APPLE POWDERY MILDEW AND ITS CONTROL. 9 
during the latter part of the summer and fall, and thus plays no 
part in starting the disease the following spring. The natural 
method by which the fungus is able to bridge over the winter period 
is by means of another kind of reproductive bodies frequently called 
winter spores. This stage of the life history of the fungus develops 
only occasionally in most localities, but is particularly abundant in 
the Pajaro Valley. Ifa careful examination of vigorous twig infec- 
tions be made about the first of July or shortly thereafter, it will be 
seen that on many of them irregular, dark, smoky-looking patches 
have developed. (See p, in Pl. IV, figs. 1 and 2, and Pl. VI, fig. 2.) 
These patches contain 
great numbers of closely 
crowded, globose, dark- 
brown bodies,.each having 
a cluster of 4 to 11 long, 
stiff, brown, hairlike ap- 
pendages on the upper side 
and varying numbers of 
short, tortuous, irregular 
processes on the under side. 
(Figs. 3 and 4.) These 
bodies are called perithe- 
cia, and within each one 
there is developed a single 
saclike body (fig. 4, a), in 
which eight so-called asco- 
spores, or winter spores, 
are produced.1 These win- 
ter spores are long lived 
and remain dormant until 
the following spring, when 
they are liberated, fall on Fis. 4.—A perithecium of Podosphaera leucotricha, 
: 3 showing details of the perithecium wall and the 
the young foliage, and G1VE basal appendages. At a is shown a single ascus 
rise to the first mildew in- which contains eight ascospores. Magnified 312 
4 times. (After Grout.) 
fections of the season. 
Careful observations have led the writers to the conclusion that in 
the Pajaro Valley this method of bridging over the winter season 
amounts to practically nothing in the matter of starting the first 
infections of the following year. It may be that relatively a very 
small percentage are established by this means, but the really impor- 
tant source is in what the writers have termed the dormant-bud in- 
fections. It can be easily seen that in such serious twig infections 
1In Podosphaera oryacanthae there is a tendency for the perithecia to be more scat- 
tered. The appendages, which are more or less equatorially placed, are of only one type. 
They are spreading and dichotomously branched at the tips. 
46689°— Bull. 120—14 2 
