APPLE POWDERY MILDEW AND ITS CONTROL. 



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. If a 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 /;, in PL IV, figs. 1 and 2, and PI. 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 



thev are liberated fall On ^ IGl ^ - — -^ perithecium of Podosphaera leuootricha, 



, ... j showing details of the perithecium wall and the 



the young IOliage, and give basal appendages. At a is shown a single ascus 



rise to the first mildew in- which contains eight ascospores. Magnified 312 



„ . . times. (After Grout.) 



lections 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 



1 In Podosphaera oxyacanthae 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 



