378 AMERICAN GRAPE CULTURE. 



The reader will get a pretty clear idea of the 

 subject (and a general idea is all we can give 

 him here) if he bears in mind that the sporules 

 of this parasite are almost constantly floating 

 in the air, waiting for a favorable moment for 

 attack, and that the vine, to a very consider- 

 able degree, is able to resist its attacks so long 

 as its vital force remains unimpaired and in full 

 vigor. Any cause whatever that impairs or 

 lessens vital action favors the attack of the 

 parasite. Hence sudden atmospheric changes 

 from heat to cold, cold rains following hot, dry 

 weather, cold nights following hot days, ex- 

 treme drought, prolonged rains, and similar 

 causes that lower the action of the plant more or 

 less suddenly, are followed by attacks of mildew. 

 It is generally first seen on the leaf, next on the 

 wood, and last on the fruit, though some- 

 times this order of attack is changed. It soon 

 enters the tissue of the leaf, and gradually de- 

 stroys it. It also penetrates the cells of the 

 wood, giving them an inky appearance. The dis- 

 ease has then become what we shall call consti- 

 tutional, and admits of no cure except amputa- 

 tion at some point below the parts diseased; 

 even this must not be too long delayed, for we 

 have found the disease to run through the cells 



