Horse Chestnut Leaf Miner
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Watch it before you act
At this level the damage rarely costs the crop. Note the date, check twice a week, and act only if it reaches new growth or the weather turns favourable for it.
Carried by insects
An insect moves it plant to plant, so controlling the carrier matters as much as treating the symptom. Check for the vector before assuming the plants infected each other directly.
Safe to compost
No pathogen carries over on this material, so it can go on an ordinary heap with the rest of the garden waste.
When and where to look
Scout the planting while temperatures hold between 10°C and 30°C.
The signs that lead here
About
The horse chestnut leaf miner (Cameraria ohridella) turns whole avenues of chestnuts brown by July across central Europe. It looks catastrophic and worries people every summer, but it does not kill established trees — the leaves have already done most of their year’s work by the time the damage shows.
Symptoms
Elongated pale blotches between the leaf veins from June, each containing a tiny caterpillar. Blotches merge until leaves are entirely brown and brittle, and the tree may drop its leaves in August as though autumn had come early.
Organic Treatment
Rake up and compost or destroy the fallen leaves in autumn — the pupae overwinter in the leaf litter, and clearing it under a garden tree measurably reduces next year’s first generation. Hot composting kills the pupae; a cold heap does not.
Chemical Treatment
Spraying a mature tree is neither practical nor proportionate. Trunk injection exists for high-value specimens but is a professional operation and rarely warranted for a pest that does not kill.
Prevention
Keep the tree healthy with mulch and water in drought so it tolerates the annual defoliation. Red-flowered horse chestnut (Aesculus × carnea) is markedly less affected if you are choosing a new tree.
Related problems
Horse Chestnut Leaf Miner risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.