MILDEWS. RUSTS AND SMUTS 
41 
springing from the base of the perithecium and spreading 
horizontally, simple or vaguely branched, often almost 
wanting. 
The genus is well characterised by the presence of only 
a single ascus in the perithecium, and the vague appendages. 
KEY TO THE SPECIES 
Mycelium persistent, forming dense, white, felt-like patches, 
in which the perithecia are imbedded, pannosa. 
]\Iycelium persistent, forming dense, brown, felt-like patches, 
in which the perithecia are imbedded. mors-uvae. 
Mycelium forming thin patches, which usually soon dis¬ 
appear . humuli. 
Sphaerotheca pannosa, Lev. Mycelium persistent, form¬ 
ing dense, whitish, satiny patches on the more persistent 
parts of the host-plant ; perithecia more or less immersed 
in the persistent mycelium, subglobose, blackish, about 
100 [JL chain. ; ascus broadly oblong to subglobose, 88—115 
p, averaging 100 X 60—75 p ; spores 8, 20—27 x 12—15 p. 
Syn. Podosphaera pannosa, De Bary. 
Parasitic on plants belonging to the order Rosaceae, as 
Rosa canina, R. arvensis, R. damascena, R. glauca, and many 
other species of Rosa, also on Prumis persica. 
This fungus is very common on wild roses, and is also 
a source of constant anxiety to rose growers. The conidial 
condition forms a thin white, pulverulent layer on the young 
leaves, but it is on the young shoots, sometimes also on 
the fruit, that the mycelium forms dense, felt-like, whitish 
patches, on which the perithecia appear in due course. 
Sphaerotheca humuli. Burr. Patches formed on both 
surfaces of leaves, mycelium white, usually soon dis¬ 
appearing, but at times persisting in white patches on the 
upper surface of the leaves ; perithecia scattered or densely 
crowded, 50—120 p. diam.., appendages very variable in 
number and length, usually long and coloured, straight or 
contorted, sometimes almost absent ; ascus broadly ellip¬ 
tical to subglobose, 45—90 X 50—72 p, ; spores 8, 20—30 
X 12—18 p,, averaging 22 X 15 p. 
Syn. Sphaerotheca castagnei. Lev. 
This fungus is parasitic on plants belonging to various 
orders of plants, as Rosaceae, Geraniaceae, Violaceae, 
Uritcaceae, etc. In this country it is best known as the 
very destructive hop mildew. 
Var. fuliginea, Salmon. Perithecia about 50 p in dia¬ 
meter, appendages usually short, pale brown, crooked, but 
