PART IV. 3
follow as the aftermath of such attack. Any factor which puts a
severe strain on a coffee bush may, therefore, cause dieback of the
branches and, in practice, several influences, either single or in
combination, are known to be able to do this. Overbearing, at least
in Kenya, is the commonest cause of the malady and is the one chiefly
responsible for the condition already mentioned in which there are
upper and lower regions of healthy branches and an intermediate
dessicated zone. The explanation of this appears to be somewhat as
follows. The youngest primaries situated near the apex of the bush
have not yet reached the bearing stage, and have, therefore, not been
subjected to the strain involved in maturing a crop. Those at the
base, although they may be carrying a heavy load of berries, have had
time to produce a system of secondary and tertiary branches to assist
in the work of providing the food materials necessary to the
development of the crop. The intermediate branches below the apical
zone, while just old enough to come into bearing, have not the
advantage of such secondary and tertiary growth so that, if the crop
is sufficiently heavy, the strain proves too great and dieback results.
Two parasitic influences which are frequently important as
indirect causes of dieback are severe attacks of leaf disease (Hemileia
vastatriz) and thrips. These cause extensive defoliation, and unless
special measures, such as heavy manuring, are taken to assist the
recovery of the affected bush, dieback invariably follows. In districts
where leaf disease is prevalent a bush which is bearing a heavy crop
succumbs much more readily to the disease than one which is not, so
that the strain of overbearing is considerably accentuated by attack by
the fungus and subsequent dieback is rendered the more certain.
Insufficient rainfall may in itself be a cause of dieback but it is more
usually only a contributory factor, although frequently an important
one. Neglect of proper cultivation has a similar effect by depleting
the soil of moisture and also by preventing proper soil aeration. This
is well seen in the extreme case of a derelict plantation, containing
rank growths of weeds; in such places the bushes are invariably badly
affected with dieback. Any circumstance which prevents the proper
development of the root system, such as faulty planting or the
presence of hard pan, may also lead to dieback.
It has been stated in the foregoing that dieback is never directly
due to parasitic agencies. Many diseases and pests may, of course,
cause the death of branches which they attack but the effect of such
direct parasitism is confined in general to the parts attacked and
the phenomena observed belong to a different category from those
characteristic of true dieback. Thrips has been mentioned as a
cause of severe defoliation leading to dieback; when, however, these
insects suck the juices from young branches the parts affected are
killed as the direct result of such attack and their deaths are not due
in any way to a general strain thrown on the bush as a whole.
Certain fungi, notably Colletotrichum coffeanum and a species of
Phoma, are almost invariably present on twigs which have succumbed
to dieback; they become established on such twigs after the event,
however, and are not the original cause of it. These fungi exist quite
harmlessly and in considerable amount in every coffee plantation, as
is shown by the fact that one or other may be confidently expected
to develop on any perfectly healthy coffee twig brought into the
laboratory and merely kept in a moist chamber for a few days. Only
two cases have been observed in six vears in which it seemed at all

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