May, 1910.] 
The Ohio Powdery Mildews. 
167 
Key to the Genera (After Ellis & Everhart). 
Class — Ascomycetae. 
Order — Perisporiales. 
Family — Erysiphaceae. 
1 . Appendages of perithecia simple, and similar to the mycelium 2 
Appendages different from the mycelium 3 
2. Only on ascus in the perithecium Spacrotheca 
Several asci in the perithecium Erysiphe 
3. Appendages coiled at tip Uncinula 
Appendages branched at tip 4 
Appendages not branched, swollen at base, with tip straight. . . . 
Phyllactinia 
4. Only one ascus in perithecium Poaosphaera 
Several asci in perithecium Microsphaera 
Sphaerotheca, Sw. 
Key to the Ohio Species. 
1 . Mycelium thick, forming persistent patches containing the perithecia 2 
Mycelium thin or not persistent 3 
2. Mycelium brown. Outer and inner walls of perithecium not 
separating mors-nvae 
3. Perithecia 60-70 /t in diameter; ascus 60 x 42 to 70 x 50 ft, inner 
and outer walls of perithecium readily separating. . phytoptophila 
Perithecia 58-120 j u in diameter; ascus 45 x 50 to 72 x 90 y, 
inner and outer walls of perithecium separating with 
difficulty 4 
4. Cells of perithecium averaging 15 ,u humuli 
Cells of perithecium averaging 25 ft humuli var. fuliginia 
Sphaerotheca humuli (DC) Burr. Perithecia 58-1 20/< in 
diameter ; cells 10 to 20 y\ appendages usually long, straight, sep- 
tate, dark brown ; ascus usually without stalk, 45 x 50 to 72 x 90 y ; 
spores 8, 12 x 20 to 18 x 25 y. 
Host plants in Ohio Herbarium: Geum canadense, Medina Co., Rubus 
odoratus, Cuyahoga Co., E. Claassen ; Rosa (cult), Columbus, J. G. Sanders; 
Taraxacum taraxacum, Auglaize Co., J. D. Simkins; Agrimonia eupatoria, 
Lancaster, W. A. Kcllerman. 
Sphaerotheca humuli fuliginea (Schl.) Salm. Perithecia 
averaging smaller than humuli; cells averaging 25 y\ spores 
12 x 20-15 x 25 y. 
Host plants in Ohio Herbarium: Prenanthes alba, Cuyahoga Co., E. 
Claassen; Bidens frondosa, Columbus, F. L. Stevens, Cuyahoga Co., E. 
Claassen ; Taraxacum taraxacum, Cuyahoga Co., E. Claassen, Columbus, 
F. L. Stevens. 
Spharotheca mors uvae (Schw.) B. & C. Mycelium dense; 
perithecia abundant, 75-110 y in diameter; cells 10-25 y \ appen- 
dages few, pale, brown, tortuous, length 1-5 times diameter of 
perithecium; ascus 50 x 70-02 x 92 y. spores 12 x 20-15 x 25 y. 
Common on cultivated gooseberries throughout^Ohio. 
