farms with shade trees had significantly more infected trees than farms without shade.
Increased CWD incidence was also observed on farms where livestock were tethered to coffee trees or animals were allowed to graze in coffee fields, as compared to farms where livestock (cattle, goats, sheep and pigs) were not tethered or allowed to graze in coffee fields. Extensive wounding of stems was observed as well on big roots where pigs had been digging up soil. Most probably these wounds led to increased infection.
Infected planting material/seedlings could have played a role in the dis- semination of the wilt pathogen. It is widely believed that CWD crossed to
Jinja, Mayuge and other districts in Eastern Uganda, mostly through volun- teer seedlings collected from forests in Mukono district where CWD was also rampant on forest coffee. The farmers are attracted by the low cost of these seedlings compared to good seedlings from known sources. The seedlings may appear healthy but could be infected or contaminated with G. xylarioides.
Symptoms may develop later when transported to other areas for planting.
Abundant inoculum in the form of perithecia is readily produced around the collar region and a few centimetres below the soil surface following the death of the seedlings. This inoculum is washed by rain into soil or spread by wind to initiate infection to healthy seedlings planted in the same field or further.
As a soil-borne pathogen, any material that carries soil is suspect. Nurs- ery operators often collect topsoil from the forests, as this soil is considered very fertile. It has been demonstrated that unsterilized soil collected from forests may have enough contamination with the disease pathogen to cause infection to seedlings (Flood, 2005). The seedlings/clones raised in soil con- taminated by the wilt pathogen can be transported far and wide and cause new infections when they die, and perithecia are formed abundantly around the collar and just below the soil surface.
Coffee plantations had significantly more CWD than other production systems, and coffee husks were used for mulching in many coffee plan- tations. A number of farmers who lost their coffee have attributed this to mulching with coffee husks. It was also claimed that the disease got to Mu- kono in coffee brought from the infected districts in western Uganda and hulled in Mukono district. Many farmers took the husks for mulching their coffee fields. Mukono district is among the districts that had a high incidence of CWD. Because the coffee husks were suspected carriers for the pathogen, its use for mulching was stopped by the local leaders. The recommendation is maintained to date.