| dc.description.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 | en |
