Tomato Plant Problems: UK Diagnosis Guide

Tomato blight is caused by the same pathogen as potato late blight — Phytophthora infestans — and is the most destructive tomato disease for UK outdoor growers. The RHS notes that in some years it can cause almost total yield loss in susceptible cultivars grown outside. The initial symptom is rapidly spreading, watery rotting of leaves that collapse, shrivel and turn brown within days. In humid conditions, a fine white fungal growth is visible on the underside of infected leaves at the lesion edges. Blight risk is highest during the classic 'Smith Period' — two consecutive days of humidity above 90% at temperatures above 10°C.

Stop it coming back:There is no fully blight-proof outdoor tomato, but varieties bred for partial resistance significantly reduce losses. Plant in a sheltered, airy spot; stake and tie in plants promptly so air circulates; remove the lowest leaves touching the soil.

compost blighted foliage — Phytophthora spores survive in the compost heap and reinfect next year's crop. Always bin blighted material.

Tomato leaf mould is a fungal disease that mainly affects greenhouse-grown tomatoes — it is rarely seen on outdoor crops. The RHS notes it can develop rapidly to cause significant yield loss in poorly ventilated greenhouses. The distinctive symptom is pale yellow patches on the upper leaf surface with a corresponding velvety olive-brown or purple-brown mould on the underside. The disease spreads by spores in humid, still air. Warm greenhouses with high humidity and poor ventilation see the worst outbreaks.

Stop it coming back:Ventilation is the key prevention measure. Even during cool or wet weather, maintain some airflow through the greenhouse. Leaf mould thrives in still, humid air above 20°C.

confuse leaf mould with blight — leaf mould is slow-progressing, mainly on greenhouse plants, and produces distinctive velvety mould on leaf undersides. Blight progresses much faster and also rots fruit.

Blossom end rot appears as a dark, sunken, leathery patch at the blossom end (bottom) of the fruit. The RHS explains it is caused not by a calcium-deficient soil but by a failure of calcium to reach the developing fruit — often because irregular watering disrupts the water flow that carries calcium from roots to the distant fruits. It is most common on the first truss of the season, on plants grown in containers, and when a dry spell is followed by heavy watering. The plant is not diseased — subsequent trusses often develop normally once watering is regularised.

Stop it coming back:For container-grown tomatoes, automated drip irrigation virtually eliminates blossom end rot. For border-grown plants, consistent hand-watering and mulching achieves the same result.

apply additional calcium spray (calcium chloride) as the first response — in most cases the issue is water uptake, not soil calcium levels. Fix the watering first.

Magnesium deficiency is extremely common in UK tomato crops, particularly in container-grown plants that are fed regularly. The RHS notes that interveinal yellowing on older leaves is one of the most frequently observed nutrient problems on tomatoes. Tomato plants are heavy fruit producers and magnesium is mobilised from older leaves to support fruit development — this is a natural process that becomes a deficiency when soil or feed magnesium is insufficient. Regular tomato feeds help, but many do not contain enough magnesium, especially for heavily cropping plants.

Stop it coming back:Begin fortnightly Epsom salt sprays when the first truss of fruit sets, rather than waiting for symptoms to develop. This is particularly important for indeterminate (cordon) varieties grown in containers.

confuse magnesium deficiency with blight or leaf mould — magnesium deficiency causes slow yellowing between the veins on older leaves with no mould, no water-soaked patches and no rapid progression.

Many tomato growers mistake physiological leaf roll for a disease. The RHS notes that temperature fluctuation — warm days and cold nights — is the most common cause, especially in early summer. Leaves roll upward (primary leaflets curl inward towards the midrib) as a stress response. It can also follow heavy pruning (pinching out) or periods of very rapid growth. The rolled leaves remain green, firm and healthy — they are not yellowing, spotted or collapsing. The plant typically continues growing normally.

spray for disease or remove rolled leaves if physiological leaf roll is the cause — rolled, green, firm leaves are not diseased, and removing them reduces the plant's ability to ripen fruit.

Tomato fruit splitting is caused by irregular water supply during the fruit ripening stage, as the RHS explains. When water uptake suddenly increases after a dry period — whether through rain, heavy irrigation, or moving a plant from dry to wet conditions — the inner flesh expands faster than the outer skin can accommodate. The result is radial splits from the stalk end, or circular 'concentric' cracks around the shoulder of the fruit. This is a physiological disorder, not a disease, and is common in UK outdoor crops in late summer when weather is unpredictable.

Stop it coming back:The RHS acknowledges that fruit splitting is difficult to prevent entirely in outdoor UK conditions because rainfall is out of the gardener's control. Consistent mulching and stake-and-tie management to keep plants upright are the most practical measures.

leave split fruits on the plant — they quickly rot and are at risk of secondary fungal infection. Harvest and use immediately or discard.

Glasshouse whitefly is one of the most troublesome pests of greenhouse tomatoes in the UK. The RHS notes that it is rarely a problem on outdoor tomatoes. The adults are tiny, white, moth-like insects that rise in clouds when the plant is disturbed. Both adults and nymphs feed by sucking sap from leaf undersides, causing yellowing, and excreting honeydew which encourages sooty mould. Populations build rapidly in warm greenhouses and can overwhelm a crop. Biological control using the parasitic wasp Encarsia formosa is the most effective management approach in greenhouses.

Stop it coming back:Introduce Encarsia formosa preventatively at planting time in the greenhouse — early biological control prevents populations from building to damaging levels. Available by mail order from biological control suppliers.

use broad-spectrum insecticides once Encarsia has been introduced — the parasitic wasp will be killed along with the whitefly, and the whitefly population will recover faster.