112 THE AMERICAN APPLE ORCHARD 



will lead to a few inferences of considerable practical 

 importance. 



1. All spraying must be preventive. The fungus 

 must be killed in the spore stage. There is no such 

 thing as curing a fungous disease. 



2. Early spraying does the most good. After the 

 spores have germinated and the fungus has gained en- 

 trance to the plant, spraying does no good. Late spray- 

 ings are useful only in killing later crops of spores, 

 but as a rule these are comparatively unimportant. 



3. Spraying must cover the entire tree. A fungous 

 spore is less than the ten-thousandth of an inch in di- 

 ameter, but the copper sulphate has to touch it to kill 

 it. 



Scab or Black Spot — The apple scab is the worst 

 disease which the orchardist has to meet. It is the most 

 widespread, and on the whole the most destructive. It 

 does great damage to the foliage in some cases, though 

 it is more conspicuous on the fruit. The leaves when 

 attacked by scab show irregular browned patches of 

 dead tissue. Sometimes these are so abundant as to 

 give the whole tree a half burnt appearance. On the 

 fruit the fungus causes black patches, which sometimes 

 crack open, making the fruit unsightly and unsalable. 

 Different varieties are susceptible in differing degrees 

 to the attacks of scab fungus. Fameuse, ^Mcintosh and 

 all apples of that type are especially subject to injury. 



The scab, once established on a tree, works in the 

 small twigs and throughout the growing tissues. It 

 also continues to develop in the fruit after picking, so 

 that fruit put into storage in reasonably good condition 

 but affected with scab will deteriorate rapidly, espe- 

 cially if the storage room is warm. 



