Previously undescribed features on the surfaces of powdery
mildew conidia were revealed by the scanning electron microscope
(SEM), reinforcing differences observable by light microscopy (LM). Four
distinct patterns were observed on the septa and 10 on the
outer wall, thus categorizing 15 anamorph taxa. Anamorphs of
Leveillula, Phyllactinia and Pleochaeta,
all had smooth to moderately
verrucose septa, but each bore a different distribution of warts on
their verrucose outer wall. Annular septa and an echinulate outer
wall were unique for the Oidium anamorph of Blumeria.
The remaining powdery mildews, all with Oidium states, had
either whorled
or fibrillar septa. Those with whorled septa are separated into three
newly proposed subgenera: Fibroidium for the anamorphs of
Podosphaera and Sphaerotheca having conidia with fibrosin
bodies and a smooth outer wall; Octagoidium for the dendritic
patterned,
eight-sided conidia of Sawadaea; and Setoidium for
the anamorph of Cystotheca reflecting the setae on the mycelium.
Three new
subgenera are proposed for those with fibrillar septa: Reticuloidium
for conidia with a roughcast outer wall of Erysiphe sect.
Golovinomyces; Graciloidium for the unswollen conidia
of
Arthrocladiella; Striatoidium for conidia of E.
sect. Galeopsidis with a striate
outer wall. The name Pseudoidium is retained as a subgenus
for the anamorphs of E. sect. Erysiphe, Microsphaera
and Uncinula which
had conidia with a scaly, fibrous outer wall. The patterns on
the outer wall of conidia were frequently modified by secondary
creasing. These wrinkling patterns, also observable under the LM, proved
useful in identification of anamorphs, especially on
herbarium specimens. SEM, LM and host range data were used to construct
keys for the prediction of the anamorphs of all the
powdery mildew genera found in the U.K. The findings, together with
teleomorph criteria, also prompted a revision of the Erysiphaceae.