BLACK KNOT 



137 



tima, Wang., Primus americana, Marsh, Prunus virginiana, 

 L., Prunus serotina, Ehrb., and Prunus pensylvanica, L. 

 The fungus is readily recognised by the formation of 



Fig. 26. Plowrightia morbosa. i, portion of a plum 

 branch showing the first stage of the disease ; 2, plum 

 branch with 'black knot,' the stroma bearing perithecia; 

 3, conidiophores bearing conidia, X 300; 4, ascus con- 

 taining eight ascospores, x 300. 



black, rough excrescences on the branches; these are 

 often several inches in length, and sometimes cause the 



