Chestnut Blight

Chestnut Blight is caused by Cryphonectria parasitica, an ascomycete in the Cryphonectriaceae. It colonises the inner bark and cambium of chestnut trees, producing conspicuous cankers whose most diagnostic feature is a scatter of tiny orange to cinnamon-coloured fruiting bodies — stromata — erupting through the bark surface.

Native to eastern Asia, where it lives quietly on Chinese and Japanese chestnuts, the fungus was introduced to North America around the beginning of the twentieth century and transformed the forests of the Appalachian region, where American chestnut had been a dominant canopy tree. It later reached Europe, where sweet chestnut is the main host.

The fungus is also notable for hosting its own virus. Hypovirulent strains carrying a mycovirus grow more weakly, produce paler cankers and less orange fruiting, and are a well-studied example of a virus altering a fungus's behaviour in nature.

Cankers

Infected bark first shows a small sunken or slightly swollen orange-brown patch that expands into an elongated canker, often splitting lengthwise. The bark within the canker cracks and the stem above a girdling canker wilts, browns and dies back while sprouts frequently emerge from below.

Fruiting bodies

Scattered across the canker surface are stromata: pinhead-sized, orange to yellow-brown cushions pushing through the bark. In humid weather these exude curling orange tendrils of conidia. Older stromata contain embedded flasks producing two-celled ascospores.

Mycelial fans

Lifting bark at the canker margin reveals fan-shaped mats of buff to tan mycelium spreading through the inner bark — one of the most reliable field characters.

Spore print

Not applicable; spores are extruded from stromata in tendrils or discharged microscopically.

The fungus lives in the bark of chestnuts — American, European sweet, Chinese and Japanese chestnut — and can also colonise chinquapin and occasionally post oak, live oak and other hardwoods. It enters through wounds, branch stubs and insect injuries in the bark of trunks and branches.

It is native to eastern Asia and is now established throughout eastern North America and much of southern and central Europe. Cankers and stromata can be found in any month, but spore release peaks in warm, wet weather; conidia are spread by rain splash and by birds and insects walking over the sticky tendrils, while ascospores travel on the wind.

Cryphonectria parasitica is a necrotrophic bark parasite. Spores germinate in wounds, and the mycelium grows through the inner bark and cambium, killing the conductive tissue in an expanding ring. When the canker encircles the stem the portion above it can no longer be supplied and dies back, though the root system usually survives and sends up new sprouts that become infected in turn — the reason chestnut persists as an understorey shrub across much of its former range.

On its Asian hosts, which are more resistant, the fungus behaves more like a superficial bark coloniser and rarely girdles stems. Hypovirus-infected strains are transmitted between fungal individuals by hyphal fusion, and where these strains circulate freely, cankers heal over and callus rather than expanding.

Nectria and Neonectria cankers on hardwoods bear bright red to coral, spherical fruiting bodies in clusters rather than the flatter orange stromata of chestnut blight, and they usually form concentric target-shaped cankers. Diaporthe and Botryosphaeria cankers produce black, not orange, pinhead fruiting bodies.

On chestnut itself, ink disease caused by Phytophthora attacks the root collar and lower trunk, producing dark oozing stains and no orange stromata or buff mycelial fans. Sunscald or frost cracks split bark without any fruiting bodies at all; the fan-shaped mycelium under the bark margin is the decisive character for chestnut blight.