BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. 



tree vigor and susceptibility to mildew was shown and experiments 

 were reported which demonstrated the possibility of foliage stimula- 

 tion through spraying with various compounds. 



THE CAUSAL ORGANISM. 



The cause of apple powdery mildew is the fungus Podosphaera 

 leucotricha (E. and E.) Salm. Another species of Podosphaera, P. 



oxyacanthae (DeC.) De- 

 Bary, has been reported as 

 the cause of this mildew 

 in some eastern localities, 

 but the writer has never 

 found it on the apple in 

 the Pacific Northwest. 



The two species can 

 not be distinguished ex- 

 cept in their perfect 

 stage, when marked dif- 

 ferences are observed. 

 In the case of Podo- 



Pig. 1.— Perithecium and ascus of Podosphaera Sphaera leucotricha there 

 leucotricha (E. and E.) Salm. (The former is are £W0 tVPeS of aDPend- 

 magnified 80 times and the latter 420 times.) ■ yi , , . 



ages on the penthecia. 

 The basal appendages are short, twining, and rootlike, while the 

 apical appendages are long, slender, stiff, and rarely divided at 

 the tips. Perithecia of P. oxyacanthae have appendages placed more 

 or less equatorially and all dichotomously branched at the tips. 

 They are usually all of one type, but rootlike basal appendages are 

 also sometimes found. The 

 distinction between the two 

 species is shown in figures 1 

 and 2. 



It is probable that much of 

 the confusion which has 

 arisen in regard to the iden- 

 tity of the species involved is 

 due to the fact that in most lo- 

 calities the perfect stage is pro- 

 duced but sparingly. How- 

 ever, in the Wenatchee Valley this stage is abundant, and repeated 

 examinations of perithecia have never revealed other than Podo- 

 sphaera leucotricha on the apple. P. oxyacanthae has, however, fre- 

 quently been found by the writer on the native bitter cherry (Prunus 

 emarginata (Dougl.) Warpers) and on the western chokecherry 

 {Prunus demissa (Nutt.) Walpers) growing in close proximity to 

 apple trees. 



Fig. 2. — Perithecium and ascus of Podosphaera 

 oxycanthae (DeC) DeBary. (The former is 

 magnified 80 times and the latter 420 times.) 



