FUNGICIDES, PREPARATION AND APPLICATION 431 



That a fungicide must be non-injurious to the host is self- 

 evident. Hosts, like pathogenes, vary in their sensitiveness 

 to fungicides. Some, like the peach, are notably susceptible 

 to injury from almost any toxic substance, while others, 

 like the apple, will tolerate a wide range in kind and con- 

 centration of fungicides. Varieties and even individual trees 

 exhibit great variation in their susceptibility to injury, so 

 that only after extensive testing may one say with safety 

 what kinds and strengths are to be applied to the different 

 fruits. Moreover, the liability to injury depends on a number 

 of variable conditions, such as the stage of seasonal development 

 of the host, or its vitality, climatic conditions, especially tem- 

 perature, moisture and the like. It is common knowledge 

 among growers that in rainy seasons bordeaux mixture is apt 

 to be very injurious to apples, while in dry seasons little or 

 no injury follows its application. Foliage on apple-trees of 

 low or impaired vitality is much more liable to lime-sulfur 

 injury than that of healthy vigorous trees. Grapes of Ameri- 

 can origin are almost uniformly injured by the application 

 of sulfur in any form while showing no evidence of injury from 

 the application of copper fungicides. European grapes, on 

 the other hand, are not injured by sulfur. Young, expanding 

 leaves of the apple will tolerate a much more concentrated 

 solution of lime-sulfur than will the fully expanded and mature 

 leaves later in the season, strange as this may seem. 



Adhesiveness, especially where the fungicide is to act in a 

 protective way, is a very necessary requisite of a good fungi- 

 cide. The spores of most pathogenic fungi are disseminated 

 and infect the host during rains. Not only must the fungicide 

 be present on the host in advance of the rains, but at least 

 enough of it must remain, in spite of the washing of the rain, 

 to destroy or inhibit the growth of the spores. Adhesiveness 

 may be due to the gelatinous nature of the fungicide as in the 

 case of bordeaux mixture, or to the finely divided character 



