Chestnut Blight Identification Guide

The First Impression

This fungus is not found by looking at the ground. You spot it by noticing something wrong with a tree. Walking a mixed hardwood slope where chestnut sprouts persist in the understorey, your eye catches a branch or a slender stem whose leaves have died and turned rusty brown but have stayed attached, hanging in the canopy while everything around them is green. That flagging leads you to the stem, and somewhere below the dead portion there is a patch of bark that is the wrong colour and the wrong shape — sunken and cracked, or swollen and split, and washed orange to cinnamon over an area from a few centimetres to a metre long. Below it, a cluster of vigorous new shoots is often pushing out from the living bark. That sequence — dead leaves held on a branch, a discoloured canker below them, sprouts below that — is the recognition pattern.

Reading the Canker

Get close to the affected bark and read it in order.

Outline and relief. Cankers are elongated along the stem and usually roughly diamond-shaped. On thin, smooth young bark they tend to be sunken, with the dead tissue collapsing and cracking longitudinally. On older, thicker bark, or where the tree is responding strongly, they can be swollen and knobbly instead, with the bark heaving and splitting into plates.

Colour. Living bark on a young chestnut stem is smooth and olive to grey-brown. Cankered bark shifts to orange-brown, cinnamon or rusty tan, the colour deepening where the fungal stromata are densest. On mature bark the colour change can be hard to read and the cracking is the better clue.

Girdling. Follow the canker around the stem. When it encircles the stem completely, everything above it dies, which is the origin of the flagged dead leaves. Look for the sharp boundary on the stem between dead tissue above and living tissue below.

The Character That Settles It: Mycelial Fans

Slice under the edge of the cankered bark with a knife and peel a flap away. Beneath it, on the inner bark and cambium, Cryphonectria parasitica produces flat, buff to tan or orange-yellow mycelial fans — thin, radiating, papery sheets of fungal tissue spreading out from the canker centre in a distinct feathery pattern. They are unmistakable once seen, and no other common canker fungus on chestnut produces fans of that size and colour. If the bark lifts cleanly and shows fans, you have your answer.

The Fruiting Structures

The reproductive structures are small but readable with a hand lens.

Stromata. Erupting through bark lenticels and cracks are numerous small cushions, roughly 1 to 3 mm across, coloured orange, cinnamon, yellow-brown or buff. They stud the surface of the canker in dense colonies and give it its overall orange cast. Each is a compact mass of fungal tissue that contains the spore-producing chambers.

Conidial tendrils. In wet, humid weather each stroma extrudes conidia in a sticky matrix, which emerges as a bright orange to amber, curling, thread-like tendril, sometimes several millimetres long, coiled like a wood shaving. A canker covered in these after rain is one of the more striking sights in forest pathology. In dry weather they collapse into hard orange crusts.

Perithecia. Embedded in the same stromata, particularly on older cankers, are the sexual fruiting bodies: black, flask-shaped, with long necks whose openings show as fine dark dots on the surface of the orange cushion. Cut a stroma with a razor and the black flasks are visible as dark specks against orange tissue in cross-section.

Microscopic Confirmation

There is no spore print to take — this species has no cap, gills or free-falling deposit, so the standard print technique simply does not apply here, and that in itself distinguishes it from every gilled or poroid fungus on a trunk. What you do instead is one of two things.

Place a piece of cankered bark in a closed container with a damp paper towel and leave it at room temperature for a day or two. Orange conidial tendrils will extrude from the stromata, and a smear of that material under a coverslip shows conidia: tiny, one-celled, cylindrical, hyaline rods only a few micrometres long, in enormous numbers.

For the sexual state, crush a stroma bearing black necks in a drop of water, and look for asci each containing eight ascospores. The ascospores are hyaline, ellipsoid, and two-celled with a central septum, roughly 7 to 11 by 3.5 to 5 micrometres. Two-celled hyaline ascospores from an orange stroma on chestnut bark is the microscopic signature.

Substrate, Host Range and Season

The primary host is chestnut — American chestnut most famously, along with chinkapin and, less severely damaged, European and Asian chestnut species. Post oaks, live oaks and a few other oaks can carry cankers, and the fungus can survive as a bark inhabitant on several other hardwoods without killing them. In much of eastern North America the tree survives only as root-collar sprouts from old stumps, so most cankers are found on thin stems one to ten centimetres thick in the understorey rather than on large trunks.

Cankers are visible year-round. Spore production is driven by moisture: conidial tendrils appear within hours of rain through the growing season, and perithecia mature on older cankers, releasing ascospores into the air in wet weather.

Separating It From Look-Alikes on Morphology Alone

Neonectria canker. Produces clusters of tiny, superficial, bright red to red-orange spherical perithecia sitting directly on the bark surface rather than embedded in a stroma, and typically forms concentric target-shaped cankers on beech, maple and birch rather than elongated diamond cankers on chestnut. The spore bodies are round and free-standing; Cryphonectria's are buried in a cushion.

Cytospora and Valsa cankers. These also extrude orange to amber spore tendrils in wet weather, which causes real confusion. The difference is in the stroma: Cytospora stromata are grey to black conical structures with a pale disc, largely immersed in the bark, whereas Cryphonectria builds a conspicuous orange to cinnamon cushion that stands proud of the surface. Cytospora is also mainly on poplar, willow, spruce and stone fruit.

Hypoxylon and Biscogniauxia canker. On stressed oaks, the bark sloughs to reveal a hard, flat crust that is olive-buff at first and matures to a sooty black surface. Black and crust-like, never orange and cushioned.

Diaporthe and Diatrype species. Immersed black stromata with pimple-like necks, no orange colour, and no mycelial fans under the bark.

Simple mechanical wounds and sunscald. Sunken and cracked, but with no orange stromata, no tendrils and no fans beneath the bark.

A Field Sequence That Works

1. Spot dead leaves retained on an otherwise living stem.
2. Trace down to a sunken or swollen, orange-tinged, cracked canker.
3. Look for sprouts below the canker margin.
4. Peel the bark and confirm buff to orange mycelial fans.
5. Hand-lens the surface for orange stromata, with black necks in older ones.
6. Confirm the host is chestnut or a related hardwood.