6 BULLETIN 712, F. S. DEPAETMENT OF AGRICULTURE. 



tlie perithecia under a cover glass for microscopic examination, but 

 after soaking in water for 24 hours they were much more easily 

 broken, indicating that rainfall would facilitate natural ascospore 

 dispersal. However, rainfall adequate for this purpose is always 

 lacking in the Wenatchee Valley at this season. 



Eepeated attempts were made to inoculate tender young foliage 

 with ascospores obtained from perithecia of the previous season's 

 growth. A suspension of spores was sprayed on Jonathan leaves 

 with an atomizer. Some of the leaves were inclosed in glassine bags 

 in order to increase the humidity, while others were left exposed. 

 The work was done between the middle of May and the first of June, 

 when the disease was actively spreading in the orchards. The results 

 were always negative. It would appear from these studies that the 

 ascospores play little, if any, part in spreading the disease and that 

 they are unnecessary to the overwintering of the fungus. This con- 

 clusion is in agreement with the results of the investigations of 

 Ballard and Volck (12) in the Pajaro Valley of California and the 

 earlier work of Galloway (3). 



DESCRIPTION OF THE DISEASE. 



In the Wenatchee Valley the writer has found the disease in its 

 conidial and XDerithecial stages on twigs, foliage, and fruit of the 

 apple, and occasionally also on pear twigs and foliage. It sometimes 

 attacks the blossoms, and in this case frequently the entire blossom 

 cluster, with its attendant leaves, is involved. Affected flowers are 

 dwarfed and hypertrophied and set no fruit. The petals are de- 

 formed, are of a greenish or yellow color, and soon become covered 

 with a powdery coating of spores. (PL I, fig. 3.) 



The first appearance of the disease is usually manifested in small 

 grayish or white feltlike patches of mycelium on the under sides of 

 the leaves, which become crinkled and curled. (PI. II, fig. 1.) Often 

 the presence of the fungus can be detected before the appearance of 

 the felted mycelium by the mottled color of the infected leaves, ac- 

 companied* by a corrugation of their surface. The xoatches covered by 

 the fungus rapidly enlarge, and generally the entire leaf becomes cov- 

 ered with the felted mycelium and a powdery coating of spores. 

 When very young leaves are attacked they have a tendency to increase 

 in length but not in breadth and gradually become somewhat folded 

 longitudinally. (PI. I, fig. 1 and PI. II, fig. 1.) Infected leaves be- 

 come gi'easy and eventually so brittle and parched that they fall from 

 the tree. The mildew spreads rapidly down the petiole of the leaf 

 to the twig, which becomes covered with mycelium and spores. Twig 

 growth is soon checked, and in severe infections the twig is killed 

 outright. (PI. II, fig. 2.) In the Wenatchee Valley, trees covered 

 with terminals killed by powdery mildew are frequently seien after a 

 season of severe infection. 



