APPLE DISEASES 115 



Cause of powdery-mildew. 



The pathogene is a fungus known as Podosphcera leucotricha 

 [ = Sphcerotheca Mali (Dub'y) Burr.]. Sometimes a closely related 

 species, P. Oxyacanthce (DC) DeBary, attacks the apple, causing 

 much the same difficulty as that just described. But in the apple 

 regions of New York and California where the disease has been 

 carefully studied it has been found that P. leucotricha is by far 

 the more common form present. 



The mildew that is found on the leaves and twigs is made up 

 of interlaced fungus hyphae. Certain of these threads, or 

 hyphse, become erect and bear chains of conidia. In mass 

 they give to the affected organ a powdery appearance. These 

 spores, or conidia, are blown to other leaves and twigs through- 

 out the growing season, causing infections. A spore falling on 

 a leaf or twig soon germinates by protruding a germtube. 

 From the latter, mycelium develops copiously, growing out in a 

 radiating fashion. At frequent intervals the mycelium sends 

 haustoria into the epidermal cells of the attacked part ; through 

 these organs the parasite obtains food. Soon a new crop of 

 conidia is developed, as already described. During the latter 

 part of the summer dark-brown, globose perithecia are formed 

 among the hyphal threads. These appear to the naked eye as 

 irregular, smoky patches on the twigs. The fungus is carried 

 over winter by these bodies. Each perithecium has several 

 appendages, and a single ascus containing sexual spores. These 

 ascospores are capable of producing infection with the advent 

 of the growing season. It is said that in the Pajaro Valley 

 where powdery-mildew is so prevalent on the apple the asco- 

 spores do not commonly bring about first infections in the spring. 

 In that region as well as in others a dormant-bud infection 

 occurs. The lateral and terminal bud-scales are penetrated 

 by the fungus ; here it remains dormant until spring, at which 

 time growth is resumed. Hence infected shoots appear as soon 

 as the leaves develop. Conidia mature rapidly on these shoots 



