DISEASES OF THE PLUM 



LEX R. HESLER 

 Assistant Professor of Plant Pathology, Cornell University, Ithaca, N. Y. 



BLACK KNOT 



The black knot, which is caused I 

 Plowrightia morbosa (Schw.) Sacc., 

 one of the most common diseases of th( 

 plum. It receives its name from the cha] 

 acteristic black excrescences or galls on th( 

 branches. The young swellings are at 

 first covered by the bark. As the knots 

 increase in size, the bark ruptures and th< 

 swollen surface becomes olivaceous ; event- 

 ually, however, the galls become coal- 

 black, hard, and brittle. The knots ar< 



usually confined to one side of the twig or limb, so that death o1 

 the affected portion does not always immediately follow attack. 

 On the other hand, in more severe cases the whole circumferenc 

 of the branch is involved, and the food supply intended for th( 

 more distant parts is cut off. In such cases whole trees are finall 1 

 killed (Fig. 393). 



The disease originated in America and is peculiar to our native 

 wild species of plums and cherries, although it is frequently d( 

 structive to certain cultivated varieties of plums and sour cherries 

 The knots are due to the work of a fungus, although for a 

 time it was believed that insects were the cause of the trouble. 



The olivaceous ruptured surface of the knot is at first cover( 

 with the summer spores, or conidia, which are borne on short U] 

 right threads and are capable of causing infection during the sai 

 season in which they are formed. This infection occurs froi 

 May until the latter half of the summer. In August the knottec 

 portion of the branch turns black and becomes brittle ; the surfai 

 is then covered with very fine, pimply protuberances, each of whiel 

 has an opening at its apex. These openings lead to flask-sha] 



[11903 



