358 



MANUAL OF FRUIT DISEASES 



Symptoms. 



The disease affects only the woody parts of the host, and usu- 

 ally only the twigs, although the pathogene may spread from 

 affected spurs into the larger limbs or the body of the tree. 

 The normal form of the diseased part is strikingly changed 

 (Figs. 100, 101, 102, 103 and 104). The knot is usually fusi- 

 form, but sometimes it may be the same diameter throughout 



its length, in which case the 

 knot terminates abruptly. As 

 the name suggests, black knots 

 are produced. A knot begins 

 at any of the following places : 

 (1) near the tips of twigs 

 (Fig. 100) ; (2) in the crotches 

 of younger limbs at the union 

 of the consecutive growths of 

 two seasons; (3) on small 

 spurs which commonly bear 

 the fruits (Fig. 100) ; (4) near 

 the axil of a leaf ; and (5) in 

 the crotches of limbs four or 

 five years old. Knots vary 

 from one-half of an inch to 

 a foot or more in length and 

 from a fraction of an inch to 

 two inches in circumference. Usually the knots do not extend 

 around the limb (Fig. 103), although in some cases they com- 

 pletely surround the affected part (Fig. 104). When their 

 course is long, they tend to proceed spirally about the stem 

 (Fig. 101). In the spring young knots are olive-green in color 

 and at this time are solid but rather pulpy. As the season 

 advances the knots become harder, more brittle, and their 

 surfaces become black. Frequently older knots are attacked 

 by insects which destroy the central part in the knot, leav- 



FIG. 100. Small black-knot swellings 

 about buds. 



