PART IV. Ey 
ventilation and high humidity in the plantations are essential, and 
these follow on the provision of excess shade and the presence of such 
obstacles to the passage of air as dense forest immediately adjoining 
the coffee. The worst case of pink disease seen in Kenya occurred on 
coffee where shade, provided chiefly as protection against hail, had 
been so overdone as to produce conditions of semi-darkness in the 
plantation. When the shade was reduced to about a quarter, pink 
disease was diminished to negligible proportions. That low humidity 
is a controlling factor for pink disease is shown by the fact that on 
other plantations it occurs only on the few rows of coffee immediately 
adjacent to thick forest. Further out, where the air is able to circulate 
more [reely, the bushes are often quite free from the disease. - 
The influence of climate on Kenya coffee is strikingly seen in yet 
another direction. On many plantations at altitudes of 6,000 feet and 
upwards the malady known locally as ‘‘ black tip > may frequently 
be observed over large acreages. Black tip is a purely physiological 
phenomenon and is characterised by the deaths of the growing points 
of the young shoots. Too sudden change of temperature from that 
of the day to that of the night is undoubtedly one of the principal 
factors involved but the effect of the sun’s rays on the tender tissues 
of the growing points, acting through drops of rain and dew in the 
early morning or after showers, also has a considerable influence in 
producing the effects observed. Resulting from the deaths of the 
terminal growing points, numerous buds near the branch tips are 
stimulated into growth. These make progress for a short time only 
to be affected in turn by the same unfavourable influences. The killed 
tips of the branches turn black and the leaves in the vicinity are 
usually dwarfed and crinkled and of an unhealthy yellowish 
appearance while the bushes as a whole assume a typical conical 
shape. The introduction of quite a light shade has a marked beneficial 
effect both by affording protection from the sun and by reducing the 
rate of change of temperature in the plantation. Incidentally it is 
very noticeable that the shoots produced in the interior of the bushes 
are unaffected by the malady. | 
| One notable example of the effect of climate on the distribution 
of disease is supplied in the case of wheat which is grown in Kenya at 
altitudes from 4,500 to 10,000 feet. This crop is subject to attack by 
three rusts, namely black rust (Puccinia graminis), orange rust (P. 
friticina), and yellow rust (P. glumarum). The first two occur 
wherever wheat is grown, but the last appears to be sharply limited 
to those districts which are situated above the 7,000 feet contour. 
It has been stated (2)* that certain early maturing wheats which 
are susceptible to black stem rust may be grown successfully in Kenya 
if sown early enough to allow of their nearing the ripening stage before 
the onset of rust attack. This is correct in theory and, in fact, was 
successfully put into practice by a few farmers for a number of years. 
An earlier attack of rust in 1922, however, caused considerable damage 
to such susceptible wheats and their use has now been discontinued 
practically entirely in favour of resistant varieties. 
Sufficient has been said to emphasise the importance of climatic 
factors on the prevalence or otherwise of fungous diseases in the crops 
of the country. Other influences, however, have also to be considered. 
In nature there is a constant war between host plants and their 
parasites but amongst wild plants a fairly even balance has been 
* See list of references on page 220. 
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