S40 



SPECIAL PLANT PATHOLOGY 



crease in size and turn reddish in color, assuming the form of a bracket, 

 or shelf. The lower surface is beset with pores, or tubes, on the 

 walls of which the spores are borne. This bracket fruit may grow 



many years, and it adds a ring on the 

 outside when new growth com- 

 mences. The fruit bodies may occur 

 singly or in groups of two or three 

 together. They are rough on top 

 and appear to be covered with a waxy 

 substance, which has hardened and 

 cracked. It is brittle and readily 

 soluble in alcohol and xylol. The 

 lower surface is smooth with regular 

 pores. ' 



Plum (Prunus americana. Marsh) 



Black-knot {Plowrightia morbosa 

 (Schw.), Sacc). — The black knot 

 was at first mainly confined to the 

 New England states, but it now ex- 

 tends across the northern United 

 States to the Pacific coast with 

 areas free from the disease in the 

 middle west and southwest. Several 

 species of plums and cherries are sus- 

 ceptible. 



The disease appears as wart-like 

 excrescences on the smaller and 

 larger branches of plum trees (Fig. 

 194) which it either surrounds com- 

 pletely kilUng the terminal part of 

 the branch, or only part way round 

 when the branch continues living 

 The common name is well given, because 



'VON ScHRENK, HERMANN: The "Bluing" and the Red Rot of the Western 

 YeUow Pine, with Special Reference to the Black Hills Forest Reserve. U. S. Bureau ' 

 of Plant Industry Bull. 36, 1903. 



Fig. 194. — Black-knot of plum, 

 Plowrightia morbosa, on cultivated 

 plum, Cold Spring Harbor, L. I., July 

 26. 1915. 



and fruit-bearing (Fig. 194). 



