What Your Tomato Leaves Are Telling You

Before you reach for a fungicide or a feed, the leaf usually tells you which one you need. Match what you see against the chart below. Most tomato leaf problems are nutrient or watering problems, not diseases, and treating the wrong thing leaves the real cause carrying on.

For late blight and blossom end rot, see the tomato blight page. For the full growing calendar, see the tomato growing guide, which explains why growing under cover is the main decision for tomatoes in Scotland. Under cover, leaf mould and watering problems are the ones to watch: leaf mould mainly affects greenhouse crops, and containers dry out quickly in hot weather.

Quick diagnostic chart

Yellowing between the veins: magnesium deficiency

The veins stay green while the tissue between them yellows, sometimes with reddish-brown tints, on the older leaves first. Older leaves then become brown with dead patches before dropping early. It is commonly seen on tomatoes, apples, plums, grape vines, raspberries, roses and rhododendrons.

Either the soil is short of magnesium, particularly a light, sandy soil, or a high-potassium feed is getting in the way, since plants take up potassium in preference to magnesium. That second route matters for tomato growers, because a potassium-rich tomato feed is what most people apply through summer to encourage flowering and fruiting. Overusing it can cause magnesium deficiency, particularly in tomatoes.

For a short-term fix, spray Epsom salts as a foliar feedA liquid fertiliser applied directly onto a plant's leaves rather than the soil, absorbed faster than a root feed. Full definition → in summer at 20g per litre of water, wetting the leaves, and repeat two or three times a fortnight. Only apply it when it is dull and overcast, as it can scorch leaves in sunny weather. For the long term on light, free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → soil, apply magnesium sulphate (Epsom salts or kieserite) to the soil at 30g per square metre, and avoid overfeeding with high-potassium feeds. See fertiliser types explained.

Scotland's west is wetter than its east, and the Highlands are among the wettest parts of the UK. Rain washes nutrients out of light soils and out of containers, so a light, free-draining plot or a pot of multi-purpose compost in a wet area loses nutrients faster than clay, which holds on to potassium.

Curling leaves: water, temperature or aphids

Curling leaves can be a sign that a tomato plant is short of water. Other causes are variable temperatures at night and aphid attack, so check under a curling leaf for aphids. Leaves that curl upward and inward into a tube are listed by University of Minnesota Extension as leaf roll, a non-disease (abiotic) disorder. Plants in containers dry out quickly and may need watering daily in hot weather. Most Scottish tomatoes grow under cover, where a growing bag can dry out fast on a sunny day, so under-watering is a common cause of curl. A leaf that is dry, brittle and crisp at the edges, with dry compost a couple of centimetres down, points to under-watering. Curl without crisping that settles as the nights warm points to a day-and-night temperature swing. Curl with waterlogged compost may mean over-watering. Ventilate under cover on warm days and keep the temperature steady (RHS). Re-wet a very dry plant gradually, because a sudden flood after a dry spell can split the fruit. Fluctuating moisture levels cause problems with the fruit too, such as splitting or blossom end rot, so keep the watering even rather than letting the compost swing between dry and soaked.

If watering and temperature both check out, consider potassium deficiency: yellow or purple-red leaf tints with browning that begins at the edges of mature leaves, and poor flowering and fruiting. A potassium-rich fertiliser corrects it, and potassium is easily washed out of light, sandy or chalky soils, while clay soils hold on to it.

Dark spots: early blight is uncommon in British gardens

If a leaf shows dark spots and you wonder about early blight, it is probably something else. Early blight is a different fungal disease from late blight, caused by Alternaria solani and A. alternata. It is found widely in North America, but it is not a common problem in British gardens. Its appearance can be confused with the symptoms of the much more common magnesium deficiency, so check the magnesium entry above first. Early blight spots start small and dark and then enlarge, and typically show target-like concentric rings on the older leaves, while magnesium shortage gives flat yellowing between the veins. Late blight, the same pathogen as potato blight, is covered on the tomato blight page. It is the fast one: a rapidly spreading, watery rot that makes leaves collapse and turn brown, with brown lesions on the stems and watersoaked patches on the fruit, in warm, wet weather (RHS). No fungicides are available to gardeners against it. Remove infected material and bury it deeply or send it for green waste collection rather than composting it. Over the higher ground of the west Highlands annual rainfall exceeds 3,500mm, against about 500mm in the driest parts of eastern England (Met Office), so wet-weather diseases are more likely to take hold in the west.

Yellow blotches and grey-brown mould: leaf mould

Tomato leaf mould mainly affects greenhouse crops and is rarely seen outdoors. It shows as yellow blotches on the upper leaf surface with a pale, greyish-brown mould on the underside directly below. It can start in April or May but is more usually found in June or July, and can develop quite suddenly. Heavily infected leaves turn brown and shrivel but do not fall. Infection is greater in warmth and high humidity.

Ventilate to avoid an excessively moist atmosphere, avoid wetting the leaves when watering (especially in the evening), avoid an over-dry growing medium, and try to keep temperatures below 21°C. Trim off some of the lower leaves once fruit has set to encourage air circulation, and pick off infected leaves as soon as you see them. At the end of the season dispose of affected plants and all debris, and disinfect the greenhouse structure. 'Shirley' and 'Cumulus' are resistant cultivarsA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition →, though resistance may not hold against every strain.

Pale, washed-out leaves: nitrogen deficiency

Nitrogen deficiency gives spindly, stunted plants with pale yellow leaves and occasionally pink tints. The older leaves go first, but the yellowing spreads to younger foliage. That separates it from magnesium deficiency, where the veins stay green. Nitrogen is easily washed out of soil by rain, and out of compost in containers. Potting compost in a container can run short of nitrogen within a few months. In containers a general-purpose liquid feed corrects it quickly. In the ground, mulch with garden compost or well-rotted manure for a small, steady supply, or apply a nitrogen-rich fertiliser to the label instructions for a short-term fix. Overapplication damages plants.

Rule out a virus first. More than 20 viruses affect tomatoes, causing mosaic patterns of lighter green on the leaves, distorted or bubbly leaves, stunted growth and reduced yield. Hormone weedkillers can cause similar stunting and distortion, but plants usually recover from that and do not usually recover from a virus. There are no chemical controls, so remove affected plants promptly.

White powdery coating: powdery mildew

Powdery mildews are a group of related fungi that cause a white, dusty coating on leaves, stems and flowers, sometimes with distorted or stunted growth. Plants suffering regular but intermittent drought stress are more susceptible, so water regularly during dry periods, and give plants space, since still, humid air favours the disease. Remove infected shoots promptly and destroy fallen infected leaves in autumn, and do not reach for a fungicide, which the RHS does not recommend. The powdery mildew page covers what to do in full.

Common questions

Sources