Common Tomato Plant Diseases: Prevention and Treatment

Most tomato leaf diseases start on the lowest leaves and work upward, and nearly all of them are easier to prevent than to stop. The wilts and viruses can't be cured at all; the fix is to pull the plant and grow a resistant variety next time. Use the table to narrow it down, then read the entry.

What you see	Likely cause
Brown spots with target-like rings, oldest leaves first	Early blight
Many small spots with dark edges and pale gray centers, lower leaves	Septoria leaf spot
Large gray-green, greasy blotches, white fuzz underneath in damp weather; plant collapses within days	Late blight
Lower leaves yellow, often on one side of the plant; brown streaks inside the stem	Fusarium wilt
V-shaped yellow wedges at leaf edges, slow decline, worse in cool weather	Verticillium wilt
Small dark spots on leaves, scabby or speck-like spots on fruit	Bacterial spot or speck
Leaf edges browning, white "bird's-eye" spots on fruit	Bacterial canker
Bronzed young leaves, ring spots, stunting	Tomato spotted wilt virus
Light and dark green mottling, distorted leaves	Mosaic virus

Early blight

Alternaria linariae (long called A. solani).

Symptoms: brown spots up to about 1/2 inch with concentric rings, often with a yellow halo, starting on the oldest leaves. Infected leaves yellow and drop, the bare stems leave fruit open to sunscald, and dark, leathery patches can form at the stem end of the fruit.
Prevention: rotate, mulch, stake plants, water at the soil and remove leaves that touch the ground. The fungus survives on old plant debris.
Treatment: pick off spotted leaves when the plants are dry. A protectant fungicide (copper, chlorothalonil or a Bacillus-based biofungicide) applied at the first spots and repeated as the label directs protects new growth; it does not clear existing spots.

Septoria leaf spot

Septoria lycopersici.

Symptoms: dozens of small round spots, about 1/8 inch, with dark borders and gray or tan centers dotted with tiny black specks. It starts on the lower leaves, usually after the first fruit set, and can strip a plant from the bottom up. It does not infect the fruit.
Prevention: the same as early blight. The fungus also overwinters on nightshade-family weeds such as horsenettle, so keep them out of the garden.
Treatment: remove spotted leaves; use the same protectant sprays as for early blight.

Late blight

Phytophthora infestans, a water mold rather than a true fungus, and the cause of the Irish potato famine.

Symptoms: large, irregular, gray-green blotches that turn brown, with white fuzzy growth on the undersides in humid weather; dark, greasy streaks on stems; firm brown patches on green fruit. In cool, wet weather it can kill a plant within a week.
Prevention: spores blow in on the wind, so outbreaks are sporadic. Buy healthy transplants, destroy volunteer potatoes, and choose resistant varieties such as 'Defiant PhR' or 'Iron Lady' if late blight is common where you live. When it is reported nearby, a protectant fungicide (chlorothalonil or copper) applied before infection is the only spray that helps.
Treatment: none. Pull infected plants, bag them and put them in the trash, not the compost. It also attacks potatoes.

Fusarium wilt

Fusarium oxysporum f. sp. lycopersici. More on the Fusarium wilt page.

Symptoms: lower leaves yellow, often on just one side of a leaf or one side of the plant, then wilt and die. Slice the lower stem lengthwise and you'll see brown streaks in the water-conducting tissue. Worst in hot weather and in light, acidic soils.
Prevention: grow resistant varieties. F on the label means resistance to race 1, FF to races 1 and 2, FFF to all three. The fungus lives in soil for years, so rotation alone won't clear it. Keeping the pH at 6.5–7.0 reduces the damage.
Treatment: none. Remove the whole plant, roots included, and don't compost it. Clean tools and stakes before using them elsewhere.

Verticillium wilt

Verticillium dahliae. See also Verticillium wilt.

Symptoms: yellow, V-shaped wedges at the edges of lower leaves, browning veins and a slow decline rather than a collapse. It favors cooler weather than Fusarium does.
Prevention: grow varieties marked V. The fungus also infects potatoes, eggplant, peppers, strawberries and many weeds, and it survives in soil for years. Rotating with corn, grains or grasses, which it does not infect, helps.
Treatment: none; remove infected plants.

Bacterial spot

Several Xanthomonas species.

Symptoms: small, dark, greasy-looking spots on leaves that can run together and make the foliage look ragged; raised, scabby spots on the fruit. Favored by warm, rainy weather.
Prevention: the bacteria arrive on seed and transplants and spread by splashing water and on hands. Start with clean seed and healthy plants, stay out of the patch when the leaves are wet, water at the soil and rotate.
Treatment: no cure. Copper sprays slow the spread somewhat, though many strains tolerate copper. Remove badly infected plants.

Bacterial speck

Pseudomonas syringae pv. tomato.

Symptoms: dark leaf spots with yellow halos and tiny, pinpoint black specks on green fruit. It looks like bacterial spot but prefers cool, wet weather, so it tends to show up early in the season.
Prevention and treatment: the same as bacterial spot. The specks on the fruit are only skin-deep.

Bacterial canker

Clavibacter michiganensis.

Symptoms: leaflets brown and curl at the edges, often on one side of a leaf; plants wilt; light streaks or cracks on stems; small white spots with raised brown centers ("bird's-eye spots") on the fruit.
Prevention: it comes in on infected seed and transplants and spreads on hands and pruning tools. Buy clean seed and plants, and disinfect stakes and cages.
Treatment: none effective. Remove infected plants and keep tomatoes out of that bed for three years.

Tomato spotted wilt virus (TSWV)

Symptoms: bronze or purplish flecks on young leaves, dark ring spots, stunted or one-sided growth; fruit with pale rings or blotchy, uneven color.
Spread: carried by thrips, which pick it up from infected weeds and ornamentals. It is a bigger problem in the South.
Prevention: resistant varieties are the best defense; look for TSWV on the label, as on Bella Rosa. Control weeds around the garden.
Treatment: none. Pull infected plants promptly so thrips don't spread it further; insecticides do little to stop transmission.

Tomato mosaic and tobacco mosaic virus (ToMV, TMV)

Symptoms: mottled light and dark green leaves, often puckered or fern-like, stunted plants and uneven ripening.
Spread: not by insects. These viruses travel on hands, tools, clothing and seed, and they last a long time on surfaces. Tobacco products can carry TMV, so smokers should wash their hands with soap before handling plants.
Prevention: choose varieties marked ToMV or TMV, and wash hands and tools when moving between plants.
Treatment: none; bag and discard infected plants. If aphids are present and new leaves turn narrow and stringy, the cause is more likely cucumber mosaic virus, which aphids do spread.

Prevention that works against most of them

Rotate. Keep tomatoes, potatoes, peppers and eggplant out of a bed for three years. They share many of the same diseases (see the nightshade family).
Choose resistant varieties. It is the only real defense against the wilts and viruses.
Keep leaves dry. Water at the soil in the morning, and stake or cage plants so air moves through them (how to support tomatoes).
Mulch to stop rain splashing spores from the soil onto the lower leaves.
Remove the lowest leaves once plants are 2–3 feet tall.
Clean up. At the end of the season, remove all tomato debris, bag anything diseased, and disinfect stakes and cages with a solution of 1 part household bleach to 9 parts water.
Use fungicides as protection, not a cure. Start at the first symptoms or before weather that favors disease, follow the label, and observe the days-to-harvest interval. Copper is the main organic option.