APPLE DISEASES 115 
Cause of powdery-mildew. 
The pathogene is a fungus known as Podosphera leucotricha 
[= Spherotheca Mali (Duby) Burr.]. Sometimes a closely related 
species, P. Oxyacanthe (DC) DeBary, attacks the apple, causing 
much the same difficulty as that just described. Butintheapple 
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 hyphe. Certain of these threads, or 
hyphz, 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 
