211 PART IV.
(4) There is some evidence to show that roguing out of the early
diseased plants (which occur in well-marked patches) checks the later
spread.
(5) In trials we have found no variety showing markedly greater
resistance than the Virginia Bunch variety. Attempts to select
resistant lines within standard varieties are in progress
Mr. RITCHIE said that this disease had been in Tanganyika for
a very long time indeed. Under unfavourable weather conditions and
when ground nuts are planted on exhausted soils a higher infection is
noticed. In cases where there was a high incidence, it could not
apparently be always correlated with suctorial insects.
Mr. FULLER observed that entomologists have still a lot to find
out about the relation ‘between insects and Rosette.
SOME FACTORS INFLUENCING THE OCCURRENCE AND
DISTRIBUTION OF PLANT DISEASES IN KENYA.
Mr. McDONALD read the following paper (referred to in Agenda
as T.C.(C) E.& M.3):
From the point of view of the investigation of biological problems
connected with agriculture, Kenya presents a most interesting and
varied field. This is particularly the case in the study of the causes
which determine the occurrence and distribution of plant diseases.
Every factor of importance bearing on this subject may be placed
in one or other of the following three groups:
1. Climatic.
2. Economie.
3. Cultural.
Of these groups the first exerts the profoundest influence in
Kenya. The general climate of a country is due primarily of course
to its particular position on the earth’s surface, but local geographical
features may modify the climate of different districts. In Kenya
these secondary features are so pronounced and varied that they
produce a really remarkable diversity of climatic conditions within the
borders of the colony.
To appreciate this it is necessary to know something about the
geography of the country for which purpose the line of the railway
from Mombasa on the Coast to Kisumu on Lake Victoria will serve as
a convenient basis. Beginning at Mombasa, which is practically at
sea level, the altitude increases fairly steadily till it reaches 5,500 feet
at Nairobi, 800 miles inland. The next 30 miles are much steeper as
far as Uplands, at the top of the Kikuyu Escarpment, where the
altitude is over 8,000 feet. After Uplands there is a sudden fall of
about 2,000 feet to the floor of the Rift Valley which runs north and
south across the whole country. On the other side of the valley there
is another steep ascent to over 8,000 feet at Mau Summit and
thereafter the country falls away, steeply at first and then more
gradually till the Lake is reached at Kisumu at about 3,700 feet. The
main mass of the highland country lies chiefly to the north of the
railway between Nairobi and Kisumu, the highest point at which
farming is carried on being in the neighbourhood of 10,000 feet.

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