- PART IV
prevailing in different parts of the same farm, indeed even in small
groups of plants or, ultimately, in the immediate vicinity of single
plants of field or plantation crops are susceptible of considerable
modification in certain directions. Thus rank growths of weeds may
assist in maintaining a higher degree of humidity in the atmosphere
surrounding the crop plants and, if allowed to persist into the dry
season, produce, by depleting the soil moisture, soil conditions
characteristic in certain respects of a district of less rainfall. In
plantation crops the presence or absence of wind-breaks or shade trees
and the amount of pruning carried out may alter considerably the
humidity and the daily range of temperature which the trees
experience.
A few instances from actual experience will help to illustrate
some of the points it is desired to make. Coffee, which is one of the
chief crops of the country, is particularly useful for the purpose. It
1s grown in Kenya at altitudes of 4,500 feet to 7,000 feet and is subject
to several diseases, some of which are very considerably limited in
severity and distribution by climatic factors.
Leaf disease, or rust, of coffee requires warmth and moisture to
enable it to do serious damage. Temperature, however, is much the
more important factor and, although the disease becomes very
destructive in a warm district with heavy rainfall, actual rain is not
an essential. Heavy dews are sufficient to enable the leaf disease
fungus to attack its host. Thus in Kenya, while the worst cases of
leaf disease occur in the lower coffee districts with high rainfall,
scarcely less important outbreaks frequently occur in other districts
of similar altitude where the rainfall is often so deficient as to cause
anxiety to the farmer. On the other hand the disease is rarely
common above 6,000 feet except where climatic conditions have been
modified in favour of the fungus by the provision of excessive
windbreaks, or by allowing the plantation to become neglected. It
has been stated that there is evidence in India and also in Kenya that
the virulence of the attack of leaf disease lessens after the first onset
(1) (2).* Experience of recent years does not appear to support this
view as far as this country is concerned. Quite mature plantations
continue to suffer extensive defoliation whenever climatic conditions
are favourable to the fungus.
Coffee berry disease which attacks green coffee berries is
apparently peculiar to Kenya and is a malady which is favoured by
very different conditions from those which encourage leaf disease.
The former requires almost constantly wet conditions but can tolerate
comparatively low temperatures. Thus it is only serious at certain
hgh altitudes with good rainfall distributed evenly over the year and
it is not anticipated that it will ever become established to the eas:
of the Rift Valley, at any rate at the lower altitudes. This berry
disease does not occur everywhere climatic conditions are favourable
and it seems therefore that other factors are involved amongst which
nutrition is probably important.
Pink disease, which kills the branches and main stems of coffee
bushes, affords a good illustration of the effects of very local influences
on general climatic factors. The disease in" Kenya is confined to
districts with high rainfall—50 inches or more—but altitude appears
to make little difference. Given sufficient rainfall, however, pink
disease does not necessarily follow. Local conditions causing deficient
* See list of references, page 220.

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