The effect of copper fungicides upon the bark microflora of coffee Arabica with particular reference to the pathogenic strain of colletotrichum coffeanum

Abstract

In recent years many coffee growers in Kenya have expressed the view that Coffee Berry Disease caused by a pathogenic strain of Coil etotri chum coffeanum Nook has been aggravated by the use of copper fungicides. Evidence to support or refute th~ ~ has however been lacking and one of the principal objects of the work described in this thesis was to determine whether copper fungicides could conceivably have any deleterious effect. The pathogenic strain of C. coffeanum lives in the maturing bark of coffee shoots and it has been considered that the level of activity of the fungus in the bark is a major factor in determining the severity of the disease. Evidently it is possible that copper fungicides might in someway create an environment in the bark which would be more favourable to the pathogenic strain. To examine this problem quantitative methods were developed for assessing the sporulating capacity of the fungi colonising the maturing bark. For much of the work a dilution plate technique was satisfactory but since spores of the pathogeni c strain often constituted only a tiny fraction of the total spores produced by the bark it was necessary to develop a more sensitive bioassay for this particular organism using detached green coffee berries. It could be shown that there was a linear relationship between the log spore concentration of inoculum applied and the probit for percentage infection of berries. Factors affecting this relationship (temperature, cultivar, age of berry and type of lesion) have been examined. An initial survey of fungi on the maturing bark of trees from sprayed and unsprayed coffee farms showed that even twel ve months after spraying had ceased there were significant differences in the composition of the fungal population, and of particular importance was the finding that the pathogenic strain of C. coffeanum was increased 5-10 fold. In order to examine the reasons for this large difference a detai Ied study was made of the fungi occurri ng on the bark at di fferent stages of maturation on sprayed and unsprayed branches. All strains of C. coffeanum including the pathogenic strain showed maximal activity in the early stages of bark maturation, other fungi tending to show peaks of activity at later stages of maturation. The short term effect of spraying is to reduce activity of all fungi though C. coffeanum strains were among the Ieast affected.. Fifteen months after spraying, however, all fungi tended to show remarkable recovery particularly in the later stages of bark maturation. The ccm strain of C. coffeanum recovered to about the same level as in unsprayed coffee but the ~ strain was appreciably reduced whilst large increases in sporulation occurred in the ccp and CBD strains. Of the fungi found in the bark relatively few showed antagonism to the pathogenic strain and these were generally most abundant in the later stages of bark maturation and were thus spatially isolated from the main site of activity of C. coffeanum. They were, moreover, substantially increased as a result of spraying and consequently would not appear to play an important role in regulating the activity of the CBD strain. There was no evidence of major antagonism between the different strains of Colletotrichum, and detailed analysis of small areas of bark showed clearly the coexistence of all four strains. Preliminary studies on the effect of copper on the survival of spores of different Colletotrichum strains indicated that the cca strain was the most sensitive and, though there was little difference between the others, the CBD strain was slightly more resistant. It is considered that the most likely explanation to account for the large increase in the CBD strain is an alteration in competitive relationships between strains of C. coffeanum though the possibility that other groups of microorganisms (bacteria, actinomycetes) may be having an effect cannot be overlooked. Moreover it is now known (Gibbs, unpub.) that an increase in the level of the CBD strain can be brought about by many fungicides other than copper. This indicates that the increase in CBD must be attributable to a general fungicide action rather than a specific effect of copper fungicide and the ways in which this may operate are briefly discussed