190 MANUAL OF FRUIT DISEASES 



Cause. 



The causal fungus, Exoascus Cerasi, is very closely related to 

 the fungus causing peach leaf-curl and to that of plum-pockets 

 (see pages 277 and 373, respectively) . Its full life-history is not 

 known. The mycelium invades the twigs, living in them from 

 year to year. Both the bark and wood are attacked. The 

 pith-cells, medullary-ray cells and outer cells of the bark 

 (hypodermis) are greatly increased in number as a result of 

 stimulation set up by the parasite. The sclerenchyma fibers, 

 on the other hand, are fewer or lacking. The stimulating effect 

 of the fungus brings about a condition whereby, instead of 

 flower-buds being produced as in ordinary cases, abnormal 

 twigs are developed, giving the broomy aspect already described. 

 The mycelium invades the leaves, causing them to curl and fall. 

 Finally, before defoliation, the fungus forms a fruiting layer of 

 asci with ascospores on the lower surface of the leaves. These 

 bodies give to this surface the whitish appearance already 

 mentioned. 



Control. 



Fortunately the disease is not usually of sufficient impor- 

 tance to require urgent attention. Little experimental work has 

 been done on which to base recommendations for American 

 conditions. Since the brooms bear neither flowers nor fruit 

 and are a source of trouble, they should therefore be removed. 

 The cut should be made several inches below the lowest point 

 in the diseased portion. It is said that this method readily 

 controls the disease. 



REFERENCES 



Stewart, F. C. Witches' brooms on cherry trees. New York (Geneva) 



Agr. Exp. Sta. Kept. 14 : 532-533. 1896. (See also Garden and 



Forest 8:269. 1895.) 

 Jackson, H. S. Diseases of drupaceous fruits. Leaf curl or witches' 



broom. Oregon Crop Pest and Hort. Bienn. Kept. 1911-1912 : 



250. 1913. 

 Atkinson, G. F. Leaf curl and plum pockets. Exoascus cerasi 



