Stromata: developing throughout late spring and summer on living leaves, producing conidia in summer and autumn, and ascospores from fallen overwintered leaves the following spring. The conidiomata sometimes persist in apparently viable condition until the ascomata are mature.
Anamorph: conidial stromata to 10(-35) mm diam., irregularly shaped but usually roughly circular, yellowish-brown in very young lesions but quickly becoming orange to reddish brown, becoming darker in the central region, locally hologenous (developing within and throughout the leaf tissue) but with the surrounding leaf hardly affected, containing a large number of conidiomata; composed of upper and lower layers of plant tissue 40-50 µm thick whose cells are filled with bright orange-brown material, an intermediate layer 300-500 µm thick of almost completely occluded angular to vertically elongated fungal cells. Conidiomata 150-250 µm diam., roughly spherical, the ostiole showing on upper surface of stromata, very inconspicuous. Conidiomatal wall very poorly developed, not clearly distinguishable from the stromatal tissue. Conidiogenous cells developing over the entire inner surface of the wall, often laterally from sequential cells of short conidiophores to 10 µm long and ~2 µm wide; derived from a thin layer of textura angularis with hyaline thin-walled cells 3-5 µm diam. Conidiogenous cells 13-24 x 2-3 µm, at first usually cylindrical but gradually tapering towards the upper region, which is slightly irregular in appearance due to successive conidial scars; usually proliferating sympodially. Conidia (22-)28-42 x 0.5-0.75 µm, the lower part (to approximately the mid point) very narrowly lanceolate to fusiform, the upper part filiform (~0.25 µm wide), sigmoidally curved to hooked, the base ± truncate, hyaline, aseptate, apparently smooth-walled.
Teleomorph: stromata 1-5 mm diam., irregularly shaped but usually roughly circular [where the leaf dimensions allow], strongly raising the upper surface of the leaf, flat or slightly concave on the upper surface, formed throughout infected leaf tissues, reddish brown to black, sometimes faintly verrucose (a feature of the leaf epidermal architecture), the ostioles sometimes inconspicuous but appearing as small black dots on paler stromata, often somewhat sunken; composed of pigmented outer layers 20-40 µm thick and a hyaline inner layer containing the ascomata. Ascomata ± spherical. Paraphyses rather sparse, to 7 µm diam., gradually tapering towards the apex, very thin-walled, strongly inflated between the septa. Asci 94-118 x 10-5-12 µm, narrowly clavate, very long-stalked (to ~60 µm), very thin-walled at all stages, the apex obtuse, with an apical ring 2-3 µm diam. and ~0-5 µm thick, 8-spored. Ascospores arranged biseriately, (10.5-) 11.5-14 x 3-5-4.5(-5.5) µm, cylindric-ellipsoidal, occasionally obovoid, occasionally slightly curved (bean-shaped), hyaline, aseptate, thin- and smooth-walled, without a gelatinous sheath.
Not formally assessed. At one time widely distributed throughout England and Wales, but now apparently confined to SW England, west Wales and SW Scotland. Further survey work is ongoing. The species seems to be quite common in Cornwall. It might therefore be assessed as Vulnerable due to decline in its range. This may be due to climatic changes, and/or changing pest management practices in plum orchards.
Polystigma fulvum is similar, but occurs on Prunus padus and has yellow rather than reddish stromata. There are no recent records of the species in GB&I, and it may be extinct in our region.
Apparently now restricted to Prunus spinosa in GB&I, with old records on P. domestica and hybrids such as the damson and bullace. The anamorph stroma forms in living leaves during the summer and early autumn, and the teleomorph is formed in overwintered leaves the following spring.
Recent reports are from the extreme west of the British Isles: VC1 W Cornwall, VC2 E Cornwall, VC44 Carmarthen and VC 46 Cardigan - and now also from Lismore (VC 98 Argyll). Formerly throughout England and Wales. It still seems to be present in western Ireland.
A biotrophic parasite, which appears not to cause a major deleterious effect on the health of its host plants.