PLUM DISEASES 



361 



FIG. 103. Black-knot ; cross- 

 section of a knotted twig- The 

 enlargement has not completely en- 

 circled the twig. 



January to June, depending upon the locality, and which are 



discharged upon maturity. The further history of these spores 



is similar to that of the summer 



spores previously described ; they 



propagate the parasite. The 



original portion of the knot is 



thus matured, but the fungus in 



the bark may continue to grow 



at either end, thereby increasing 



the length of the old knot. In 



this manner a single knot may 



in time extend a long distance 



on a branch, its course tending to 



proceed spirally about the stem. 

 Control. 

 Eradication of the causal fungus is the recognized method of 



most practical value in the control of black-knot. In following 



this principle, it should be 

 remembered that (1) the sum- 

 mer spores are produced abun- 

 dantly during the late spring 

 and early summer, (2) the 

 winter spores develop from 

 midwinter to spring, and (3) 

 there are many kinds of plums 

 and cherries attacked by these 

 spores. Therefore, prune out 

 the knots before either kind of 

 spores is matured, that is, in 

 the fall or early winter, before 

 January. Look for them in the 

 wild as well as in the cultivated 



species of plums and cherries. An annual fall inspection of the 



trees and the removal and destruction of all the knots is the most 



FIG. 104. Black-knot ; cross- 

 section of a diseased twig. The 

 knot has completely encircled the 

 twig. 



