Rose Black Spot

Rose Black Spot is caused by Diplocarpon rosae, an ascomycete in the Drepanopezizaceae. Its usual visible stage is asexual: dark, cushion-shaped masses of spore-producing tissue (acervuli) that erupt through the leaf cuticle within a spreading black lesion. The sexual apothecial stage is rare and seldom observed in gardens.

The fungus is a specialist of the genus Rosa and is one of the most familiar fungal signatures on cultivated plants anywhere in the temperate world. It has many physiological races, which is why one rose variety in a bed can be heavily spotted while a neighbour stays clean.

The key visual feature is the outline of the spot. Unlike most leaf-spot fungi, which produce lesions with clean, sharply defined edges, black spot lesions have rays of fungal growth extending outward under the cuticle, giving a fringed, star-like or feathered margin.

Leaf lesions

Rounded to irregular black or very dark brown spots, 2-15 mm across, appearing first on lower leaves. The margins are diffuse and radiate outward in fine fringes, and a broad yellow halo commonly spreads around each spot until the whole leaflet turns yellow and drops.

Fruiting structures

Within a mature lesion the surface is dotted with minute raised black pustules — acervuli that split the cuticle and release masses of two-celled conidia in a slimy matrix during wet weather. A hand lens shows these as glistening dark specks.

Canes

On young green stems the fungus produces purple-black, slightly raised blotches that can persist over winter as a source of new spores in spring.

Spore print

Not applicable. Conidia are produced in slime and dispersed by rain splash, not shed as a print.

The fungus is restricted to roses — garden hybrids, shrub roses, climbers, and wild species such as dog rose. It colonises the upper surfaces of leaves, leaf stalks, sepals and young canes.

It is found wherever roses are grown, in gardens, parks, hedgerows and wild scrub across Europe, Asia, North America, Africa and Australasia. Spotting begins in late spring, build through warm humid summers and peak in late summer and autumn. The fungus overwinters as mycelium in cane lesions, in dormant buds and in fallen leaves beneath the plant.

Diplocarpon rosae is a hemibiotrophic parasite of living rose tissue. Spores need a film of water on the leaf for several hours to germinate, then the germ tube penetrates the cuticle directly and the mycelium spreads in a radiating pattern between the cuticle and the epidermal cells — the reason for the characteristic fringed lesion edge.

Repeated cycles of conidial production and rain-splash dispersal occur throughout the growing season, so lesions climb the plant from the lowest leaves upward. Leaf loss and cane infection carry the fungus into winter, and mycelium surviving in canes and litter starts the following season's epidemic when new leaves emerge.

Rose downy mildew produces angular purple-brown blotches bounded by leaf veins, with a faint greyish fuzz on the underside, rather than rounded black spots with fringed edges. Cercospora leaf spot of rose forms smaller, sharply defined spots with pale tan centres and distinct purple borders — the clean margin is the giveaway.

Rose rust makes bright orange pustules on the leaf underside before any darkening occurs, and anthracnose produces small spots with pale grey centres that often fall out to leave shot-holes. Spray or sunscald damage causes irregular blackening without discrete rounded lesions or the yellow haloes typical of black spot.