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. 21C