Sugar-beet powdery mildew (Erysiphe betae).

UNLABELLED: Summary Erysiphe betae causes sugar-beet powdery mildew, a serious fungal foliar disease resulting in sugar yield losses of up to 30%. The fungus occurs world-wide in all regions where sugar beet is grown and it also infects other edible beet crops, e.g. beetroots (garden beets). Unlike other powdery mildews, E. betae has so far received relatively little attention from pathologists and the precise mechanisms by which it infects its host remain unclear. Sources of genetic resistance have been identified in cultivated and wild Beta germplasm and molecular markers developed linked to Pm, the only single major R gene described so far, and also to QTL. TAXONOMY: Erysiphe betae (Vañha) Weltz.-Kingdom Fungi, Subdivision Ascomycotina, Class Pyrenomycetes, Order Erysiphales, Family Erysiphaceae, Genus Erysiphe. IDENTIFICATION: Superficial persistent mycelium; unbranched erect conidiophores; conidia ripen singly, are hyaline, ovoid, 30-50 microm x 15-20 microm; cleistothecia globose, dark brown/black, 80-120 microm in diameter; mostly 4-8 asci per cleistothecium, mostly 2 or 3 spores per ascus. HOST RANGE: A monophagous parasite specific to Beta species. Disease symptoms: Infected foliage and inflorescences bear numerous powdery, white colonies. Under favourable environmental conditions the colonies coalesce, host tissue develops chlorosis and usually senesces early. Cleistothecia develop on heavily infected leaves in late summer and are small black/dark brown globose bodies resting on the mycelial surface. CONTROL: Chemical control and partial genetic resistance.