POWDERY MILDEWS OF FLORISTS’ CROPS 
Powdery mildew diseases frequently affect various ornamental plants 
of greenhouse and garden. They are sometimes very destructive and 
commonly troublesome. Among such plants which most often suffer from 
the powdery mildews are roses, phloxes, sweet peas, willows, hawthorns, 
lilacs, honeysuckles, bittersweet and dogwoods. 
SYMPTOMS 
The leaves are usually the organs affected, although stems, blossoms 
and even fruits may be diseased. Powdery mildews are usually most 
common and conspicuous in gardens and borders toward the latter part 
of the season. The white mealy coating which is formed on the affected 
organs is usually very striking. The minute black perithecia of the patho- 
gene frequently appear in great numbers late in the summer or early au- 
tumn, in some cases standing out sharply against the white mycelial mats 
on which they rest. Where the mycelium is sparse or webby, the black 
perithecia may not be easily detected. In many cases they are but rarely 
formed. A tendency to stunt or dwarf the host is commonly to be ob- 
served. ‘This is much more striking in some cases than in others. 
On the rose. Examine the diseased shoots provided. oBSERVE:— 
1. The white felt, covering large areas on the canes and running 
out over the thorns; in some cases localized about the base of large thorns. 
2. The powdery and less felty character of the white coating of 
the leaves. Which surface is affected? 
3. The curling and dwarfing effect on the leaves, especially 
marked in hothouse-roses and in ramblers. 
4. The abnormal coloration sometimes exhibited by the leaf 
under and about the mildewed spot. ; 
5. The dwarfing of the entire tips or branches of some shoots, 
most frequently observed in ramblers. This results from bud-infection 
explained later. 
6. The white mycelial felt, coating young buds and hips. The 
buds are often so stunted that they fail to open or the affected hips are 
dwarfed and do not ripen. 
Make pDRawINGs to show the symptoms exhibited in the material 
studied. 
On phlox. All above-ground parts of this host are likely to be affected. 
The mildew-spots are most prominent on the leaves. Examine the speci- 
mens provided. OBSERVE:— 
7. The felty white mycelial patches on the leaves. How do 
the patches on the two surfaces of the leaf differ? 
8. The purple coloration often developed beneath and about 
the spots. ; 
9. The yellowish color of the mycelial mat in the older patches 
and their tendency to coalesce and cover the larger part of the leaf-surface. 
10. The brown centers of many of the spots due, as may be seen 
with the hand-lens, to the perithecial fruit-bodies of the pathogene. 
11. The mycelial patches on stems and inflorescence; less felty, 
often hardly discernible but usually covered with the brown perithecia. 
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