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 Podosphcera Oxyacanthce. 

 It thrives best during warm, dry weather ; therefore outbreaks 



FIG. 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 



