280 MANUAL OF FRUIT DISEASES 



usually killed, but the growing tip may develop a healthy shoot, 

 leaving a swollen canker-like lesion at its base. An exudation 

 of gum often accompanies the lesions on the twigs. The 

 flowers and young fruits are often attacked, but because they 

 soon drop away this symptom of curl is seldom observed. In 

 general, the sickly yellow, curled foliage, and the final defolia- 

 tion of the trees the latter part of June, followed by a refolia- 

 tion, are the most striking symptoms of this disease. 



Cause. 



The peach leaf-curl disease is caused by the fungus Exoascus 

 deformans, so named because it forms its asci on the outside 

 of the host, and because it deforms the leaves. The mycelium 

 of the pathogene grows between the cells of the leaf, stimulating 

 them to abnormal increase in size and number, and robbing 

 the leaf of its green chlorophyl. When the spores are to be 

 produced, the mycelium invades the cuticle of the leaf, and at 

 once gives rise to sacs (asci) which bear from 3 to 8 ascospores. 

 The presence of the asci on the upper surface of the leaf gives 

 the leaf its silvery appearance. At maturity, the ascospores 

 bud extensively within the ascus, giving rise to spores suggesting 

 conidia. The ascus then empties its contents through a crack 

 at the top. Thus far the life-history of Exoascus deformans is 

 definitely known ; but where these ascospores and their budded 

 descendants go is unknown. The fate and habits of the fungus 

 during the summer, fall and winter are not clearly understood. 

 It has been held that the mycelium is perennial in the twigs, 

 growing out into the leaves and young shoots in the early spring. 

 But this is doubtful, since thorough spraying in the fall or 

 before the buds open in the spring will control the disease. 

 Since the fungicide cannot penetrate the twigs to kill the 

 fungus, it can only be surmised that the inoculum comes from 

 some external source. It is the opinion of most investigators 

 that the fungus hibernates on the bud-scales in the form of 

 spores (kind unknown), and that these spores germinate during 



