240 



MANUAL OF FRUIT DISEASES 



withers, turns brown or red, and finally shrivels into a mummy 

 (Fig. 63). Diseased fruits shell very easily. 



Cause. 



The downy-mildew pathogene is a fungus, Plasmopara 

 Vitwola. It has both a sexual and an asexual stage, the former 



FIG. 63. Downy-mildew on grape-berries. 



represented by oospores, the latter by conidia. It is generally 

 agreed that the oospores carry the fungus through the winter 

 in the old fallen leaves. In the spring these oospores germinate, 

 forming a conidiophore, on which is borne another spore, a 

 conidium. The conidium in turn, at maturity, germinates, 

 but instead of forming a germtube, its contents break up into 



