BLACK KNOT 



137 



tima^ Wang., Frumis aviericana^ Marsh, Primus virginiaiia. 

 L., Prumis serotina^ Ehrb., and Frumis petisylvanica, L. 

 The fungus is readily recognised by the formation of 





A 



V'V(+;!y ''^' 



1. .; v.> 



%i 



^ 



/^'' 



Fig. 26. — Plowri^htia viorbosa. 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 



