6 FUNGOUS DISEASES OF PLANTS 



physiological disturbances in the host itself. 1 The normal physiology 

 of the host requires attention in order that a proper comparative study 

 may be made. The conditions which predispose the host plant to 

 attack, or, in other words, the conditions favorable to the penetration 

 of the fungus and its development within the host are most funda- 

 mental from the standpoint of pathology, and also in order that 

 control measures may be properly developed. It is a direction in 

 which future work promises most profitable returns. Very little of 

 lasting value has been done towards determining the exact condi- 

 tions under which the host plant is most susceptible to attack. It 

 is well known in the case of certain forced plants that the undue 

 suffusion of the plant with water, whether due to lack of ventilation 

 or to a combination of causes, is a certain factor in inducing disease. 

 Under such conditions many fungi are able to gain entrance and 

 become the cause of epidemics, whereas, under more normal con- 

 ditions, they may remain as harmless inhabitants of dead materials. 



Every season shows differences in the prevalence of the more 

 injurious fungous diseases. One season the brown rot of the peach 

 may affect only extremely sensitive varieties, and the following 

 season it may cause the loss of those most resistant. Again, some 

 varieties of the host may be, under most conditions, but slightly 

 predisposed to attack, notable instances being those of the very 

 slight predisposition of the Kieffer pear to the blight, or in the 

 resistance of certain American varieties of grapes to the downy 

 mildew. Such cases might be multiplied indefinitely, and, in fact, 

 there is scarcely a known fungous disease of the variable cultivated 

 crops with reference to which all varieties of the host plant are 

 equally susceptible. This important fact has led to the selection 

 and to the production through hybridization of varieties which shall 

 at once possess both the qualities desired from the standpoint of 

 their own fruits or other products, and which shall, at the same 

 time, carry with them the high resistance necessary to enable them 

 to compete with the fungous pests. 



The effect of the fungus upon the host may be, further, merely 

 to modify the quality of the product, such as the sugar or starch 

 content, without seriously affecting the appearance of the economic 

 product. In fact, the different means whereby the effect of the 



1 See Ward, H. M., Disease in Plants, 1901. 



