98 Diseases of Economic Plants 



to exercise all care not to bruise the tree. An effective meas- 

 ure, however, consists in pruning out and burning all black 

 knots. This course, if followed persistently and thoroughly, 

 will remove all danger. Experiments show that spraying 

 very materially lessens the spread of black-knot, and while it 

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

 an orchard for other reasons really diminishes this risk. 



Plum-pockets i^^' ^^^ (Taphmia pruni (Fcl.) Tul. and T. 

 communis (Sad.) Gies.). — The disease is widespread in 

 Europe and the United States, and in certain localities it is 

 destructive. The name "plum-pocket," or "plum-bladder," 

 arises from the curious hollow deformity of the plum, which 

 appears soon after the flowers fall. 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 affected 

 fruits vary from 2-5 cm. in length, and can readily be dis- 

 tinguished from the healthy fruit by their pale yellow color. 

 As they age, they become coated over with a fine powder, the 

 spores of the fungus. Later the pockets turn black and fall. 



The attack of the fungus is, to some extent, local in char- 

 acter; thus a single tree in an orchard may bear "pockets," 

 sometimes every fruit on the tree being affected, while sur- 

 rounding trees are normal. A tree once affected may con- 

 tinue to bear pockets in succeeding years. Treatment, in 

 such cases, consists in cutting and burning the affected 

 branches or trees. To some extent the disease is also due to 

 yearly surface infection, which may be prevented by spraying 

 as for peach curl. 



Leaf-spot ^^"^^ (Coccomyces prunophorce Higg., Cylindro- 

 sporium) . — Leaf -spot or shot-hole disease, similar to that of 

 plums and cherries, produces discoloration of the leaf tissue, 

 the spot often bearing a red or purple border. If abundant 

 several spots may coalesce as they enlarge. The tissue in- 

 volved dies, turns brown, and eventually drops from the 

 leaf, leaving circular or ragged holes. The disease is also 

 frequent upon petioles. 



The attack is insidious, and the disease often does much 



