ASCOMYCETES 



227 



seasons, given trouble of serious nature, particularly where the 

 nursery stock are planted very close together. The mildew covers 

 both surfaces of the leaves and frequently involves the whole 

 twig. Spraying with Bordeaux mixture and potassium sulfide is 

 effective. 



XXV. POWDERY MILDEW OF PEAS 

 Erysiphe Polygoni De C. 



This fungus is distributed throughout the world. It is the most 

 common and one of the most variable of the Erysiphacese. The 

 species has been listed upon con- 

 siderably more than three hundred 

 hosts of diverse genera and orders. 

 Among these the succulent and her- 

 baceous plants predominate, but the 

 fungus occurs upon some woody 

 hosts. 



As a mildew of garden peas (Pisum 

 sativum) this fungus may become a 

 nuisance, especially when an attempt 

 is made to grow these plants during 

 the late summer. It is most preva- 

 lent during moist seasons, and more 

 destructive in some Atlantic and 

 southern states. Upon this host the 

 fungus forms a rather dense, persist- 

 ent mycelium, frequently covering 

 stems, leaves, and pods. The co- 

 nidia are developed in profusion. The 

 perithecia, averaging 90 /x in diam- 

 eter, are also produced in large num- 

 ber during a later period, commonly 

 after the plants have begun to dry 



up. When the mildew attacks young plants the crop is generally a 

 total loss. The fungus also attacks beans and certain vetches. 



The perithecia contain ordinarily 6-8 asci, each with 2-3 spores. 

 The appendages are very variable, even upon the same host, under 

 similar conditions. 



Fig. 92. Mildew of Cherry 



