DRUPACEOUS FRUITS 141 



all danger. Experiments show that spraying very materi- 

 ally lessens the spread of the black knot, and while it might 

 not pay to spray to prevent this disease, spraying in an 

 orchard for other reasons really diminishes this danger. 



Plum pockets (Exoascus Pruni Fckl.) . — The name "plum 

 pocket," or " plum bladder," arises from the curious hollow 

 deformity of the plum, caused by a fungus. The pulp and 

 stone of the fruit are replaced by a thin, soft, inflated shell, 

 and in place of the seed merely a hollow cavity exists. 



The disease appears soon after the flowers fall. The 

 affected fruits vary in size from 2-5 cm. in length, and can 

 be readily distinguished from the healthy fruit by their 

 pale yellow color. As they age, they become coated over 

 with a fine powder, consisting of the spores of the fungus 

 which caused the malformation. Later the pockets turn 

 black and fall. 



The disease is local in character. A single tree in an 

 orchard may bear " pockets," sometimes every fruit on the 

 tree being affected, while surrounding trees are normal. 

 A tree once affected continues to bear pockets in succeeding 

 years. Treatment consists in cutting and burning the 

 affected branches or trees. 



Leaf spot (Cylindrosporium Padi Karst). — The leaf spot 

 or shot-hole disease attacks plums and cherries, causing 

 a portion of the leaf tissue to become discolored, the spot 

 often bearing a red or purple border. If abundant several 

 spots may coalesce as they enlarge. The tissue involved 

 dies, turns brown, and eventually drops from the leaf, 

 leaving circular or ragged holes. This character gives rise 

 to the common name " shot-hole disease." The disease 

 is also frequent upon petioles. 



