540 



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 {Primus 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. 



„, , , , , 104) which it either surrounds com- 



FiG. 194. — Black-knot of plum, ^ .... . , 



Plowrightia morbosa, on cultivated pletcly killmg the termmal part of 

 plum, Cold Spring Harbor, L. L, July the branch, or Only part way round 



when the branch continues living 

 and fruit-bearing (Fig. 194). The common name is well given, because 



Won Schrenk, Hermann: The "Bluing" and the Red Rot of the Western 

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

 of Plant Industry Bull. 36, 1903. 



