FUNGOUS DISEASES. 55 



from any point of view the desirability of vigorous plants. Yet, 

 in the case of the strictly parasitic fungi already referred to, it is 

 very doubtful if vigor alone is sufficient to prevent disease. Re- 

 sistance to strictly parasitic fungi seems to be more commonly a 

 character; a character like earliness or something of that nature, 

 wholly independent of vigor, and any condition of the environment 

 may operate to assist the fungus or operate to make the host more 

 susceptible. Some of the most vigorous varieties have been pe- 

 culiarly susceptible to certain diseases. Before adopting the view 

 that all ills flee before vigor we must make ourselves clear as to 

 what vigor means. If it is S}Tion}'mous with resistance to disease 

 then of course all plants subject to disease under any conditions are 

 non-vigorous. Many wild or native prototyj^es of certain highly 

 robust, cultivated varieties when grown side by side with the latter, 

 may show more, or may show less resistance to disease, wholly 

 independent of robustness. It does not at all hold that climatic 

 factors which favor the fullest development of the host may not also 

 encourage the fungus. Moreover, factors unfavorable to the host 

 may be similarly unfavorable to the fungus. In the case of fungi 

 whose weapons of attack are most effective where the plant is least 

 active, as when the leaves have been injured by drought or other 

 causes, or when the fruit is maturing, it is clear that any environ- 

 mental factor promoting a healthy growth of all parts throughout 

 the season would decrease disease. Sanitation, so-called, is in ail 

 cases essential. Passing now to some of the chief factors of the 

 environment we may take up what seem to be particularly important 

 instances in which environmental conditions predispose. 



The Chief Factors of the Exvironment. 



As some of the chief factors of the environment now known 

 which may promote or inhibit fungous diseases, we may enumerate 

 water (precipitation, humidity, soil moisture); light; acidity and 

 alkalinity of the soil; nutrition; and temperature. We may 

 examine some of the effects of these factors upon cultivated plants 

 and upon certain disease-producing organisms; but it must be 

 understood that we are not always able to distinguish between the 



