Common tomato problems and how to tell them apart

Watch on Greenhouse tomatoes, cucumbers and chilli peppers

Brush a greenhouse tomato and a cloud of tiny white insects lifts off the leaves and settles again. The adults sit under the youngest leaves near the growing tip. The young are flat translucent scales fixed to the leaf undersides, and they do most of the feeding. They drop sticky honeydew onto the trusses below, and black sooty mould follows. Glasshouse whitefly breeds all year under cover, weakens the plant and can carry viruses, so catch it before the leaves turn sticky.

Watch on Soft shoot tips and young leaves

Aphids gather on soft growing tips and the undersides of young tomato leaves, in green, black or pink colonies that sit still rather than flying up when disturbed. They breed fast in warm weather and coat the leaves below in honeydew, which then grows sooty mould. Direct feeding damage on an established plant is usually small. The greater risk is virus, since aphids can move cucumber mosaic and other viruses between plants quickly. Predators help more than a late spray.

Watch on Leaves, worst under glass in hot dry air

Red spider mite makes tomato leaves look dull, then finely speckled with pale dots. It thrives in the hot dry air of a greenhouse, so plants near the glass or a vent often show symptoms first. Fine webbing across the shoot tips means the colony is already large. Turn a leaf over and tap it onto white paper to find the moving specks. Left alone under cover, the leaves bronze and drop, and the plant can lose most of its foliage in a month.

Watch on Young leaves and unfurling shoot tips

Thrips rasp tomato leaves and leave silvery or bleached patches with tiny black dots of excrement. The youngest leaves show it first and may come through puckered and scarred. An established plant can grow through light feeding. The reason to act is virus risk: thrips carry tomato spotted wilt, and a bronzed young plant with ringed fruit may already be infected. Blue sticky traps show whether thrips have arrived.

Watch on Leaves and green fruit, in the Americas

Tomato hornworm is a finger-thick green caterpillar up to 10 cm long, with pale diagonal stripes and a horn at the tail. It can strip whole tomato branches in a day or two and gouge green fruit. It hides well against the foliage, so dark pellet-shaped droppings on the leaves and soil are often the first sign. It is found across the Americas. Leave any hornworm carrying upright white cocoons, because a parasitic wasp has already stopped it feeding.

Watch on Leaves and fruit

Smaller caterpillars chew ragged holes in tomato leaves and bore into fruit, leaving dark pellets of frass on the foliage below. Under cover, the tomato moth caterpillar is a common cause. It is green or brown and feeds at night. In warmer regions, corn earworm and armyworms also tunnel into fruit. Count the fleshy prolegs to separate a true caterpillar from a sawfly larva, since the treatments differ. Hand-picking and a fine cover catch most of them, and any caterpillar dotted with small cocoons is already parasitised.

Watch on Leaves

A leaf miner is a grub living inside the leaf. It eats the green layer while both leaf surfaces stay intact. On a tomato it shows as pale winding tunnels or blotches, with a dark thread of droppings inside and a small grub at the growing end when the leaf is held to the light. Light mining does little harm to an established plant. Because the grub sits inside the leaf, a surface spray will not reach it. Remove mined leaves while numbers are low.

Watch on Outdoor foliage and fruit

Late blight ends more outdoor tomato crops than anything else. Dark brown blotches spread from leaf tips and edges, brown streaks run up the stems, and firm leathery patches develop on the fruit, often from the shoulder down. In warm humid weather the whole plant can collapse within a week or two. There is no cure once it arrives, and marked fruit keeps rotting indoors. Grow a resistant cultivar, keep the foliage dry and raise tomatoes under cover where you can.

Watch on Stems, leaves and fruit under glass

Grey mould is a fuzzy grey-brown coat that puffs spores when touched. It starts on dead or wounded tomato tissue before moving into healthy growth. It often begins on a faded flower, a leaf scar or a pruning snag, then rots the stem or fruit. It can also leave pale ghost rings on otherwise sound skins. Still, damp air under glass favours it, so a heated but unventilated greenhouse is a high-risk place. Ventilation, spacing and clearing dead material help more than spraying.

Watch on Leaves, mainly under glass

Powdery mildew coats a tomato leaf in white or pale grey patches that wipe off on a finger, spreading until the leaf yellows, browns and drops early. It is most common under glass, on plants whose roots have run dry while the air stays humid, which is the reverse of most fungal disease. The plant is weakened rather than killed, though heavy defoliation cuts the crop short. Watering the roots well, mulching and moving air through the greenhouse hold it back better than reducing water, which is the common mistake.

