Black Knot Identification Guide

First impression and the setting

Unlike most fungi in a field guide, this one is easiest to find in winter. You are walking a hedgerow, a woodland edge, an old orchard or a scrubby fence line, the leaves are down, and the bare branches of a wild cherry, chokecherry or plum carry what look like lumps of charcoal welded around the twigs. They are elongated rather than round, running along the branch rather than sitting on it, and they are the deepest black anything in the hedge.

Before anything else, identify the host. Apiosporina morbosa occurs on Prunus and effectively nothing else: chokecherry, black cherry, wild plum, domestic plum and their relatives. Check the twigs for the characteristic features of the genus, which include horizontally lenticelled bark on younger stems, simple alternate leaves with toothed margins and a pair of small glands near the leaf base if any foliage is present, and the smooth, glossy, reddish bark of young stems. A black elongated swelling on a non-Prunus host is something else entirely.

Reading the gall itself

Walk up and look at an established knot. It is a hard, brittle, coal-black swelling that completely encircles the twig or runs along one side of a larger branch. Typical lengths are 2 to 15 cm, though old knots on main limbs can exceed 30 cm and fuse into long ridges. Diameter is usually two to five times that of the branch it sits on.

The surface is rough, irregularly ridged, deeply cracked and warty, like clinker or weathered slag. Tap it: it is hard and sounds dry. Break a piece off with a knife point and the fracture is brittle and matt black through most of its thickness, with the wood of the branch visible as a pale core at the centre. That structure, a black crust of fungal tissue wrapped around and fused into swollen host wood, is the essential architecture of the gall.

Note where the branch is dead. Twigs distal to a girdling knot are usually dead, leafless and often bearing dried leaves, and older knots frequently sit at the base of a dead branch tip. That combination of a black swelling and a dead terminal is very distinctive from a distance.

The first-year stage, which is easy to miss

Search the same tree for younger material, because the early stage looks quite different and is genuinely helpful.

In the first season after colonisation, the affected twig develops only a slight, soft, elongated swelling with the bark still intact but stretched and slightly split. Through late spring and summer this swelling becomes covered in a velvety olive-green to olive-brown layer, which is a felt of asexual spores. Rub it with a fingertip and it smudges. By late summer or autumn the olive coat darkens to brown then to black, and by the second spring the knot has hardened into the familiar brittle black form. Finding both stages on one tree confirms the identification and shows you the progression.

Reading the fertile surface

This is an ascomycete, so there are no gills, pores or teeth to examine, and the usual spore-print routine does not apply. What you examine instead is the black stroma, the compact mass of fungal tissue forming the crust.

Take a mature, weathered knot in early spring, which is when the sexual stage matures. Wet the surface and study it with a hand lens. Embedded in the black stroma are numerous flask-shaped chambers, the perithecia, showing at the surface as low rounded bumps each with a tiny central pore. On a good specimen the surface reads as finely pimpled rather than plain.

To take this further, shave a thin sliver from the crust with a razor blade and mount it in water. You should see the flask chambers in section, packed with elongate sacs, each holding eight two-celled spores. The two cells are unequal, one noticeably larger than the other, with a constriction at the septum. That unequal-celled spore is the origin of the genus name, and it is the confirming microscopic character. Ascospores are discharged into the air during wet weather in spring, timed to the emergence of young shoots.

If you want a crude substitute for a spore print, seal a freshly collected spring knot in a damp chamber over a glass slide overnight and look for a fine scatter of pale two-celled spores on the glass.

Substrate, season and ecology

Knots are perennial structures. The fungus survives in the host tissue and the gall enlarges year on year, extending along the branch and into new wood, so a single infection can develop into a metre of intermittent black ridging over a decade. Look for material in any season, but time visits for late winter and early spring if you want mature perithecia, and for early summer if you want the olive conidial stage.

Ecologically, the fungus stimulates the host to produce abnormal quantities of wood and bark tissue, which is what creates the swelling. The knot is therefore composed of both host and fungal material. Girdling of the branch interrupts water transport and kills the shoot beyond, and heavily colonised trees develop a characteristic gaunt architecture of stubby dead tips. Old dead knots are commonly bored by beetles and colonised by secondary fungi, and are sometimes overgrown by a pinkish-white mould that changes the surface colour entirely, so aim to examine several knots of different ages rather than judging a tree from one.

Separating it from look-alikes on morphology alone

Insect galls are the commonest confusion. They are typically smooth, rounded or spindle-shaped, brown or tan rather than black, and cutting one open reveals a hollow chamber or a larval tunnel rather than solid black tissue over a pale woody core. Host also helps, since most conspicuous woody galls occur on oaks and willows.

Crown gall produces a rough tan to brown corky mass, but it forms at the root collar or on the lower stem rather than out along the twigs, and it is never coal-black.

Target cankers caused by Neonectria form sunken, concentrically ridged lesions where the bark and wood are sunken and killed rather than swollen outward, so the branch is thinner at the lesion, not thicker.

Eutypa and Cytospora cankers produce sunken, cracked, discoloured bark with resin or gum flow and again no true swelling.

Burr knots and adventitious root initials on fruit trees form clusters of stubby tan-brown protuberances, soft and corky rather than brittle and black.

Finally, sooty moulds and black algal films can darken bark, but they wipe off with a damp cloth and leave no swelling behind, whereas a black knot is a structural part of the branch.

Field checklist

Host in the genus Prunus, elongated swelling wrapped around a twig, hard brittle coal-black crusty surface over a pale woody core, dead shoot beyond, and, on the same tree, younger swellings coated in olive-green velvet. That is Apiosporina morbosa.