2. PART IV. 
struck during the course of ages. Sometimes some local condition 
may favour temporarily one side or the other, but soon again the 
normal is restored. In the case of cultivated crops, however, the 
conditions of nature have been rudely disturbed especially by the 
crowding together of large numbers of plants of the same species or 
variety. In the wild state such plants were probably separated from 
one another by a considerable number of others of a variety of kinds 
so that parasites feeding on particular species had to traverse 
appreciable, and in some cases considerable, distances before 
encountering another suitable host. In cultivated crops the proximity 
of plants of the same kind facilitates greatly the spread of disease. On 
the other hand, however, a compensating gain is derived by cultivated 
crops from the fact that they are grown under much less competitive 
conditions, so that their general vigour and consequent resistance to 
diseases tend to be increased. 
In a new country like Kenya, situated at great distance from its 
principal markets, the number of crops which can be grown profitably 
ts considerably limited and this has led to the growing of many 
successive crops of the same kind on the same land. One farmer 
is known to have grown last year, his eleventh successive maize crop 
on the same field and this is by no means a record for Kenya. 
Considering that in spite of lack of manuring, a crop of 10 bags to the 
acre was still obtainable from the land, it will readily be appreciated 
that the temptation to continue the system is a strong one. 
Nevertheless such methods of farming add greatly to the difficulties 
of disease control and operate adversely in two ways. In the first 
place they tend to increase the range of distribution of diseases. 
Most fungi have little difficulty in tiding over the period from one 
season to the next so that when such parasites are provided year after 
year with just the food they require, there is every opportunity and 
encouragement for their spread to previously uninfected places in the 
vicinity. An excellent example of this was seen in this country some 
years ago in the case of flax. - In many places this crop became 
infected with the wilt fungus which started in small patches in the 
fields. When crop after crop of flax was grown on such infected land, 
the patches of disease grew larger each year until many acres became 
so heavily infected that flax could not be grown on them. The same 
disease exists in Ireland, but it is not a great source of trouble there 
because in that country the crop is only grown once in a seven years 
rotation. Head smut of maize which exists in certain parts of Kenya, 
is another disease which is likely to give considerable trouble in future 
if no crop rotation is practised. Experience in this and other countries 
suggests that quite a short rotation (probably of three years) would 
be sufficient to keep head smut in check. 
The second evil effect of lack of rotation is the tendency towards 
the weakening of the resistance of the crop to diseases through the 
depletion of essential food materials in the soil. So many other 
factors help to obscure the effect of this that it is difficult to give 
instances to illustrate it. In maize, however, the amount of disease 
in Kenya has been steadily on the increase of recent years (although 
the loss due to this cause is still not very great) and I am confident 
that continuous drain on soil nutriments has played a considerable 
part in bringing about this increase. The Helminthosporium leat 
blight of maize comes to mind as an instance of disease which is 
probably specially favoured by the reduced resistance of the plant 
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