BULLETIN OF THE 



c 



No. 120 



Contribution from the Bureau of Plant Industry, Wm. A. Taylor, Ch 

 September 3, 1914. 



APPLE POWDERY MILDEW AND ITS CONTROL IN 

 THE PAJARO VALLEY. 1 



By W. S. Ballard, Pathologist, Fruit-Disease Investigations, and W. H. Volck, 

 County Horticultural Commissioner of Santa Cruz County, Cat? 



INTRODUCTION. 



Apple powdery mildew is a fungous disease which attacks the 

 foliage and young twig growth of the apple. Occasionally it occurs 

 on pears, more particularly on nursery stock in damp or foggy locali- 

 ties, and at times it seriously injures quinces. The disease is quite 

 widely distributed over the world and may be caused by either of 

 two very similar fungi, namely, Podosphaera leucotrieha (E. and E.) 

 Salm., and P. oxyacanthae 

 (DC) de Bary. 3 



Powdery mildew of the 

 apple occurs at least occa- 

 sionally in nearly all parts 

 of the United States and is 

 reported from some Prov- 

 inces of Canada. It is 

 widely distributed in Eu- 

 rope and occurs also in 



Australia, New Zealand, Fig. 1. — Map of the United States, the shaded area 



and Japan. In the Cen- lowing where apple powdery mildew occurs oc- 



1 _ casionally and the black spots where serious out- 



tral and eastern United breaks are common. 



States it has been known for many years as a more or less 

 serious disease of nursery stock, but it is only during the last few 

 years that it has been reported as doing any considerable damage in 

 bearing orchards. In the western United States, however, from 



1 This bulletin is intended to assist apple growers in dealing with a fungous disease 

 which is becoming a serious menace to their industry. It is suitable for distribution in 

 the apple-growing sections west of the Rocky Mountains and in certain limited areas east 

 of the Rocky Mountains, as shown on the map. 



2 These investigations were carried on cooperatively between the Office of Fruit- 

 Disease Investigations of the Bureau of Plant Industry and the office of the county 

 horticultural commissioner of Santa Cruz County, located at Watsonville, Cal. 



3 See also page 5. 



46689-°— Bull. 120—14- 



