APPLE POWDERY MILDEW. 5 



DISSEMINATION OF THE FUNGUS. 



In the Wenatchee Valley the spread of the fungus during the 

 growing season is largely dependent on the prevalence of dews. 

 Light rains occasionally occur, but moisture for most of the spore 

 germination is found in the dews, which are of almost nightly oc- 

 currence. While the humidity of the air in the Wenatchee Valley is 

 usually very low, it is much higher in the orchards which are being 

 irrigated. The daily range of temperature varies between 25 and 

 30 degrees F. during most of the early part of the season, and often 

 it is even greater; consequently, dews are frequent and occur in the 

 orchards when other sections are free from them. 



Spore dissemination is doubtless accomplished by the agency of 

 the wind. During the early part of the season strong winds are of 

 common occurrence and often blow steadily for days. The branches 

 of the trees are whipped about, and the conidia are scattered around 

 and lodged on tender j^oung leaves and shoots. With the coming of 

 nightfall and dews, favorable conditions for spore germination arise 

 and infection proceeds. 



Beetles were often found feeding on the mildew, and they may be 

 responsible for some of the spread of the fungus ; but they were never 

 found in sufficient numbers to indicate that they were important 

 agents in its dissemination.^ 



The perfect stage of Podosjyhaera leucotAcha begins to appear as 

 early as the middle of June, when black speckled patches of peri- 

 thecia form in the felted mycelium on twigs, leaf petioles, and mid- 

 ribs (PI. I, fig. 1), and occasionally on the fruit. 



The perithecia have been observed from year to year in an at- 

 tempt to determine definitely whether they play any appreciable 

 part in the dissemination of the fungus. The dates of the earliest 

 collection of perithecia in the Wenatchee Valley during the course 

 of this investigation are as follows: 



In 1915, on June 22, three months after the appearance of the first leaves. 

 In 1916, on June 13, two months after the appearance of the first leaves. 

 In 1917, on June 29, two months after the appearance of the first leaves. 



Twigs bearing perithecia were brought into the laboratory at fre- 

 quent intervals in the spring for examination to determine the date 

 at which the spores were shed. None of the perithecia were found 

 ruptured until fully a month after conidia had become abundant on 

 the current season's growth. Frequently no ruptured perithecia 

 could be found until after new perithecia had developed on the cur- 

 rent season's growth. Early in the spring it was difficult to break 



'^ Identification of the insects discovered feeding upon the mildew has been made by 

 Mr. E. J. Newcomer, of the Bureau of Entomology, as follows : The most abundant, 

 a ladybird (Psyllohora J)orealis Casey) ; the next common, a small, pointed, black and 

 brown beetle (Pentaria nuMUa Le Conte) ; and the rarest, Anthicus nitudulus Le Conte. 



