338 



BULLETIN 226. 



any more than favorable weather creates a corn crop. A fungus can- 

 not develop unless the spores get on the host plant any more than a 

 corn crop can be grown without planting the seed. But there are usually 

 plenty of spores on hand so that all they need is weather favorable to 

 their growth. Unusually wet weather during the blossoming period 

 favors the growth of the scab fungus. For the same reason it does 

 more damage in shady, unpruned and undrained orchards.' Dense 

 tops prevent the evaporation of the moisture. Open tops allow the 

 air to circulate freely, and they therefore quickly dry out after a 

 rain or dew. 



Some varieties arc more affected than others. The Snow, Spitzenburg 



and Maiden Blush 

 are particularly sub- 

 ject to scab. The 

 Greening and Twenty 

 Ounce are more af- 

 fected than the Bald- 

 win. Golden Russet 

 and Hubbardston are 

 among the more 

 resistant varieties. 

 The difference is 

 probably due to the 

 more tender outer 

 skins of some vari- 

 eties. The fungus 

 must penetrate the 

 outer skin before it can do any damage. It is interesting to note that 

 while the Greening apples are much more affected than the Baldwin, yet 

 the Baldwin foliage is very much more affected than that of the Greening. 

 Relation to other fungi. The scab fungus is often confused with other 

 fungi that secure a foothold in the wounds caused by the scab.. Part 

 of this confusion is doubtless due to the fact that the scab is almost 

 always called " the fungus." It would be well if farmers would call 

 it the apple-scab, in order to distinguish it from -the hundreds of other 

 fungi. The scab fungus causes the dark spots on the apple that may 

 later change to rusty spots. It is never white. The white moulds that 

 sometimes grow on these same spots are other fungi that could not have 



cl 



FIG. 82. Cross-sections of apple leaves, i. Healthy leaf. 

 2, The upper surface attacked by the scab fungus. 5. 

 A later stage in the growth of the fungus. 



