10 Sept., 1918.] 



Downy Mildew. 



573 



spore of similar type to ordinary summer spores, or Couidia, but con- 

 siderably larger, hence the term " Macroconidia." These germinate in 

 water in the same manner as the ordinaiy summer spores, i.e., by Zoo- 

 spores.* Should rain continue, these are splashed on to the lower leaves 

 of the vine, which they penetrate through the stomata or breathing 

 pores, forming in due course, after the lapse of the incubation period 

 (7 days), the characteristic oil-spot; should weather conditions continue 

 favorable, sporulation shortly occurs in the shape of the appearance of 

 the patches of white down on the under surface of the leaf. Subsequent 

 invasions occur, as has been previously described, whenever weather con- 

 ditions and absence of protective copper render fresh infection possible. 



Conidia germination likewise is only possible in water (rain or dew- 

 drops) ; it is influenced to a very considerable extent by' temperature. 



Fig. 6. 



Penetration of the vine tissues by the Mildew Fungus. The 

 germ tubes from the zoospores are entering through the stomata 

 or breathing spores of the vine. Above— zoospores from 

 macroconidia Below — zoospores from conidia (ordinary summer 

 spores). After Ravaz. (Highlj^ magnified. 



Below 46 deg. F., and above 85 deg. F., it does not occur. Germination 

 takes four or five hours at 46 deg., 40 minutes at 77 deg., and 

 twelve hours at 85 deg. It appears to be most active in the dark. 

 Zoospore germination takes place rapidly — 20 to 25 minutes after ex- 

 pulsion from the conidum their germ tube has already sprouted. It 

 follows that, at the most favorable temperature (77 deg. F.), contami- 

 nation may take place in an hour and a half. 



Sporulation of oil-spots (appearance of the white down) depends on 

 moisture (in dry air it does not take place), and also on temperature; 

 it scarcely occurs below 55 deg. F., nor above 82 deg. F. 



The above notes from Professor Ravaz's recent article (Progres 

 Agricole, 19th May, 1918), will give an idea of the moisture and tem- 

 perature requirements of the different stages of the mildew fungus, and 

 show the extent to which the evolution of the disease depends on the 



* Conidia usually emit four to six Zoospores ; Macroconidia send out a far larger number — 15 to 20. 



