PART IV. . J)
under continuous cropping. It is difficult, nevertheless, to distinguish
between the evil effects due to this cause and those due to the
accumulation of infective material in the soil in the debris from
previous crops. In either case lack of rotation is the important factor
and there seems to be no reason why losses due to maize diseases
should not continue to increase while present methods persist.
The influence of cultural factors has not been dealt with separately
because, except in so far as they have a modifying influence on local
climatic conditions, they present few points which are not common
to most countries. One instance, however, may be of interest,
namely, that of overbearing in coffee. This factor causes more loss
than is generally realised and plays an important part in the case of
two diseases. Bearing a heavy crop is a great strain on a coffee bush
and one which is accentuated by deficient rainfall. The strain is
particularly felt by young coffee bearing a heavy maiden crop before
it has time to establish a system of secondary branches to assist in the
work of food manufaeture. It is very noticeable, therefore, that those
bushes in a plantation which are bearing most heavily succumb much
the most readily to attack by leaf disease. In fact overbearing may
just make the difference between quite an insignificant attack by the
fungus and a very severe one. Severe attack by leaf disease is almost
invariably followed by considerable dying back of the branches in the
following season, but this is also a very frequent consequence of
overbearing itself even where no leaf disease oceurs. The obvious
method of avoiding losses due to overbearing is partial decropping of
heavily laden bushes but, since in Kenya the temptation to make
hay while the sun shines is about as great as elsewhere, overbearing
as an influence affecting the occurrence of disease might be considered
as much an economic as a cultural factor.
Having now traversed a range of problems connected with the
incidence of disease in Kenya, it may be interesting in conclusion to
mention one which has been under investigation for a short time only.
This concerns streak disease of maize which derives its name from its
chief symptom, which is the presence of white or yellowish streaks
which run parallel with the leaf veins and alternate with the normal
green colour. This is associated with considerable dwarfing of the
plant and reduced yield. The nature of the disease and its method
of transmission were first described by Mr. Storey, the Natal
mycologist, about two or three years ago (3)* and (4). He showed
that it was of a virus type and conveyed from a diseased plant to a
healthy one through the agency of a leaf hopper. He also found that
the same disease occurs in sugar cane and certain wild grasses. In
Kenya, however, different factors, which are as yet very imperfectly
understood, appear to be operating. The particular leaf hopper which
proved to be the vector in Natal is not known to occur in Kenya and
experiments with other leaf hoppers have not yet been successful in
bringing about transmission of the disease. Moreover the disease has
not shown any inclination to spread from a maize plot to an
immediately adjacent patch of sugar cane at the laboratories and it
has not yet been found on any local grasses. (Nore: Since this paper
was read streak disease has been found on sugar cane in Kenya.)
Investigations in Kenya are still in the early stages, but some very
interesting facts have been discovered. So far the disease appears
to be confined to the red soil of the Kvambu and Fort Hall districts.
* See list of references on page 220.

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