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Hypophyllous on Celtis laevigata. Similar to Erysiphe macrospora, however the ascospores are around 8-10 μm shorter (about 1/3 shorter). Documentation for E. parvula is lacking, so I assume that the name parvula might refer to the small spores. While I can't find a description of the ascospores, the chasmothecial measurements and ascus count are a good fit, along with the host species.

Measurements
Chasmothecia
90–112 μm (briefly mentioned as 90–100 μm in older literature)
Asci
50–55 × 33–38 μm; ~5–6 per chasmothecium
Ascospores
18–19.5(–21.5) × 10.5–12(–13.3) μm; Q=1.5–1.8; ~6 per ascus

Images
Slides dyed with LPCB, scale bars 10 μm
1,2: Chasmothecia with numerous curled Erysiphe-type appendages
3: Erumpent asci
4,5: Habit

Hypophyllous on Modiola caroliniana. Not easily confusable due to the host; microscopy matches literature anyway. Both teliospores and basidiospores highly variable in size and shape.

Measurements
Teliospores
37–80 × 10–21 μm
Teliospores in literature
45–87.5 × 8.0–17.0 µm (Albu et al. 2019)
31–81(–95) × 10.5–20 (–25) µm (Aime & Abbasi 2018)
Basidiospores
9–12.5(–17.8) × 6–9(–10.5) μm

Images
All slides in LPCB, scale bar 100 μm for image 1, 50 μm for image 3, 10 μm otherwise.
1: Cross section of telium, teliospores visible on lower surface
2: Epiphyllous habit
3: Various teliospores
4: Sterigmata, two with attached basidiospores
5: Spent sterigmata
6: Promycelia/metabasidia in various stages of growth from teliospores
7: Basidiospores
8: Notched teliospore apex
9: Hypophyllous spermogonium
10: Early or broken teliospore germ tube
11: Additional habit

Beautiful, dichotomously branched chasmothecial appendages. Took 20+ hours to stitch together the focus stacks manually.
Mycelium and fruiting bodies are strictly hypophyllous on Quercus pagoda leaves, as shown in habitus pictures 5 and 6.
Literature

The most often described Erysiphe sp. from oak leaves is E. alphitoides; however, the chasmothecial appendage lengths range from 0.75 to 2 times the diameter of the chasmothecium. The appendages in this observation consistently only measure 0.5 to 0.75 times the chasmothecial diameter.
E. quercicola is similar to E. alphitoides, but with shorter appendages that match the range of this observation. E. quercicola would be a good candidate apart from the ascospore dimensions, which are smaller than this observation's ascospore dimensions by ~30%. Additionally, both E. alphitoides and quercicola are only dubiously present in North America, and may be limited to Europe and/or South America.
Identification
Erysiphe abbreviata is a North American species that has been collected from Quercus pagoda. It has notably larger ascospores than the other Erysiphe spp. mentioned, and shares the short appendages of E. quercicola; Braun et al. 2007.
Image annotations
1) Chasmothecium with appendages – lactophenol cotton blue (LPCB)
2) Chasmothecial appendage, dichotomously branched ~5x – LPCB
3) Erumpent ascus with ascospores – Melzer's reagent (MLZ)
4) Ascospores – MLZ
5) Habitus with chasmothecia visible on underside of Q. pagoda leaf
6) Close view of habitus
7) Conidium with initial appressorium growth – LPCB
8) Lobed appressorium – LPCB
9) Chasmothecium detail with early, unbranched appendages – LPCB
10) Uncropped view of all appendages – LPCB
11) Chasmothecia among stellate trichomes – MLZ
12) Ruptured chasmothecia detail – Water

All scale bars are 10 μm.
Average measurements vs. literature range
Ascospores:
24.14 × 14.13 μm || 20–32 × 12–21 μm
Asci:
55.62 × 38.29 μm || 50–70 × 35–55 μm
Chasmothecial diameter:
82–124; x̄:108 μm || 70–125(–130) μm
Conidia:
32.21 × 14.41 μm || 25–45 × 10–20 μm
Hyphal width:
4.42 μm || 2–6 μm

Literature measurements from:
Braun, Uwe & Shi, Ainong & Mmbaga, M. & Takamatsu, Susumu & Divarangkoon, Rangsi & Chen, P.. (2007). Erysiphe abbreviata on cherry bark oak - Morphology, phylogeny and taxonomy. Mycologia. 99. 655-63. 10.3852/mycologia.99.5.655.