360 



MANUAL OF FRUIT DISEASES 



ceeds to such an extent that the normal form of the limb 

 is strikingly changed (Fig. 102). The newly forming knot 

 can often be detected in the fall, when it appears as a slight 

 swelling (Fig. 100). It is, however, more conspicuous in the 

 spring ; at this time it enlarges and the bark is ruptured, thus ex- 

 posing a yellowish surface (Fig. 101). This color does not prevail 



long, but the fungus 

 grows to the ex- 

 terior and thereon 

 develops its sum- 

 mer spores which 

 give to the knot a 

 velvety olivaceous 

 appearance. These 

 spores are dissemi- 

 nated in April, May 

 and June, being car- 

 ried by the wind to 

 suitable places for 

 initiating a new 

 knot. As the sea- 

 son advances the 

 gall gradually 

 changes color ; by 



tne rst * ^ep- 

 tember black dots 



appear over the 



surface, and within another month the whole knot is perfectly 

 black and presents the appearance which is so commonly seen 

 (Fig. 102). If one examines such a knot closely, it will be ob- 

 served that it is then covered by innumerable small elevations 

 which project from its irregular surface (Fig. 102). Each of 

 these elevations constitutes a winter condition of the fungus 

 a perithecium in which a second kind of spores develops from 



FIG. 102. Black-knot ; two-year-old knots with 

 black roughened surface. 



