Pests & problems

Blight, blossom end rot, leaf mould and whitefly

What goes wrong with tomatoes, what it looks like, and what the sources say to do.

Jump to: Blight | Blossom end rot | Splitting | Leaf mould | Whitefly | In North America

Blight

Tomato blight attacks the leaves and fruit, causing rotting. It is most common in warm, wet weather. RHS: How to grow tomatoes Tomatoes are prone to blight outdoors, but rarely get it in a greenhouse. RHS: How to grow tomatoes

UMN: one inch of rain or watering a week is ideal. Avoid overhead sprinklers, because wet leaves are more prone to disease. University of Minnesota Extension: Growing tomatoes in home gardens

Blossom end rot

Blossom end rot is a round patch, greenish brown to black, at the flower end of the fruit (the end furthest from the plant). It becomes sunken as it grows. RHS: Blossom end rot Blossom end rot isn’t a pest or a disease: it is caused by poor growing conditions. RHS: Blossom end rot

It is caused by a lack of calcium in the fruit, but the RHS says soils very rarely lack calcium. The real problem is an irregular water supply. RHS: Blossom end rot Plants in pots or growing bags are most at risk. Plants in border soil are less likely to be affected. RHS: Blossom end rot

Affected fruits can’t be saved, but steady moisture protects the next ones. In hot spells, watering twice a day is better than once with double the volume. RHS: Blossom end rot The RHS says calcium sprays on the leaves don’t work, because the fruit doesn’t take up calcium that way. RHS: Blossom end rot

Garden Organic links blossom end rot to too little or irregular watering, and says it is most common when the first trusses are forming. Garden Organic: Tomatoes growing guide

Splitting

The RHS says splitting and cracking in late summer is difficult to prevent, and links it to up-and-down moisture levels. RHS: How to grow tomatoes Water regularly to keep the soil or compost evenly moist. Up-and-down moisture levels can cause splitting or blossom end rot. RHS: How to grow tomatoes

Leaf mould

Tomato leaf mould is a fungal disease, mostly of greenhouse tomatoes: yellow blotches on top of the leaf, with greyish-brown mould underneath. RHS: Tomato leaf mould To prevent leaf mould: ventilate well, avoid wetting the leaves, try to stay below 21°C (70°F), and pick off infected leaves as soon as you see them. RHS: Tomato leaf mould

The RHS lists ‘Shirley’ and ‘Cumulus’ as resistant cultivars, though new strains of the fungus can break down resistance. RHS: Tomato leaf mould · checked 30 Oct 2026

Glasshouse whitefly

Glasshouse whitefly are tiny sap-sucking insects, about 1.5mm long, that fly up in clouds when a plant is disturbed. Their sticky honeydew lets black sooty moulds grow. RHS: Glasshouse whitefly Each female whitefly can lay more than 200 eggs, and at 21°C (70°F) the life cycle can take about three weeks. RHS: Glasshouse whitefly

Whitefly often arrive on new plants, so check new purchases. RHS: Glasshouse whitefly An established control is a tiny parasitic wasp, Encarsia formosa. Whitefly young that it has parasitised turn black, so you can see it working. RHS: Glasshouse whitefly

In North America

In Minnesota, UMN Extension says early blight and Septoria leaf spot are common tomato diseases, while late blight doesn’t turn up every year. University of Minnesota Extension: Growing tomatoes in home gardens Tomato hornworms are large caterpillars that feed on leaves and fruit. University of Minnesota Extension: Growing tomatoes in home gardens

Britannica lists bacterial wilt, early blight, mosaic virus, Fusarium wilt, nematodes and tomato hornworms among tomato pests and diseases. Encyclopaedia Britannica: tomato

UMN: a resistant variety won’t get the disease; a tolerant one will, but it spreads more slowly and is less serious. Seed catalogues use codes to show which is which. University of Minnesota Extension: Growing tomatoes in home gardens UMN: you can still have a satisfactory crop even when plants and fruit show some disease and pest problems. University of Minnesota Extension: Growing tomatoes in home gardens

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