1 i O MANUAL OF MODERN VITICULTUKE. 



seedlings growing in soils where mildewed leaves had been 

 Iwried in Autumn, we have also found it in other isolated 

 parts where mildewed leaves had been buried, but where no 

 seedlings had grown. 



After the invasion has started, the disease is propagated 

 by the conidia during the whole summer if the atmospherical 

 conditions are favorable. The only two conditions neces- 

 sary for the development of mildew are moisture and heat. 



Light showers followed by warm weather, abundant dew 

 followed by a bright sunny day, sea winds, greatly increase 

 the growth of conidiferous filaments, and are the main 

 causes of the germination of these conidia in the drops of 

 water suspended on the leaves. The importance of these 

 causes has been proved by well-established facts. Vines 

 have been completely invaded in 24 hours after foggy 

 weather or heavy dew, so much so that vignerons have often 

 attributed the cause of the destruction of their vines to this 

 excess of humidity, which is called neble in the South of 

 France, me/in in Medoc, sun scald in the state of Missouri, 

 and mehl-thau (flour dew) by the Germans. Finally, it has 

 been noticed that on vines planted under the shelter of trees 

 preventing radiation and, therefore, dew which always 

 follows it, the mildew does not develop. Low vines are more 

 attacked than high-crowned vines, and suffer most. 



The temperature most favorable to the germination of 

 conidia is 77 to 86 Fahr. When the temperature falls 

 below 77 the germination takes place more slowly ; for 1 

 instance, at 62 it takes place irregularly, and only after two 

 or three days when the conditions of humidity are favorable 

 and constant. The germination may even be completely 

 stopped by a temperature of 57 Fahr. However, below this 

 temperature the conidia do not always lose their germinat- 

 ing power. Conidia submitted to a temparature of 32 

 Fahr. have even germinated when progressively brought up 

 to a temperature of 73 to 77 Fahr. 



In dry surroundings germination does not take place, and 

 the spores die. This explains the beneficial effects of north- 

 west winds in the South of France and siroccos in Algeria. 



The disease is propagated from one year to another by 

 means of winter spores or oospores, which are formed, as we 

 have seen, inside the leaf. These oospores are very resistant 

 to the action of outside agencies. They withstand the 

 lowest temperatures reached in regions where the vine grows ; 

 drought and excessive humidity do not deprive them of their 



