On infected leaves the fungus makes thin, white, irregular patches on both surfaces. The
mycelium (the surface growth) is amphigenous (present on both leaf faces) white, thin and irregular, and may fade or persist.
Hyphae are straight to slightly curved,
hyaline, thin-walled and smooth, about 4–6
μm wide. Small attachment structures (hyphal
appressoria) are solitary, nipple-shaped to slightly lobed and can be inconspicuous.
Conidiophores (spore-bearing stalks) arise from the upper part of the mother cell, are erect and straight, and reach about 125 μm in length. The basal "foot cells" are cylindrical and straight, typically (28–)31–53(–64) × (4–)5–7 μm. The
asexual spores (
conidia) are produced singly and are mostly cylindrical to oblong (occasionally
ellipsoid), measuring 33–48(–54) × (12–)14–16(–18) μm, with a length/width ratio (L/W) of 2–3.6. The sexual morph has not been observed.
Erysiphe alphitoides,
E. castaneae, and
E. castaneigena are somewhat similar in
morphology, but can be distinguished by the dimensions and form of their conidia and foot cells. It differs from
E. alphitoides (conidia 25–40(–45) × 13–25 μm; foot cells 15–40 μm long; conidia often ellipsoid–doliiform; L/W 1.4–2.3) and
E. castaneae (conidia 28–35 × 14–18 μm) in its larger, mainly cylindrical conidia and longer conidiophores/foot cells; it differs from
E. castaneigena (conidiophores to about 90 μm; conidial L/W 1.6–2.8) in having longer conidiophores and higher conidial L/W. ==Host and distribution==