Conidial morph: not known.
Ascostromata perithecia, 50-90 µm diam., mostly aggregated in clusters of up to 20, erumpent through the cuticle and becoming ± superficial, amphigenous, initially in small pale areas on living leaves with the colonies becoming blackened as the leaf becomes moribund; globose to conical, glossy black. Peridium thin, composed of two to three layers of thick-walled polygonal brown cells to ca 8 µm diam., the upper part setose and especially in the ostiolar region. Setae 25-40 µm long and 4-5 µm diam. near the base which is sometimes slightly swollen, sometimes slightly curved, tapering smoothly to an acuminate apex, mid to dark brown, aseptate, thick-walled. Interascal tissue of thin-walled pseudoparaphyses, tending to deliquesce at maturity. Asci (35-) 44-52 x 10-11 µm, obclavate to saccate, ± sessile, thick-walled and fissitunicate, the apex obtuse to rounded with an ocular chamber, no apical structures present, not staining in iodine, 8-spored. Ascospores 11-13 x 4.5-5 µm, greenish to olivaceous brown, clavate or occasionally clavate-rhomboid, the apex rounded and the base obtuse, straight, septate in the lower third, slightly constricted at the septum, thin-walled, smooth, without a perispore or appendages.
Coleroa robertiani is similar but has ± hemispherical non-setose ascomata and is apparently restricted to Geranium robertianum. Venturia geranii has immersed perithecial ascostromata with slightly narrower spores; it could be an immature version of C. circinans.
On living and moribund leaves of Geranium spp., including G. molle, G. pratense, G. pyrenaicum and G. rotundifolium.
S. England (VC2 E Cornwall, VC8 Wiltshire, VC17 Surrey, VC28 E Norfolk, VC29 Cambridgeshire), also Scotland (VC80 Roxburghshire).