Epiphyllous on Quercus nigra, creating a dense mat of mycelium. Around 5 or 6 asci per chasmothecium and 6 spores per ascus. Due to the length of the chasmothecial appendages, this should be Erysiphe extensa var. curta (shorter appendages than E. extensa var. extensa, but still much longer than the otherwise similar E. abbreviata).

Measurements
Ascospores
22–28.5 × 10.7–13.5 μm
Asci
50–70 × 34–43 μm
Chasmothecial Diameter
(85–)94–119 μm
Chasmothecial Appendage to Diameter Ratio
1.9–2.6
Chasmothecial Appendage Count
6–9 (maybe more, many were broken off)

Description from Literature

Mycelium usually epiphyllous, persistent, thin films on the whole surface of leaves, cleistothecia (80–)90–170 μm in Diam., 5–20 appendages, 1–2.5 times as long as the cleistothecial diameter, 3–10 asci, 45–65 × 35–50 μm, 3–8, usually 4–6 spored, 18–28 × 11–18 μm

U. Braun, Mycotaxon 15: 130 (1982).

Images
Slides in LPCB, scale bars 100 μm for images 1,2,12, and 13; 10 μm otherwise.
1-6: Chasmothecia with erumpent asci and ascospores
7-10: Dichotomously branched chasmothecial appendages
11-13: Intact chasmothecia and appendages
14-17: Spent conidiophores
18,19: Habit