Identification and classification of powdery mildew anamorphs using light and scanning electron microscopy and host range data

Abstract

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.