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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