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Downy Mildew of Grape (Plasmopara viticola). a. portion of the mycelium from a diseased grape ; 

 B. A tuft of spore stalks ; c. mature spores ; d. thick walled resting spore (from Longyear) . 



The morphology and life history of the different genera are very simi- 

 lar, and the family on the -whole is such a coherent group that it may 

 be treated as a whole in this regajrd. In fact it is practically impossible 

 to distinguish between the conidial forms, insomuch that they were for- 

 merly classified as an autonomous genus of the Hyphomycetes under the 

 name Oidium. 



The vegetative mycelium consists of numerous branching, septate, 

 white hyphae. These are always mucvh interwoven. The septa do not occur 

 at very close intervals, giving the hyphae the appearance of being divided 

 into rather long cells. 



In all the genera, except Phyllactinia, the hyphae form haustoria 

 which penetrate the cuticle and make their wav into the epidermal cells. 

 The function of these, of course, is to attack the fungus and obtain nour- 

 ishment. The genus Phyllactinia forms special hyphal branches which 

 enter the stomates or breathing pores and then send out haustoria which 

 penetrate the surrounding cells. In general it may be said that with the 

 exception of the haustoria and the hyphal branches of Phyllactinia the 

 mycelium of the Erysiphaceae is entirely superificial. 



