E. necator belonging to the Ascomycetes is heterothallic and sexual reproduction occurs after hyphal fusion between members of opposite mating types. Cleistothecia are spherical, yellow to light brown when immature becoming black with a basal concavity and diameter of 84 to 105 µm upon maturity. Cleistothecia contain four to six asci, and appendages are myceloid, septate with uncinate and coiled tips. Asci contain 4-6 ovate to ellipsoid ascospores (15-25 x 10-14 µm). Mycelia are very thin (4-5 µm in diameter). One to 10 conidia (32-39 µm x 17-21 µm) are borne in chains on the conidiophore
E. necator is an obligate biotroph growing only on grapevine tissue. All green tissue (stems, leaves, flowers and fruit) of grapevine may become infected by powdery mildew showing whitish-gray mycelium with a powdery appearance caused by the fungus. On leaves small yellow blotches on upper surface of the leaf appear, corresponding underside veins are often brown. Infection can occur on both upper and lower leaf surfaces.
On hoots red-brown to black blotches on mature canes indicate infection from earlier in the season. The berries which show severe infection delay maturity and cause the berry to split as it grows. A network pattern of scar tissue on mature berries indicates where the fungus has grown. Mycelium and conidia overwinter in dormant buds. Young shoots emerging from these buds ('flag shoots') are distorted and stunted.
The disease cycle of E. necator encompasses both asexual and sexual over-wintering stages. In temperate countries, both stages may be important, however cleistothecia appear to be the only means of overwintering for the fungus. Conidia produced from mycelium are the primary source of inoculum at the beginning of the growing season in vineyards where 'flag shoots' emerges.
Germination of conidia occurs at temperatures between 7 and 31°C and is inhibited above 33°C. Germination is greatest at 30-100% RH.
Maximum sporulation and growth occurs at 24-26°C and 90% RH. The production of conidia from these primary infections initiates the secondary spread of the disease and repetition of this cycle continues throughout the growing season. The release of ascospores is triggered by rainfall of at least 2.5 mm and infection can occur at temperatures > 10°C, irrespective of leaf wetness. The resulting colonies produce conidia for the secondary spread of the disease.