CONTROLLING VINE DISEASES 



283 



sample and test with a few drops of potassium ferrocyanide solution. 

 If the solution turns brown, there is not enough lime, but there will 

 be no change if there is enough lime. There should be some excess 

 of lime to be sure the copper is neutralized, but not too much, as the 

 lime will clog the nozzles. Lime is used to neutralize the copper so 

 it will not burn the foliage, and also to make the solution more 

 adhesive. 



Time of Spraying. No rule can be given as to the time to begin 

 spraying, except it should be done before the blight has opportunity 



FIG. 113. A good field of potatoes. Sprayer at work in the distance. 



to attack the crop. The blight may make its first appearance any 

 time from midsummer to the very end of the season, or not appear 

 at all. In hot, dry weather there is less danger than in wet or humid 

 weather, and conditions must be watched closely. 



Generally, it is best to give the first spraying when the plants are 

 less than a foot high, in order to cover all the lower leaves. Apply 

 other sprayings according to the weather or rapidity of growth 

 (Fig. 113). 



The number of sprayings given in practice varies from 3 to 10. 

 If the disease is present, and weather damp, it is generally agreed 



