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 
