APPLE POWDERY MILDEW. 7 



A serious phase of the disease is found, in the case of twigs stunted 

 but not killed. New leaves are infected as rapidly as they appear. 

 Buds formed on such twigs are infected by the overwintering myce- 

 lium, and in the spring the fungus resumes activity when the leaves 

 unfold, thus performing a function vital to the spread of the disease 

 in carrying the fungus over from season to season. On such twigs the 

 internodes are much shortened and the lateral buds are crowded to- 

 gether. (PI. II, figs. 3 and 4.) ' These buds are characterized by a 

 purplish red color and an elongated shape, and they are always de- 

 layed in opening from a week to 10 days behind normal buds. (PI. 

 II, figs. 4 and 5, and PI. III.) 



In cases of severe infection the formation of new fruit buds is pre- 

 vented and the following season is one of a light crop or no crop at 

 all. Vegetative growth, however, is favored by the absence of a fruit 

 crop ; hence, during such a season the trees are given an opportunity 

 to recuperate their vigor and resist the mildew for the time being. 

 But the following year an increased crop is produced, and mildew, 

 again being favored by reduced vegetative vigor, resumes virulent 

 activity and so tends to produce a periodicity of crops that is very 

 undesirable. 



The effects of fruit infection are to dwarf the apple and produce 

 a russeting of the skin beneath the mycelium. (PL I, fig. 2.) Fruit 

 infection usually occurs early in the season. Active infections on 

 apples after the skin has become hardened have not been observed, 

 and it is therefore probable that danger of such injury is past before 

 the apple is half grown. Infected apples often show elongated stems, 

 while the basins are generally russeted. 



SUSCEPTIBILITY OF VARIETIES. 



The varieties of the apple found to be most susceptible to the dis- 

 ease in the Wenatchee Valley are Pryor Red, Jonathan, Newtown, 

 Black Ben Davis, Grimes, Esopus {Spitzenburg) ^ Fameuse (Snow), 

 and Stayman. No variety appears to be immune, but among the 

 least susceptible are Winesap and Wliite Pearmain. Of these varie- 

 ties the Pryor Red and Fameuse are not commercially important in 

 the Wenatchee Valley. 



IMPORTANCE OF THE DISEASE IN THE PACIFIC NORTHWEST. 



Apple powdery mildew occurs in all of the apple-growing regions 

 of the Pacific Northwest, but its economic importance in any of 

 them varies from year to year. Investigations by the writer have 

 shown that the disease is of little economic importance in the Willa- 

 mette Valley of Oregon and the sections of Washington west of the 

 Cascade Mountains. In these sections infection seems to be confined 



