On our friend Antonio's farm there's a chestnut tree that catches your eye. Half the crown is green and the other half is dead. It's not lack of water, and it's not age. Antonio tells the story while pointing at the dead branch:
“A storm wrecked it. And the disease gets in right there, where the cut is.”
That disease is chestnut blight, a fungus that lives in many chestnut groves and only needs a wound to get into the tree. That's why, in Antonio's valley, pruning a chestnut is a decision you think about twice. In this article we explain what blight is, why pruning can heal a chestnut or make it sick, and how wounds are protected. And while we're at it, we'll talk about ink disease, the other big chestnut disease, which attacks through the roots.
What chestnut blight is
Blight is caused by the fungus Cryphonectria parasitica. In the early 20th century it devastated the chestnut trees of the United States, killing billions of them, and it has been present in Europe since the middle of the century. In Spain it affects almost every chestnut-growing area.
The most important thing about this fungus is that it can't get through healthy bark. It needs a way in: a pruning cut, a branch torn off by the wind, a crack, a hail wound or a knock from a machine. Once inside, it spreads under the bark, kills it and forms a kind of ring around the branch. When that ring closes, sap can no longer flow and everything above it dries out.
How to spot it
- Dead branches on a tree that is otherwise green, often with the dry leaves still hanging on.
- Cracked, sunken or reddish-orange bark in the affected area.
- Small orange dots on the bark, which are the fungus's fruiting bodies.
- New shoots growing below the damaged area, as if the tree were trying to bypass the wound.
That's what Antonio means when he says many chestnuts “have a dead part”. They don't die all at once: “it keeps drying, it gets sick”.
To prune or not to prune: the dilemma
Pruning is necessary. An unpruned chestnut builds up dead wood, its crown closes in, it bears less fruit and it becomes more fragile in the wind. But every cut is a wound, and every wound is a chance for blight. Antonio puts it like this:
“It can do it good, or the disease can get in. It's very delicate. One wound, one cut, and that's where the fungus gets in.”
Even in his own family there are two ways of seeing it. Some prune every year and some would rather not touch the trees. Antonio has seen chestnuts that ended up sick after a badly done pruning. That's why he insists pruning should be done by someone who knows what they're doing, and at the right time.
How to prune a chestnut wisely
- In dry weather. The fungus's spores travel with rain and moisture. Pruning on dry days, with no rain forecast, lowers the risk.
- With disinfected tools. Clean shears and saws, disinfected when moving from one tree to the next and, above all, after cutting diseased wood.
- With clean cuts. A clean cut, right by the branch collar, heals faster than a tear or a long stub.
- Little by little. When rejuvenating old trees, the guides recommend working in stages, removing about a third of the crown each year, instead of doing one drastic pruning.
- Protecting large wounds. On thicker cuts, a wound-sealing paste with fungicide is applied.
Wound-sealing paste
Antonio is clear about it: that storm-hit chestnut “will need someone to come and put some paste on it”, a sealant for pruning cuts. Wound-sealing paste is a product you spread over the cut to seal it. Technical guides recommend using it with fungicide on thick cuts, because it speeds up the formation of the healing callus and shortens the time the wood stays exposed.
It's worth knowing, though, that the paste won't cure blight that's already inside. Once the disease has taken hold, what you do is cut below the affected area, making sure to remove all the infected wood, and burn the remains so they don't keep releasing spores.
“You have to cut that off so it grows back.”
A treatment using the fungus itself
There's a curious tool against blight: using the fungus itself in a weakened form. There are hypovirulent strains of Cryphonectria parasitica, infected by a virus that takes away their ability to do harm. When they're inoculated into an active canker, they pass that weakness on to the aggressive fungus and the disease stops spreading. In Galicia, for example, this treatment can be requested free of charge from the Forest Health service of the Consellería de Medio Rural. If you have chestnuts with blight, it's worth asking in your region.
Ink disease, the enemy of the roots
The other big chestnut disease doesn't get in through wounds, but through the soil. Ink disease is caused by fungi of the genus Phytophthora (P. cinnamomi and P. cambivora) that attack the roots. The tree loses vigour, its leaves become smaller and yellow, and it eventually dries out. It's called ink disease because a dark ooze can appear at the base of the trunk.
Ink disease thrives in waterlogged ground, which is why chestnuts like loose, well-drained soil. It's prevented by improving drainage, avoiding tilling the soil under the trees and disinfecting tools. In low-lying areas, below about 600 metres, where ink disease is more common, the guides recommend planting resistant hybrid chestnuts instead of European chestnut.
A year-round job
Looking after a chestnut grove isn't just about harvesting in autumn. In August and September it's time to clear the land so the undergrowth doesn't take over. In winter, prune when it's time, and carefully. And all year round, keep an eye on the trees to catch in time a branch that's drying out or the deformed shoots caused by the chestnut gall wasp, which weaken the tree and leave wounds where blight can get in. As Antonio sums it up, with chestnuts “there's always work”.
Frequently asked questions
Does blight affect the chestnuts?
Not the chestnuts that do ripen, which are perfectly normal. What it does is kill branches and reduce the tree's yield.
When are chestnut trees pruned?
Usually in winter, while the tree is dormant, and on dry days. Either way, every area has its own customs and it's a good idea to follow the recommendations of your region's Forest Health service.
Can a chestnut with blight be saved?
Often, yes. By cutting off the affected part in time, protecting the wounds and, where available, applying the hypovirulent strain treatment, the tree can sprout again.
What's the difference between blight and ink disease?
Blight gets in through wounds in the trunk and branches and kills the part of the tree above them. Ink disease attacks the roots from the soil and weakens the whole tree.
From the chestnut grove to your table
A healthy chestnut tree is the result of years of small acts of care: a well-made cut, a wound protected in time, clean ground. We're working with Antonio on a way to bring his freshly picked chestnuts to your home. In the meantime, in Reserve your harvest you'll find the autumn fruit we pick the same day it ships. From the field to the box, and from the box to your home. That simple, that authentic.
Sources
- Hifas Foresta. Chestnut blight, prevention in formative pruning
- Hifas Foresta. Treatment and control of chestnut blight
- Campo Galego. Health of the chestnut tree and the chestnut
- DiCYT. Blight and ink disease, the two main chestnut diseases in El Bierzo
- Castanea. Technical manuals
- SEO/BirdLife Montes Vivos. Pests and diseases
- Castaños Urquijo. The chestnut tree (ink-resistant hybrids)
