How to recognise and prevent tomato late blight?
Tomato late blight, caused by Phytophthora infestans, shows as large brown blotches with a green-grey edge on the leaves, dark patches on the stems and firm brown spots on the fruit. Cool, wet weather favours it. To prevent it, keep foliage dry, space plants, rotate crops and remove affected parts promptly.
Symptoms
Leaves develop large, dark brown blotches with a green-grey edge that aren't confined by the veins. Stems show firm, dark brown lesions, and firm brown spots spread across the fruit. In humid weather a thin white growth appears on the affected parts.
When it appears
University of Minnesota Extension explains that the pathogen is a water mould rather than a true fungus. It favours cool, damp conditions, while long hot, dry spells stop it spreading. Tomatoes and potatoes are the main hosts.
Prevention in the home garden
Don't plant tomatoes where tomatoes, potatoes, peppers or aubergines have grown in the last 3–4 years. Water in the morning with drip irrigation or a soaker hose so the foliage stays dry. Stake or cage plants and space them so air can move. Check them often, remove diseased leaves and clear plant debris at the end of the season. Resistant varieties help, though in conditions that strongly favour the disease they may still show symptoms.
In the app
The photo check lets you snap a plant and get an indication of its health. It is only an indication, not a firm diagnosis: for serious cases, ask a professional.
Frequently asked questions
Can I compost blighted plants?
Better not: take them out of the garden so they don't linger as a source of infection.
Do I need to spray?
UMN Extension doesn't recommend spraying in home gardens where the disease is uncommon. If you do use a product, research it first and follow the label.