UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 712 



Contribution from the Bureau of Plant Industry 

 WM. A. TAYLOR, Chief 



AJr^mfU 



Washington, D. C. 



PROFESSIONAL PAPER 



October 29, 1918 



APPLE POWDERY MILDEW AND ITS CONTROL IN THE 

 ARID REGIONS OF THE PACIFIC NORTHWEST. 1 



By D. F. Fisher, 

 Assistant Pathologist, Fruit-Disease Investigations. 



CONTENTS. 



Economic importance of powdery mildew 1 



Review of the literature 2 



The causal organism 4 



Dissemination of the fungus 5 



Description of the disease 6 



Susceptibility of varieties 7 



Importance of the disease in the Pacific North- 

 west, 7 



Spray injury 9 



Orchard spraying experiments 10 



Spraying experiments in 1915 12 



Orchard spraying experiments— Contd. 



Spraying experiments in 1916 15 



Spraying experiments in 1917 17 



Injury to fruit and foliage 20 



Spray materials 21 



Summary of control measures 23 



Dormant sprays 23 



Pruning experiments 24 



General notes on the control of the disease 25 



Conclusions 26 



Literature cited 28 



ECONOMIC IMPORTANCE OF POWDERY MILDEW. 



Apple powdery mildew is usually considered as of only minor im- 

 portance and principally affecting nursery stock in the eastern part 

 of the United States, only becoming serious in restricted localities. 

 Occasionally it is severe in Utah, Colorado, New Mexico, and the 

 East on old trees, but in the apple-growing districts of the Pacific 

 coast the conditions are such that it often becomes serious. The 

 arid climate of the hot interior valleys of the Pacific Northwest has 

 proved an effective safeguard against fungous diseases in general, but 

 apple powdery mildew is endemic. 



In these regions, where deciduous- fruit growing, and especially 

 apple growing, has become such an important industry, orcharding 

 is generally carried on under very intensive conditions and the in- 



1 Acknowledgment is due Dr. Charles Brooks, of the Office of Fruit-Disease Investiga- 

 tions, Bureau of Plant Industry, for many helpful suggestions during the course of the 

 work, and to Messrs. E. J. Newcomer, of the Bureau of Entomology, and M. M. Brown 

 and L. C Carey, formerly assistants in spraying experiments, who actively assisted in the 

 field work. 



