Drupaceous Fruits 



97 



PLUM 



• Black-knot ^^^ (Dibotryon morbosum (Schw.) T. & S.). — 

 The black-knot receives its name from the swollen black 

 distortions, 3-15 cm. long, upon the branches. In a young 

 condition the galls are 

 olivaceous, but as the 

 season advances they 

 become darker and 

 eventually coal black. 

 At the same time soft 

 tissue changes to a 

 hard, brittle texture. 



When the knot com- 

 pletely encircles the 

 twig, the supply of 

 nourishment to more 

 distant parts is seri- 

 ously interfered with. 

 If less than the whole 

 of the circumference 

 be involved, the dam- 

 age is not so great, 

 but it is even then suf- 

 ficient to seriously im- 

 pair the fruiting of the 

 tree. Black-knot may 

 either kill the tree in 

 a year or two, or sim- 

 ply destroy its value. 

 All varieties of plums, 

 and nearly all cherries, 

 are subject to the dis- 

 ease, which is therefore very widely distributed. It is more 

 common in the East than in the West. 



It is probable that infection can occur only through injured 

 bark. One step toward lessening the disease is, therefore, 



Fig. 49. — Portion of a plum tree badly 

 infested with black-knot. Original. 



