514 



SPECIAL PLANT PATHOLOGY 



to the grape" leaves (Fig. 186), where it produces under ordinary 

 conditions spots of mildew, especiaUy on the lower leaf surface. In 

 bad cases, the whole lower leaf surface may be covered with the downy, 

 or cottony mass of hyphae which gives the fungus its common name. 

 The parasitic hyphae live in the intercellular spaces of the host and 

 send into the host cells smaU knob-like haustoria. The presence of 

 the mycelium seriously interferes with the normal physiologic activity 

 of the host. In light cases, the areas of upper leaf surface immediately 

 overlying the hyphae turn brown in the form of angular spots. Through 



Fig. 185. — Black-rot fungus {Guignardia BidweUii). a. Portion of an affected 

 grape showing pustules; 6, section of pustule (pycnium) showing pycnospores; c, 

 ascus with ascospores; d, ascospores. (After Quaintance, A. L., and Shear, C. L., 

 U. S. Farmers' Bull. 284, 1907.) 



the stomata emerge stiff projecting conidiophores which form short 

 stub-like branches from which fall ellipsoidal conidiospores. These 

 conidiospores are virtually zoosporangia for their protoplasmic con- 

 tents divide into a number of biciliate zoospores which escape and 

 swim about in the rain water which covers the leaf or stem, or are washed 

 down, or splashed from plant to plant during a dashing rain storm. 

 When the fungus appears on the fruit, it has been called gray rot, and 

 occasionally, the berry may be completely covered with a downy mass 

 of hyphae. 



