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. nots vary 
; : from one-half of an inch to 
as Bae ieee rir swellings 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- 
