BLACK KNOT 



137 



iima, Wang., Prunus americana, Marsh, Prunus virginiana, 

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

 The fungus is readily recognised by the formation of 



Fig. 26. — Plowrightia morboia. i, portion of a plum 

 branch showing the first stage of the disease ; d, 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 



