Black Knot

Black Knot is not a mushroom in the familiar cap-and-stem sense but a stem-inhabiting sac fungus (Ascomycota) in the family Venturiaceae, closely allied to the fungi that cause scab diseases. Its fruiting structure is a stroma: a dense mass of fungal tissue that grows inside and around the bark of a living twig, eventually bursting through and swelling into a knobby black mass.

The fungus is confined almost entirely to the genus Prunus — wild and planted cherries, plums, blackthorn-type shrubs and chokecherry. Because the stroma is perennial and grows a little further along the branch every year, old knots can reach 20-30 cm in length and completely surround the twig, which is why an infected tree often carries a distinctive line of black lumps along one limb.

What makes Black Knot instantly recognisable is the colour and texture combination: nothing else on a cherry branch is this dense, hard, charcoal-black and irregularly ridged. Older knots often become chalky or crumbly and may be overgrown by secondary moulds.

Fruiting body

The visible structure is an elongated, spindle-shaped to irregular swelling wrapped around a twig or small branch, typically 2-30 cm long and 1-4 cm thick. First-year growth is soft, olive-green and velvety; by the second season it hardens into a brittle, coal-black, deeply cracked crust with a rough warty surface.

Internal structure

Cutting a mature knot lengthwise shows black fungal tissue fused with distorted, discoloured wood. Embedded near the surface are countless minute flask-shaped chambers (perithecia) visible with a hand lens as pinprick bumps, each releasing two-celled ascospores in wet spring weather.

Host response

The branch beyond a girdling knot is typically stunted, sparsely leafed, or dead, so infected trees show an uneven, gappy crown. Bark on the near side of the knot is often cracked and raised where the stroma pushed out.

Spore print

Not applicable — spores are discharged microscopically from perithecia and cannot be collected as a print.

Black Knot occurs on the woody stems of Prunus species: wild black cherry, chokecherry, pin cherry, domestic and wild plums, blackthorn and ornamental flowering cherries. It is a stem parasite rather than a soil or litter fungus, so it is found wherever these trees grow — hedgerows, woodland edges, roadside thickets, gardens and orchards.

It is most abundant in eastern and central North America, where it is a familiar sight on wild cherries, and it also occurs in parts of Europe and Asia on native plums and blackthorn. Spores are released during wet weather in spring as new shoots elongate, and infections established then remain invisible until the following season.

The fungus is a biotrophic-to-necrotrophic parasite of living woody tissue. Ascospores land on green current-year shoots, germinate in a film of water, and grow into the bark, where the mycelium stimulates the host cambium to produce abnormal, tumour-like tissue that the fungus colonises. The resulting knot is a mixture of host and fungal cells.

The life cycle spans at least two years: the first season produces the soft green swelling and asexual spores, the second produces the black perithecial stroma that ejects ascospores. Because the stroma survives on the branch indefinitely, a single old knot can seed new infections throughout a thicket for many years, which is why wild cherry stands often show heavy, repeated colonisation.

Insect galls on cherry and plum twigs can be similar in outline but are smooth, woody and brown or grey rather than crusty black, and they usually contain a hollow larval chamber when cut. Bacterial and fungal cankers on the same hosts produce sunken, oozing, flattened lesions rather than a swollen mass that projects well beyond the bark line.

Diaporthe and Nectria-type cankers on other hardwoods form roughened bark and coloured pinhead fruiting bodies but never the thick, uniformly charcoal-black corky stroma. Sooty mould films on twigs rub off as a powder, whereas Black Knot is hard, fused to the wood and cannot be wiped away.