Chestnut blight

Cryphonectria parasitica

The disease that erased the American chestnut from four billion trees to almost none. Orange-buff cankers girdle the trunk; the roots live on and sprout, and are struck again.

Fungus No cure Airborne sporesRain splashSpread by insects Spreads to neighbors

What you'll see

A sunken or swollen orange-buff canker on the trunk or a limb, often cracked, with the bark loose over it.
Fans of buff-colored fungal tissue under the loosened bark, visible when a flap is lifted.
Pinhead orange pustules studding the canker, extruding orange spore tendrils in wet weather.
Leaves above the canker wilting, browning and hanging on the branch through winter rather than dropping.
Sprouts coming up from a living stump or root collar, growing for a few years, and then cankering in their turn.

Why it happens

A fungus introduced from Asia around 1904 on nursery stock. It enters through wounds in the bark, grows in the living tissue just beneath, and girdles the stem. Asian chestnuts co-evolved with it and shrug it off; the American chestnut had no resistance at all, and lost some four billion trees in fifty years. The root systems mostly survived, which is why the species persists as a permanent understory shrub.

Throughout the natural range of the American chestnut in the eastern United States and beyond. European chestnut is intermediate in susceptibility; Chinese and Japanese chestnuts are essentially resistant.

Treating it

There is no practical treatment for a garden tree. Remove dead wood for safety and let the roots sprout.
Do not move chestnut wood or nursery stock out of an affected area.
A mud pack over a small canker on a valued tree can slow it, and is worth trying on a single stem, but it is not a cure.
Hypovirulence — a virus of the fungus itself that weakens it — is used in research and has been patchily successful in Europe; it is not something to attempt in a garden.
Support the breeding programs rather than trying to save one tree; the answer to this disease is a resistant tree, and it is being made.

Keeping it away

Plant Chinese chestnut, or a Dunstan-type American-Chinese hybrid, if you want nuts and a tree that lives.
Avoid wounding the bark of any chestnut, since wounds are the only way in.
Do not import or move chestnut planting stock between regions.
Keep a young tree unstressed, well watered and open-grown; vigorous trees wall cankers off for longer.
Report American chestnuts that are surviving and flowering in a blighted area; they are exactly what the breeders are looking for.

Who gets it

American chestnut above all, then European chestnut, chinkapin and — very lightly — Chinese and Japanese chestnut. Post oaks are occasionally affected.

Chestnut