Anthracnose disease — identification and treatment guide

Anthracnose disease — identification and treatment guide

Anthracnose is not one disease but a group of closely related fungal infections that share a distinctive look: small, sunken, dark lesions that expand and merge, often with concentric rings and a pink or orange spore mass at the centre during humid weather. The Greek root anthrax means "coal" — a fair description of how the lesions look on a ripening tomato or pepper. The disease attacks an unusually wide host range, from common vegetables (beans, tomatoes, peppers, cucumbers, squash) to fruit (strawberries, mangoes, papayas, apples) to ornamental shade trees (dogwood, sycamore, ash, plane, oak). Because the same management principles work across hosts, gardeners benefit from knowing the disease once rather than learning a different protocol for every plant.

Quick triage: If you have dark sunken spots on ripening fruit, or brown patches along leaf veins after a wet spell, jump straight to the identification checklist below. The pest identifier also includes anthracnose alongside the other common houseplant and garden problems.

What causes anthracnose

The bulk of garden anthracnose is caused by fungi in the genus Colletotrichum, with smaller contributions from Apiognomonia (shade trees) and Glomerella (the sexual stage of some Colletotrichum species). Each crop has its dominant species — C. lindemuthianum on beans, C. coccodes on tomato, C. gloeosporioides on mango — but the cultural controls that work against one work against the others.

Three conditions trigger an outbreak:

1. Prolonged leaf wetness. Twelve or more hours of damp foliage is the single biggest risk factor. Spores need free water to germinate and infect.
2. Mild temperatures of 18-25 °C — the optimum range for most Colletotrichum species. Hot dry summers in Mediterranean climates suppress the disease; cool wet British or Pacific-Northwest summers favour it.
3. Splash-dispersed spores. Rain and overhead irrigation move spores from infected tissue (or fallen debris) onto healthy leaves and fruit. This is why a single sick plant can seed an entire row.

Anthracnose overwinters in three reservoirs that catch a lot of home gardeners by surprise: infected seed (especially in beans, where the disease can live on the seed coat for years), plant debris (fallen leaves, mummified fruit, infected stems left in the bed), and woody bark of perennial hosts. Removing visible disease in summer does little if the spores survive winter on dead material in the soil.

Identification checklist

Anthracnose looks different on each plant part, but the sunken, dark, water-soaked quality is universal.

• On leaves. Small dark spots (1-5 mm) that expand into irregular brown or black blotches, often spreading along veins or starting at leaf margins. Centres may turn papery and drop out, leaving "shot holes." Heavily infected leaves yellow, curl, and fall early.
• On stems. Sunken, dark, oval-to-elongated lesions. If a lesion girdles the stem, the tissue above wilts and dies — a common failure point on bean plants.
• On ripening fruit. This is where anthracnose is most damaging financially. On tomato, pepper, apple, mango, papaya, and strawberry, you see sunken circular spots with concentric rings. In humid weather a pink, orange, or salmon-coloured spore mass appears in the centre of each spot — these are the fruiting bodies (acervuli) releasing the next generation of spores.
• On beans. Brown sunken lesions appear on pods, often with a darker rim. Seeds inside infected pods carry the disease into next year's planting.
• On woody ornamentals (ash, dogwood, sycamore, plane, oak). Irregular brown blotches along leaf veins, sometimes with twig or shoot dieback in spring after a wet flowering period. Heavy spring infections cause early leaf drop, but mature trees usually recover by midsummer.

A useful confirmation: in moist weather, brush a clean fingertip across the centre of a suspected lesion. If it picks up a pink or orange smear, that's the spore mass — diagnostic for anthracnose.

How anthracnose differs from look-alikes

Treatment — what actually works

Anthracnose responds best to cultural control (changing conditions so the fungus can't thrive). Chemicals help only as a supplement during wet stretches.

1. Remove infected tissue immediately

Take off and bag (do not compost) every visibly infected leaf, stem, and fruit. Spores are still releasing from these surfaces even after the tissue is dead. Sterilise pruners between cuts in a 10% bleach solution or 70% isopropyl alcohol — Colletotrichum spores transfer easily on tool blades.

