How Do I Treat Anthracnose on My Watermelon Plants?

Anthracnose (Colletotrichum orbiculare) is a fungal disease that can affect watermelon at every stage — seedling, vine, leaves and developing fruit. In wet, warm summer weather it spreads rapidly, potentially ruining both the foliage and the harvest within a few weeks. Learning to identify it early, treat it promptly and prevent recurrence through rotation and cultural changes is essential in areas where summer rainfall is significant.

How to identify anthracnose

On leaves: small, circular to irregular pale green or yellow spots that enlarge and turn dark brown or black with a water-soaked appearance. In humid conditions, a pinkish or salmon-coloured spore mass (acervuli) develops in the centre of the spot. On vines: dark, elongated lesions that may girdle and kill a section of stem. On fruit: circular, sunken, dark green to black spots that enlarge in concentric rings; the surface may have a salmon-coloured spore mass at the centre in wet weather. Fruit affected by anthracnose is inedible.

Conditions that favour anthracnose

Anthracnose spores germinate in films of water on plant surfaces. Extended periods of warm, wet, humid weather — above 24°C with regular rain or overhead irrigation — are the ideal conditions. Spores are spread by water splash from soil or infected debris, from tools, and on hands and clothing. The disease overseasons in infected plant debris in the soil, in seed from diseased plants and on perennial weed hosts in the cucurbit family.

Treatment

Apply copper-based fungicide or chlorothalonil (where permitted for use on edibles) at the first sign of symptoms. Repeat every seven to ten days while wet weather continues. Remove badly affected leaves and dispose of them — do not compost them. Avoid working with plants when they are wet to reduce spread via tools and hands.

Prevention through rotation and hygiene

Remove all cucurbit debris at the end of the season — leaves, stems and fruit — as the pathogen overwinters in this material. Rotate away from cucurbits (watermelon, cucumber, pumpkin, courgette, squash) for a minimum of two years in any affected bed. Grow from certified disease-free seed. Some anthracnose-resistant varieties are available — check seed catalogues for resistance ratings.