PLANT DISEASES 251 



of cyanide of mercury 1 at a strength of one gram to five 

 hundred cubic centimeters of water. Corrosive sublimate is 

 not the best disinfectant for wounds, and the other does not 

 work well as a disinfectant for tools. 



The limb which has been cut off must be burned at once. 

 In this way the disease can be definitely kept in check, but 

 in those regions where it is well established, eternal vigilance 

 is the price which each pear-grower must pay if he hopes to 

 keep his orchard. Sometimes the disease can be detected in 

 whiter through the shriveling of the bark, but this calls for 

 expert observation and for the amateur the surest way is to 

 wait until the leaves appear in spring, when the progress of 

 the disease can be definitely noted. 



Considerable thought is being given to resistant stocks. 

 Some varieties of Chinese pears are not susceptible to blight 

 and these are being used. Such a method does not save the 

 tops from being attacked, but it does insure that the root 

 will not be killed by the disease. 



362. Powdery mildew, found chiefly on the grape, has 

 been known for three-fourths of a century and is a constant 

 problem to grape-growers not only in the United States but 

 throughout Europe. 



It can be seen readily on the leaves of diseased vines, 

 occurring on both the upper and lower surface as whitish 

 blotches, more or less rounded in shape. It does not confine 

 itself to the leaf, however, but attacks all parts except the 

 root. Blossoms which are mildewed do not set their fruit, and 

 fruit itself, if it be affected, soon ceases growth and falls, or 

 develops irregularly and fails to ripen. 2 



Thousands of tons of sulfur are used every year in the 

 grape-growing sections of the United States in the control of 

 this disease. This sulfur is carried on the back of the 



1 Both substances are very poisonous! Young children should never 

 handle either. 



2 Stevens and Hall : Diseases of Economic Plants. 



