The histology of resistance to coffee berry disease (CBD) was studied in artificially inoculated berries and hypocotyls of 6-week old seedlings of a number of varieties of Coffea arabica L. In resistant varieties a phellogen was quickly formed some cell layers below the site of infection and progress of the fungal invasion was effectively blocked by a complete barrier of suberized cells. Such barriers were absent or incompletely developed in CBD susceptible varieties. A highly significant correlation (r = 0.87) was found between the frequencies of complete barrier formation in berries and in hypocotyls of young seedlings, while both were also highly correlated to observed mature plant resistance (r _> 0.93). Resistance to CBD in arabica coffee may to an important extent be based on the formation of cork barriers. These cork barriers confine the pathogen to the small volume of tissue external to the barrier so that its growth is severely restricted. Such a resistance mechanism is likely to be stable (race-nonspecific). The almost identical response to infection observed in berries and hypocotyls provides further evidence that plants with resistance to CBD can be reliably preselected by the hypocotyl inoculation test.

No abstract

Data from previous trials that have been re-evaluated indicate that low levels of coffee berry disease on unsprayed coffee are associated with higher populations of microflora on coffee tissues. Samples were taken at different seasons and altitudes, and from coffee plots receiving different fungicide treatments for the control of coffee berry disease. Although largest populations occurred at the highest altitudes, the relative proportions of microflora components varied with altitude. Fungicides reduced populations of filamentous fungi on all tissues. Trees that had never received fungicide had low populations of Colletotrichum kahawae and had very low levels of coffee berry disease. The disease was also at a lower level on trees that had not been sprayed with fungicides for two seasons where microflora populations were highest. The likely effects of microflora components on coffee berry disease and their potential as biological control agents are discussed.

Vegetative compatibility using nitrate nonutilizing (nit) mutants was analysed between 44 isolates of Colletotrichum kahawae from Kenya, one each from Ethiopia and Malawi, one of Colletotrichum gloeosporioides and one of Colletotrichum acutatum. Another isolate of C. kahawae did not generate mutants and thus could not be utilized. The results showed that all the C. kahawae isolates, except a white sector mutant (VCG2), belonged to one vegetative compatibility group (VCG4). The other species belonged to their own unique groups (VCGs 1 and 3). Implications of the results and future research needs on the subject are discussed.

Coffee is of basic importance to the Kenyan economy, producing about 30 per cent of the total foreign exchange earnings. In the 1960s severe losses from coffee berry disease led to the launching of an intensive breeding programme that has resulted in a new cultivar which is disease resistant, high yielding, and compact in growth and produces good quality arabica coffee.