PART [ 
CHAPTER I 
INTRODUCTION TO FUNGI 
It is well known that the yields of the common 
farm crops show enormous variations which may be 
due to a number of causes, e.g. kind of soil, variety 
of seed, management and climate. Often, however, 
very different yields may be obtained when the above 
conditions are almost identical, and in these cases it 
is usual to attribute the loss of crop to a ‘“‘disease.”’ 
Diseases of plants then may be looked upon as causes 
which prevent normal growth. 
It is usual to classify plant diseases into two groups: 
(1) Those caused by unsuitable surroundings, such 
as unfavourable conditions of soil, or of weather. 
(2) Those caused by living agencies. 
It is this second group, the diseases caused by 
fungi and insects, with which we are here concerned. 
The two groups are not however strictly separable, 
for we often find plants weakened by some condition 
of their surroundings marked out for attack by a 
living agency. 
The mangolds at the Rothamsted Experimental 
Station often suffer from leaf spot caused by a fungus 
known as Uromyces betae, but not all the plots are 
P.F. 1 