2. Change how you water

Switch to drip irrigation or soaker hoses at ground level. If you must water by hand, water early in the morning so any splashed foliage dries within a few hours. The single biggest source of new infections after the initial outbreak is splashing wet — fix this first.

3. Space and prune for airflow

Crowded plantings stay wet longer. For tomatoes, prune lower foliage to expose the ground to airflow. For beans, plant rows in the direction of prevailing wind. For roses and ornamentals, open the centre to light and air. The goal is foliage that dries within 4-6 hours of dew or rain.

4. Mulch the bed

A 5-7 cm layer of straw, leaf mould, or wood chip mulch under affected crops prevents soil spores from splashing onto lower leaves and fruit. This single change often makes a bigger difference than any spray.

5. Apply a copper fungicide preventatively

If wet weather is forecast and the crop is high-value, copper hydroxide, copper oxychloride, or copper sulphate sprays applied before infection are moderately effective. Reapply after heavy rain. Copper is approved for organic gardens but accumulates in soil over years — use sparingly and rotate sites. Always follow product label directions for dosage, PPE, and pre-harvest interval. UK gardeners: copper oxychloride is no longer authorised for amateur use (HSE register, 2024) — copper sulphate-based products remain available.

6. Pull and replace badly infected plants

There is no point spraying a bean plant where 60% of the pods are already infected. Pull it, bag it, and concentrate effort on the rest of the row.

Preventing anthracnose next season

1. Resistant varieties. Especially for beans and tomatoes, choose cultivars listed as anthracnose-resistant. The reliable beans include 'Resistance' (pole), 'Provider' (bush), and many older landrace varieties. Tomato resistance is harder to find but improving.
2. Certified disease-free seed. Colletotrichum lindemuthianum lives on the bean seed coat for 2-3 years. Don't save seed from a plot that had anthracnose. Buy from a supplier that tests for it.
3. Three-to-four-year crop rotation. Don't plant beans where beans grew last year. Same for tomatoes — though tomato anthracnose can also live on weed hosts (nightshade family), so weeding matters.
4. End-of-season cleanup. Pull and bag every bit of plant material — including mummified fruit on the soil, which is a major overwintering site. Bare soil over winter, with a brief green-manure cover crop in late winter, breaks the disease cycle.
5. Solarise stubborn plots. In a sunny climate, cover the affected bed with clear plastic for 4-6 weeks at the height of summer. Soil temperatures above 50 °C kill most Colletotrichum spores in the top 15 cm.

Anthracnose on woody ornamentals

For dogwood, sycamore, plane, and ash anthracnose, the calculus is different. These are large mature trees — you can't spray them, and pruning is impractical for the average gardener. The good news: healthy mature trees almost always recover. A heavy spring infection looks alarming (premature leaf drop, twig dieback) but the tree refoliates by midsummer.

Long-term, the only meaningful interventions are:

• Rake and bag fallen leaves under affected trees each autumn.
• Improve soil drainage if the tree is on heavy wet ground.
• Avoid overhead irrigation anywhere near the canopy.
• For young or stressed trees, an arborist-applied trunk-injected fungicide in early spring can suppress severe infections, but the cost rarely justifies it on established trees.

If a sycamore or plane shows dieback that worsens year after year, suspect canker stain disease (Ceratocystis platani) rather than anthracnose — that one is much more serious and warrants a professional diagnosis.

Related articles

• Plant pest identifier — interactive symptom-to-pest matcher (anthracnose included)
• Powdery mildew — full treatment guide — the other major garden fungal disease
• Houseplant diseases — full reference — the broader pest and disease library
• Garden pest identification — outdoor pest companion guide
• How to grow tomatoes — prevention starts with healthy culture
• How to prune basil — pruning improves airflow for nearby crops
• Anthracnose glossary entry — the short-form botanical definition
• Open Growli for photo-based diagnosis — wider species coverage than the web tool

Reviewed and updated by the Growli editorial team. Sources: APS Plant Disease Lessons (Colletotrichum spp.), UK HSE Pesticide Register (2024), UC IPM Pest Management Guidelines, Cornell Plant Disease Diagnostic Clinic. For questions about anything here, open Growli and ask — or email hello@getgrowli.app.