Anthracnose on watermelon — symptoms and treatment

Severity: medium

What is Anthracnose

Anthracnose of watermelon is caused by the fungus Colletotrichum orbiculare. It causes leaf spots and fruit rot — a double attack that is what makes it costly. The foliage damage alone would be manageable, but the same fungus rots the melons, including some that looked sound when picked. A crop can be lost after harvest rather than in the field.

Symptoms

On the leaves, the disease appears as dark brown to black circular spots — distinctly rounded rather than blotchy, and as they multiply the leaf dies back around them. The vines follow: infected runners suffer dieback, and a plant can lose whole sections of vine while the rest looks healthy. The fruit carries the diagnostic sign. Lesions on the melon are sunken, as though a thumb had been pushed into the rind, and in humid weather they carry pink spore masses in the centre of the sunken spot. That pink is the fungus fruiting, and no other common watermelon problem produces it. Sunken lesions open the rind, and rot follows into the flesh.

• Early: dark brown to black circular spots on the leaves
• Progressing: vine dieback; spots multiply and leaves die back
• Advanced: sunken lesions on the fruit, with pink spore masses in humid weather

Causes and conditions

The fungus survives on infected crop debris and on the seed, which is why it can arrive in a clean field on a bad seed lot and why last year's vines left in place are a standing source. It spreads by rain splash and by anything that moves water and soil — irrigation, hands, tools — carrying spores into the healthy canopy. It runs in warm, wet weather: spores need free water to germinate, so rain, heavy dew and overhead irrigation all drive infection, and a dense sprawling canopy keeps leaves wet long enough for the fungus to get in.

Treatment

Crop rotation and sanitation — cultural

Rotate with non-cucurbit crops for 2 years. Remove crop debris. Use disease-free seed. These three close the routes the fungus uses to reach next season: rotation starves out debris-borne inoculum by denying it a host, debris removal takes away the material it overwinters in, and clean seed stops you planting the disease yourself. Seed is the one most often overlooked — no spray programme rescues a crop that was infected on day one.

Chlorothalonil — chemical

Apply contact fungicide preventively during warm, wet weather. Repeat every 7-10 days. This is a contact material: it protects the tissue it covers and nothing else, so it must be on the plant before spores arrive, and new growth is unprotected until the next spray. That is why the repeat interval matters. Cover the developing fruit, not only the leaves. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Use disease-free seed — the cheapest defence there is
• Rotate away from cucurbits for 2 years so debris-borne inoculum starves out
• Clear and destroy crop debris at the end of the season
• Water at the base rather than overhead, so the canopy and fruit stay dry

Frequently asked questions

Can I eat a melon with a sunken spot on it? Cut well away from the lesion and check the flesh — rot moves in through the sunken rind. Badly affected melons are not worth keeping, and they will not store.

Is it contagious to other plants? It is a cucurbit disease, so cucumbers, melons and squash nearby are at risk. That is also why the rotation must be away from cucurbits, not just away from watermelon.

When should I treat? Preventively, once warm wet weather sets in — not after the pink-centred lesions show. By then the fungus is already sporulating.