APPLE DISEASES 113 



Hedgcock, G. G. Field studies of the crown-gall and hairy root of 

 the apple-tree. U. S. Agr. Dept. Plant Indus. Bur. Bui. 186 : 

 9-96. 1910. 



Cook, M. T. Crown gall and hairy root. New Jersey Agr. Exp. Sta. 

 Circ. 34 : 3-14. 1914. 



POWDERY-MILDEW 



Caused by Podosphcera leucotricha (E. and E.) Salm. 



This mildew affects, in addition to the apple, the pear, quince, 

 cherry, plum, thorn and juneberry. Seedling nursery-stock, 

 wherever apples are grown, are likely to suffer to some extent 

 from this disease. In localities where the relative humidity 

 at times runs high such varieties as the Yellow Newtown and 

 Yellow Bellflower are liable to be severely injured by this 

 disease. In the Pajaro Valley of California no variety is 

 immune, although there is some variation in this respect. The 

 most susceptible varieties include the Smith, Missouri, Esopus 

 and Gravenstein in addition to the two already mentioned. 

 Those less seriously affected are : White Pearmain, Winter 

 Pearmain, Red Astrachan, Rhode Island and Langford. 



Apple powdery-mildew has a wide geographical range over the 

 world. It has been reported from Europe (Germany, Austria- 

 Hungary and Russia), Asia (Japan), Australia, New Zealand 

 and North America (United States and Canada). In the 

 United States it has for many years been a serious nursery 

 pest, and recently the disease has caused injury to bearing 

 orchards. This mildew is most prevalent and most serious 

 west of the Rocky Mountains, particularly along the Pacific 

 Coast where climatic conditions apparently are very favor- 

 able to the pathogene. The peculiar weather conditions 

 in the Pajaro Valley of California favor powdery-mildew, 

 and here commercial apple orchards suffer more than those 

 of anv other district in the United States. Bearing trees 



