PLANT DISEASES 239 



presence is not suspected until the spore-bearing parts appear, 

 and then it is too late to eradicate them. The oyster mush- 

 room and some of the shelf fungi are among the better known 

 of the wound parasites. The entrance of such parasites may 

 be prevented by promptly covering all wounds with paint 

 or grafting wax. 



Other plant diseases. The list of plant diseases is a very 

 long one. It includes the black knot on plum, fire blight of 



FIG. 177. Potatoes affected, by potato scab 

 From Duggar's "Fungous Diseases of Plants" 



the apple and pear, peach yellows, plum pockets, potato scab, 

 cedar apples, witches'-brooms, peach-leaf curl, clubroot of 

 cabbage, anthracnoses, and a host of others. It is usually not 

 necessary to positively identify the organism causing the dis- 

 ease in order to remedy it, since what will control one disease 

 will be likely to control all the others like it. The main thing 

 is to discover the trouble before it has had time to spread, and 

 to take prompt measures for its suppression. In a majority 

 of cases the most efficient treatment is to spray with some 



