THE POWDERY MILDEW 85 



Literature. An excellent account of this downy 

 mildew, by Professor F. L. Scribner, may be found in 

 the United States Department of Agriculture, Report 

 for 1886 (pp. 96-105). Since then much has been pub- 

 lished, in the Department reports and bulletins, in ex- 

 periment station bulletins, and in the horticultural 

 press, concerning remedial measures. 



The Powdery Mildew 



Uncinula ampelopsidis 



The powdery mildew of the grape seldom becomes 

 seriously destructive, except in the Southern States and 

 along the Pacific coast. It only thrives in very dry 

 weather, and attacks all the green parts of the vine. . 



This fungus is closely related to the powdery mil- 

 dews affecting apple and gooseberry, which are discussed 

 on other pages of this book. During summer reproduc- 

 tion takes place by means of summer spores, which ger- 

 minate upon the moist surface of leaf or fruit, the ger- 

 minating tubes sending out suckers (Fig. 42 c) which 

 penetrate the tissues of the host, and enable the myce- 

 lium to develop on the outside of the cuticle or skin. 

 In this way the mycelium may grow all over the outer 

 surface of the leaf, giving it a cobwebby appearance, 

 and drawing nourishment from it by means of the suck- 

 ers already mentioned. After it has grown for awhile 

 in this way it sends up certain vertical branches (b), 

 which soon develop transverse partitions, and thus pro- 

 duce the small oval summer spores. These are light 

 and easily distributed by the wind, and serve for the 

 rapid extension and propagation of the fungus during 

 summer. 



Later in the season the mycelium produces another 

 kind of spores the so-called winter spores. These are 

 more complicated in structure than their summer repre- 

 sentatives ; as seen under the microscope, they consist 



