Diseases of the Plum 321 



to be of any practical consequence. Perhaps the Dam- 

 sons and the slower-growing Domesticas in general 

 are most easily affected by the black knot; but neither 

 is this difference of susceptibility great enough to in- 

 fluence the man who plants plum trees. 



Black knot is, in fact, not a serious matter in the 

 plum orchard. Plum trees which are tended, even 

 moderately, will seldom suffer from it. Occasionally 

 the season seems to be especially favorable to the 

 spread of this disease and an harassing outbreak oc- 

 curs; but this is seldom the case. 



The knots or warts on the plum trees are caused 

 by a fungus, although many persons suppose the cause 

 to be an insect. This fungus grows within the tissues 

 of the branches and twigs, but breaks through in early 

 spring, causing the familiar warty appearance. These 

 warts are at first yellowish; but during May or June 

 they turn a darker greenish color and become shining 

 and velvety. This velvety appearance is caused by 

 an immense crop of spores which entirely cover the 

 exposed portions of the wart. These spores are most- 

 ly soon distributed, carrying the infection to new trees 

 and new branches. They usually lodge in the crotches 

 of small branches, or in the axils of leaves or fruit 

 spurs. Here they germinate and soon penetrate the 

 woody tissue, giving rise, next year, to new warts. 



Later in the fall the knots will be found to have 

 turned a dull black. On close examination they show 

 a fine granular surface. This is caused by the pres- 

 ence of another crop of spores. These late spores are 

 thick-skinned and resistant to the weather. They live 

 over winter and are able to originate new warts in 

 the spring. 



Besides the late spores, the fungus has another 

 resource for carrying itself over the winter. The 

 mycelium, that portion which grows inside the woody 

 21 



