Epidemic Plant Diseases. F. T. Brooks. 231 



to a slight extent, is of negligible importance. Under these 

 conditions the spores of the fungus are produced in much smaller 

 numbers and the majority of them, even if they alight on potato 

 leaves, are unable to germinate and cause infection on account 

 of lack of moisture. Correlated with a heavier rainfall, potato 

 blight is generally more severe in the west of England than in 

 the east. 



If climatic conditions are such as to prevent entirely or almost 

 entirely for a season the occurrence of a disease which is usually 

 epidemic, there may elapse an interval of years before the 

 disease again assumes serious proportions. The dry summer of 

 192 1 prevented almost entirely the appearance of potato blight, 

 which has been uncommon in most parts during 1922 and 1923, 

 notwithstanding weather conditions apparently favourable to it. 

 The reason for this may be that the sources of infection were 

 largely destroyed by the heat and drought of 192 1 and that 

 there is a necessary lag until the sources of infection are suffi- 

 ciently replenished. 



In temperate regions epidemic plant diseases are often checked 

 by the oncoming of winter, during which most of these parasites 

 are not present in an infectious condition. Mildews which are 

 propagated in an epidemic manner during the summer by spores 

 of a delicate nature, usually form another type of spore towards 

 the end of the gro\\dng season, which is enclosed in an impervious 

 case that remains sealed until the following spring. At the same 

 time the young stems and leaves which are the parts chiefly 

 liable to infection by such fungi either do not exist during the 

 dormant season or have become greatly modified in structure. 

 In the tropics, however, vegetation and weather conditions are 

 both often practically uniform throughout the year, the tem- 

 perature and rainfall varying but little in successive months. 

 In such a climate there is no close season for fungal parasites, 

 and epidemic diseases are greatly to be feared. 



Disease may affect the subterranean parts of plants as well 

 as the stems and leaves, and some of the most serious plant 

 diseases and the most difficult to control are of this nature. In 

 recent years much has been heard of wart disease of potatoes 

 {Synchytriiim endobioticum) in this country. This parasite in- 

 variably causes infection through the soil, in which the resting 

 spores may retain their vitality for many years. A disease of 

 this kind can only be epidemic when the causative parasite is 

 generally distributed throughout the soil, for there is no means 

 of active dissemination by wind currents. There are other ways 

 in which root parasites may become so prevalent as to cause 

 disease more or less of an epidemic nature. Thus if a forest or 

 woodland be cut down and replanted with trees there are several 



