annual reports. 271 



Plum and Cherry Diseases. 



Black Knot [Plowrightia morbosa (Schw.) Sacc.]. Black 

 knot of the plum and cherry is one of the most conspicuous 

 fungus diseases of New Hampshire. Early stages of 

 this disease are found in the spring as irregular knotty 

 swellings. At first these knots are yellowish brown in 

 color and have a velvety surface. This appearance is due 

 to the fact that the knots are at this time covered with a 

 layer of spore-bearing stalks. The spores thus produced are 

 capable of infecting other plants the same season. Later in 

 the year the knots have become much enlarged and have 

 turned from brown to a dead black. Upon close examina- 

 tion the surface of these knots is now found to have a rough 

 uneven appearance, due to minute, closely crowded pimples 

 or pustules. A miscroscopic study of these pustules shows 

 them to contain club shaped cells or sacs filled with 

 spores. These are the winter spores and serve to start the 

 disease anew the next season. The enormous number of 

 uncared for plum and cherry trees makes this an especially 

 difficult disease to fight. Orchardists who have kept it out 

 of their own orchards are compelled to be continually on 

 guard to prevent its introduction from neglected plants. 

 [See (a), plate 16.] 



Brown Rot [Sclerotinia fructigena (Kze. & Schm.) 

 Norton]. This is a very common disease of plums and may 

 also attack cherries, peaches and apples. The fungus may 

 be found on the leaves and the young shoots, but its charac- 

 teristic results are found upon the fruit. Here it produces 

 soft brown spots which rapidly enlarge, destroying the fruit. 

 The fruit soon takes a powdery appearance due to the abun- 

 dant production of the summer spores on its surface. The 

 rotten plums often remain on the tree nearly all winter. The 

 mycelium lives over winter in the mummified fruit, giving 

 rise to spores the next season that serve to infect the new 

 crop. 



