POWDERY MILDEW OF CEREALS AND GRASSES 
This is a very common and sometimes serious disease of wheat, rye 
and other cereals. It is also to be found commonly on various wild grasses, 
especially species of the genus Poa. 
SYMPTOMS 
The powdery mildews are detected chiefly by the pathogene-structures 
developed upon the exterior of the host. There are also some accompany- 
ing affects or symptoms exhibited by the host. The lesions are confined 
largely to the leaves and leaf-sheaths. In the material provided, 
OBSERVE :—- 
1. The densely matted white, sometimes brownish, mycelial 
patches on the surface of the leaf (upper and lower). In the fresh condi- 
tion these patches are powdery due to the abundance of conidia, hence the 
name, powdery mildew. 
2. On the wheat leaves especially, the minute black bodies 
buried in the mycelial mass, usually most abundant at the center of the 
spot. These are the perithecia of the pathogene. : 
3. The effect on the tissues of the leaf beneath and about the 
mycelial mat. Note that in some cases the entire leaf has turned brown 
and died. 
Make sKETCHES showing these symptoms. 
Where the attack is severe, there is a dwarfing of the heads or a shriveling 
of the maturing grains, or both. If material is available, study and com- 
pare with healthy heads and grains. Make sKETCHES to show the compari- 
son. 
Examine the illustration specimens of powdery mildews on various hosts 
provided and NoTE:— 
4. The marked similarity in the symptoms exhibited by all of 
them. Select one of the specimens and sKETcH. Label fully. 
ETIOLOGY 
The powdery mildew of cereals and grasses is caused by Erysiphe 
graminis DeCandolle, the conidial form of which is known as Ozdium 
monilioides Link. Like all the other powdery mildew pathogenes it is 
an ascomycetous fungus belonging to the family, Erysiphaceae. They 
all develop externally upon the surface of their host except for short 
haustoria sent into the epidermal cells or, in the case of one or two species, 
an occasional mycelial thread sent through stomata into the tissues. 
Life-history. This pathogene exhibits during its life-cycles all of the 
characteristic structures of the powdery mildew fungi. 
; The Primary Cycles are initiated in the spring. The sources of 
octane are the overwintered perithecia on the leaves and stubble of the 
ost. 
Pathogenesis. Remove some of the minute black perithecia 
from the mycelial mats on the old overwintered host-leaves. Crush in a 
drop of water by pressing on the cover-glass. OBSERVE :— 
_5. The large ellipsoidal ascospores forced from the perithecia; 
some still in the asci. Determine the number of asci in each peri- 
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