APPLE DISEASES 113 
Hedgecock, G. G. Field studies of the crown-gall and hairy root of 
the apple-tree. U. S. Agr. Dept. Plant Indus. Bur. Bul. 186: 
9-96. 1910. 
Cook, M. T. Crown gall and hairy root. New Jersey Agr. Exp. Sta. 
Cire. 34:3-14. 1914. 
PowpeEry-MILDEW 
Caused by Podosphera 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 any other district in the United States. Bearing trees 
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