The Ohio Naturalist. 
[Vol. X, No. 7, 
1 66 
THE OHIO POWDERY MILDEWS.* 
W. C. O’Kane. 
Introduction. 
A number of years ago Professor A. D. Selby published a 
paper on The Ohio Erysiphaceae, or Powdery Mildews, so far as 
they had been reported in this state at that time. 
Since then new species have been recorded, new host plants 
have been reported, and there have been changes in nomenclature 
as further scientific study has disclosed certain synonyms. 
The present paper is an attempt to bring the record down 
to date. 
In the generic keys as well as the specific descriptions the 
writer has closely followed Salmon’s admirable monograph of 
the Erysiphaceae. 
Acknowledgment is due the valuable assistance of Professor 
J. Id. Schaffner, under whose direction this paper has been 
prepared; also the suggestions and counsel of the late Dr. W. A. 
Kellerman. 
GENERAL CHARACTERISTICS OF THE POWDERY MILDEWS. 
The Powdery Mildews are familiar to observers in two distinct 
stages. 
In the earlier, or conidial, stage, the affected parts of the 
parasitized plant appear as if covered with a white, mealy pow- 
der — the summer spores given off by the rapidly growing my- 
celium. 
Later the winter spore-cases, or perithecia, are formed. These 
are dark, spherical bodies, distinguishable with the unaided eye, 
and give the affected parts of the plant the appearance of being 
more or less covered with a brown or black powder. 
The perithecia usually bear characteristic appendages. If 
the perithecium is gently crushed it is found to contain one or 
more spore-sacs, or asci, in which are the spores. The outer 
surface of the perithecium is divided into many cells. 
The mycelium that bears the conidia and the perithecia grows 
on the surface of the leaf, or stem, drawing its nourishment by 
means of short branches, or haustoria. These may penetrate 
directly into the epidermal cells; or, as in the genus Phyllactinia, 
may enter the stomata of the leaf, and so reach the inner cells. 
In either case the plant is robbed of its sap, and the mildew lives 
at the expense of its host. 
* Contribution from the Botanical Laboratory of Ohio State Uni- 
versity, 56. 
