230 Transactions British Mycological Society. 



for infection {e.g. exposure) as well as an abundant supply of 

 spores in order for disease to appear in epidemic form. 



Epidemic diseases are essentially dependent upon two sets of 

 factors for their development, namely, a condition of the host 

 favourable to invasion by the parasite, and an environment of 

 such a kind as to allow of the rapid dissemination of the spores 

 and germination of these upon the surface of the host; these 

 two sets of factors are intimately related to each other, for 

 conditions detrimental to the growth of the host are often 

 favourable to the development of the parasite. As regards the 

 host being in a state of susceptibility to attack, it is a common- 

 place that individuals, whether of animal or of plant populations, 

 vary greatly in their liability to certain diseases. It is also well 

 known, that even the most robust individual may become so 

 weakened as to become susceptible to diseases from which nor- 

 mally he is free. Plants of economic importance, most of which 

 have been in cultivation for long ages, exist in the form of 

 numerous varieties which shew marked differences in their sus- 

 ceptibility to disease. Some varieties of wheat are markedly 

 resistant to yellow rust caused by Puccinia glumarum, while 

 others are very susceptible; again in potatoes, some forms are 

 badly attacked by the blight fungus and others are scarcely 

 troubled by it. But often even the healthiest forms of cultivated 

 plants may have their vitality so weakened under some condi- 

 tions as to become a prey to a parasite which usually cannot 

 thrive on it. In India for instance Einkorn wheat, which is noted 

 for its extreme resistance to rust attacks throughout the world, 

 becomes badly attacked by black rust {Puccinia graminis) during 

 abnormally hot weather in the Ganges Valley. Climatic con- 

 ditions in this case doubtless so reduce the vitality of the host 

 plant as to render it susceptible to the parasite. 



Granted, however, that the host plant is in a susceptible con- 

 dition for attack, epidemic disease will not follow unless the 

 germs of the parasite are available in abundance and unless 

 environmental conditions favour their rapid dissemination and 

 germination upon the tissues of the host. In fact it is not too 

 much to say that weather conditions are usually all important 

 in determining whether disease appears in an epidemic form or 

 not. This accounts for differences in incidence of such plant 

 diseases in successive years. The prevalence of one of the com- 

 monest epidemic diseases, potato blight, is chiefly determined 

 by weather conditions. In a wet summer great damage may be 

 done by the vigorous spread of this fungus, and so rapid is the 

 onslaught of the parasite under suitable weather conditions that 

 potato fields sometimes become black in a day or two. In a 

 moderately dry summer, potato blight, though usually present 



