POWDERY MILDEWS OF TREES AND FRUITS 
_ _ Powdery mildew diseases have been reported on about 1500 species of 
wild and cultivated plants. Several of them are frequently very injurious 
to fruit-trees and sometimes to shade- and forest-trees. 
SYMPTOMS 
Leaves and young shoots are usually the parts of the host that are 
affected. Throughout the latter part of the summer the powdery mil- 
dews are conspicuous and give to the infected parts of the host-plant a 
whitish, mealy or dusty appearance, due partly to the superficial white 
web-like mycelium of the pathogene, and partly to the presence of myriads 
of rapidly formed white conidia. Later in the summer, and in the autumn, 
there usually appears, on the affected parts, the small black spherical 
perithecia of the sexual stage. In the autumn these are more in evidence 
than the whitish growth; the latter often disappears. These signs, the 
fruit-bodies of the pathogene, are usually the most striking evidences of 
the disease. Definite and characteristic symptoms resulting from the 
effects of the pathogene on the host are, however, not wanting in many 
cases. 
On'‘the cherry. On this host, the leaves and twigs show the effects 
of the disease. Examine the specimens and photographs provided and 
OBSERVE :-— 
1. The upward rolling or curling of the leaf-blades parallel with 
the mid-rib. 
2. The shorter and thicker internodes of diseased twigs as com- 
pared with healthy ones. 
.38. The weft-like coating of fine white hyphae on the leaves, 
especially on the under surface. The mycelium of most of the powdery 
mildew pathogenes is entirely superficial. 
4. Patches of the miycelial weft dotted. with the minute black 
perithecia of the pathogene. 
Make a DRAWING of a diseased twig to show the signs and symptoms 
exhibited. 
On the apple. The young leaves, flower clusters and shoots are af- 
fected. Examine the diseased shoots and U. 5S. Agr. Dept. Bul. 120, 
pl. I and VI, provided. oBsERVE:— 
5. The marked hypoplastic effect exhibited in the dwarfed 
foliage. . 
6. The mealy white coating of the diseased leaves,—conidia 
and mycelium of the fungus. 
7. The thick felty mycelial coating on the watersprouts collected 
in the autumn. Note the dirty white or brownish tinge as compared with 
the pure white of the growth on the leaves. ; 
8. The minute perithecia, more or less embedded in the mycelial 
mat on the shoots. 
Make prawincs to show the appearance of affected leaves and water- 
sprouts. 
On the peach. Not only the leaves and twigs, but also the fruits of 
the peach, are subject to the disease. Study the specimens and photo- 
graphs provided. OBSERVE :— 
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