178 MANUAL OF FRUIT DISEASES 
over the affected area. The internodes of diseased twigs are 
shortened and thickened (Fig. 49, right). Both surfaces of the 
leaves are liable to attack, although as a rule only one side of a 
single leaf shows mildew at a given time. Diseased leaves are 
caused to curl inward and upward in a very marked fashion. 
Cause. 
The pathogene is a fungus known as Podosphera Oxyacanthe. 
It thrives best during warm, dry weather; therefore outbreaks 
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) 
Fic. 49. — Powdery-mildew of cherry; healthy shoot on left, diseased on 
right. 
occur in seasons characterized by such conditions. A drought 
in the Mississippi Valley in 1887 and 1888 was accompanied 
by an epiphytotic of cherry-mildew. 
The fungus hibernates as mature perithecia. In the spring 
ascospores which develop within these bodies escape by the 
disintegration of the. perithecia. Those ascospores which fall 
on susceptible parts germinate in the presence of moisture, the 
germtube developing into mycelium which grows over the sur- 
face of leaves and twigs. This growth becomes profuse and 
constitutes the mildew so noticeable as a symptom of the disease. 
None of the mycelial threads enters the tissues of the affected 
plant, but small suckers, called haustoria, arise as branches 
