10 Nov., 1917.1 



Notes on Downy Mildew. 



693 



The mechanism of infection' may be briefly described : — Each one 

 of the conidia falling into a drop <if water on a healthy leaf, under suit- 

 able temperature conditions, is cai>able of causing a fresh spot — a fresh 

 infection. The time which elapses between the entry of the fungus from 

 the genninating spore until the oil spot becomes noticeable is known as 



6 



Fig. 1. 



Downy Mildew — I'lasmopara vitkula. B. and du T. 



Diagrammatic section of a vine leaf infested with Downy Mildew, 

 showing how the mycelium of the fungus (a) circulates between the 

 colls wliich constitute the normal tissue of the leaf. At s, s, .1 are 

 three stomata or breathing pores, through which tlie conidiophorcs 

 or spore-bearing filaments emerge. It is these wliich constitute the 

 characteristic white down on the under side of the leaves. Conidia, 

 or summer spores (e) are, however, only shown on one filament. 

 The round black bodies with white rim, in the interior of the leaf, are 

 oospores. — After Viala. 



the period of incubation;* it is of very considerable importance in con- 

 nexion with the treatment of the fungus. 



The germination of the conidia takes place as follows: — The interior 

 substance splits up into several small bodies called zoospores, which 

 swim about in the drop of water until they find stomata or breathing 



* We undorsfand by " period of incubation " the [x-riod wliicli elapses between the penetration of 

 zoospores and the app<-araiicc of the flrst symptoms revealinfi to the nakinl eye the presence of mycelium 

 in the orcans attaeked (oil spots on the leaves, yellow-brown colour on bunches. tcDdrilB, and tips of 

 canes). — Gy de ittvanffi, page 94. 