Watch on Whole plant, worst in warm soil

Fusarium wilt blocks the water-carrying tissue of a tomato from inside. The lowest leaves yellow first and the yellowing climbs, often on one side of the plant or one half of a leaf. The plant wilts in afternoon heat and recovers by morning, until one day it does not. Cut the stem across and a brown ring stains the wood well up from the base. It is worst in warm soil and there is no cure, so use resistant cultivars labelled F, or grafted plants, where a bed has a history.

Watch on Whole plant, in cooler weather

Verticillium wilt looks much like fusarium, but it usually appears in cooler conditions and may slow once hot weather arrives. On a tomato it yellows the older leaves from the bottom up, wilts the plant by day, and stunts the crop. A cut stem shows a brown or olive stain in the wood. The fungus lives in the soil for many years and affects many hosts, so nothing cures an infected plant. Use grafted plants on resistant rootstocks, or grow in fresh compost in containers.

Watch on Young leaves and fruit

Tomato spotted wilt shows as pale or brown target-like rings on green or ripe fruit, with bronzed young leaves and dark streaks up one side of the stem. It is carried by thrips and there is no cure. Only thrips that fed on an infected plant as larvae can pass it on, so removing infected plants and clearing weeds that harbour the virus matters more than spraying adult thrips.

Watch on Leaves and fruit

Mosaic viruses mottle tomato leaves in irregular blocks of dark and pale green, with no clear edge and nothing growing on the surface. New leaves may come through puckered, blistered or narrowed to a fern-like strap. The fruit ripens unevenly and the plant stays stunted while its neighbours grow normally. Feeding or watering does not improve it, which is the clearest clue. Aphids carry many mosaic viruses, and tobacco mosaic spreads on hands and tools, so hygiene and removing infected plants are the main defences.

Watch on Lower leaves and fruit

Alternaria, or early blight, makes dark brown spots on the oldest lowest tomato leaves. Each spot has a yellow halo and fine target-like rings inside it. The spots run together, the lower leaves collapse while still attached, and the damage climbs slowly over weeks while the growing tip stays green. Dark sunken lesions can form on the fruit at the stem end. Unlike late blight, it moves slowly and rarely destroys a crop. It travels on seed as well as on the wind, so clean seed and clearing old debris both help.

Watch on Fruit, the first truss most

Blossom end rot starts as a pale water-soaked patch on the base of a tomato, opposite the stalk. The patch darkens to brown or black, sinks a little and dries leathery while the rest of the fruit ripens normally. It is a calcium transport failure rather than a soil shortage. Swings between dry and drenched roots stop calcium reaching the fast-growing cells at the far end of the fruit. The first truss is usually worst. Steady watering and a larger volume of compost matter more than adding calcium, and long-fruited and plum types suffer most.

Watch on Flowers, before fruit sets

Blossom drop means a tomato flowers freely but sets no fruit. The flowers yellow at the stalk and fall, leaving a clean scar. Tomato pollen works only within a band of temperature. It can fail below about 13 deg C at night or above about 30 deg C by day, so both cold spells and heatwaves can leave empty trusses. Very dry or very humid air also stops pollen shaking free, and too much nitrogen encourages leafy growth at the expense of fruit. Tap the trusses at midday and switch to a high-potassium feed.

Watch on Ripening fruit

Fruit splitting happens when a tomato swells faster than its skin can stretch. Radial cracks run down from the stalk scar after a surge of water, usually when a dry spell is followed by heavy rain or heavy watering. Fine rings around the shoulder come from swings in temperature and strong sun on exposed fruit. The largest, ripest fruit split first. The crack itself does little harm, but grey mould and brown rot can enter the wound within days, so pick split fruit and use it the same day.

Watch on Leaves, older or younger by nutrient

Nutrient shortages show on tomato leaves as patterned yellowing, and the position on the plant helps identify the nutrient. Magnesium shortage is the most common. It yellows between the veins of older lower leaves and leaves a green herringbone, often with orange edges. Heavy high-potassium feed can bring it on. A whole plant that is pale and slow may be short of nitrogen. Much yellowing is not a shortage at all, since waterlogged roots, cold roots and exhausted compost can look similar. Check the roots and watering before feeding.

Watch on Whole plant in dry spells and containers

A wilting tomato may be short of water or drowning in it, and the two look identical from above. Put a finger in the compost before acting. Dry and light means drought; wet, heavy and sour means root rot, and watering it again will finish the plant. Drought browns the leaf edges crisply, yellows and drops the lower leaves, and sheds flowers and small fruit. A plant left to wilt hard again and again stops bouncing back. Water thoroughly and less often, mulch, and give containers enough volume to ride out a hot day.