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21. That the mildew-patches are confined to the under surface 
of the leaf. 
22. The rather thick felty character of the mycelial growth; 
color and extent. 
23. The numerous perithecia sitting on the mycelial mat, 
not embedded in it; larger than the perithecia of the other mildew- 
pathogenes observed; some of them immature as indicated by their small 
size and light color. 
: 24. Any evidence of injury showing on the upper surface op- 
posite the mildew-spot. 
DRAW a leaf showing the characters of the mildewed areas. 
On the willow. Many species of willow and also poplars are affected. 
The leaves are usually the only organs involved. In the specimens 
provided, OBSERVE :— 
25. The location and distribution of the spots; hypophyllous 
or epiphyllous? : 
; 26. The characteristic dense white mycelial border of the spot 
with darker center, especially in older lesions, due to the numerous black 
perithecia. Willows are often defoliated by this disease. 
27. Any evidence of injury to the tissues. 
DRAW a willow leaf showing the mildew-spots. 
ETIOLOGY 
The powdery mildew diseases are all caused by species of ascomycetous 
fungi belonging to a single family, the Erysiphaceae. Each disease above 
studied is caused by a different species of pathogene. Even within some 
of these species there are doubtless biologic species. The diseases studied 
with their respective pathogenes are as follows:—the powdery mildew, 
of cherry, caused by Podosphaera Oxycanthae (NeCandolle) de Bary; 
of aoe caused by Podosphaera leucotricha (Ellis and Everhart) 
almon ; 
of peach, caused by Sphaerotheca pannosa (Wallroth) Léveillé, var. 
Persicae Woronichin; 
of grape, caused by Uncinula necator (Schweinitz) Burrill; 
of gooseberry and currant, caused by Sphaerotheca Mors-uvae (Schwei- 
nitz) Berkley and Curtis: 
of chestnut, caused by Phyllactinia Corylea Karsten; 
of willow, caused by Uncinula Salicts (DeCandolle) Winter. 
Besides the four genera represented, Podosphaera, Sphaerotheca, 
Uncinula and Phyllactinia, two more are known, species of which are very 
common in this country. Examples:—Erysiphe graminis DeCandolle, 
on grasses and cereals and Microsphaera Alni (Wallroth) Winter, on lilac. 
(See demonstration specimens.) 
Life-history. These powdery mildew pathogenes all exhibit such 
similarity in their structures and life-habits that the following outline 
should serve for the study of any of them. As they are strictly obligate 
parasites, saprogenesis is a period of rést or a maturation-process carried 
on at the expense of stored food-reserves. 
The Primary Cycles are initiated in the spring or early summer. 
The primary inoculum is usually the ascospores from overwintered peri- 
thecia. In some cases, as in that of S. pannosa (Wallr.) Lév., or of Podo- 
