The best way to diagnose a tomato problem is to start with what you’re actually seeing, not with a list of diseases to cross-reference. This guide is structured the way you’d actually use it: you describe the symptom, you get the most likely causes, and you get the test that confirms which one it is.
Most tomato leaf problems are not emergencies. Several are harmless or self-limiting. A few - particularly the wilt diseases and some viruses - require fast action to avoid losing the plant. The symptom guide below will tell you which category you’re dealing with.
Quick Reference: Symptom to Cause
| What You See | Most Likely Cause | Urgency |
|---|---|---|
| Lower leaves yellow, dropping, brown spots with rings | Early blight (Alternaria solani) | Low - manage, not emergency |
| Leaves curl upward, no spots | Heat stress or physiological leaf roll | None - usually harmless |
| Leaves curl downward/under | Viral (TYLCV) or herbicide drift | High if viral |
| Purple/bronze undersides, dark green overall | Phosphorus deficiency or cold soil | Low - soil/temp fix |
| Brown spots, fringed rings | Early blight vs. Septoria leaf spot | Low - manage |
| Wilting despite moist soil | Fusarium wilt or bacterial wilt | High |
| Mosaic or mottled pattern | Tobacco mosaic virus (TMV) or related | High - remove plant |
| Blossoms dropping, no fruit set | Temperature extremes | Medium - timing fix |
1. Lower Leaves Yellow and Dropping, Brown Spots
The most common tomato leaf problem, and often misdiagnosed as nutrient deficiency.
Early blight (Alternaria solani): the yellowing starts on the oldest, lowest leaves first - this is important. If yellowing starts at the top of the plant, it’s a different problem. With early blight, you’ll see brown spots 1/4 to 1/2 inch in diameter, each surrounded by a yellow halo, often with concentric rings inside the spot (the “target” or “bullseye” appearance). Leaves yellow around and between spots, then drop.
Early blight is a fungal disease that spreads by spores splashed up from soil, which is why the lowest leaves are always affected first. Wet weather, overhead irrigation, and crowded plants accelerate it.
What to do: remove affected leaves (bag them, don’t compost). Apply copper-based fungicide at first sign and every 7-10 days during wet periods (UC Davis ANR, Tomato Diseases, 2023). Mulch 3-4 inches deep to prevent spore splash from soil. Stake or cage plants to keep foliage off the ground. Early blight rarely kills the plant - it’s manageable, not a crisis.
Nitrogen deficiency also causes yellowing of lower leaves without the spots. Nitrogen-deficient plants are uniformly pale yellow-green throughout the lower canopy. If you see yellow without spots or rings, test: side-dress with fish emulsion or balanced fertilizer and reassess in 7 days. If it greens up, nitrogen was the issue.
2. Leaves Curl Upward
Leaves that roll upward along their length, forming tubes, on an otherwise healthy plant.
Physiological leaf roll is the most common cause and is entirely harmless. It occurs when plants are under heat or water stress, when soil is too wet, or when plants have just been transplanted. The plant reduces transpiration by reducing leaf surface area - essentially curling the leaves to protect itself. No spots, no yellowing, no discoloration. Plants that look fine one morning and show rolled leaves by afternoon in 90°F heat are doing exactly what they should.
Confirm: check at cooler times of day (morning). Leaves that uncurl as temperatures drop are showing physiological leaf roll. No treatment needed.
Heavy fruit load can also cause leaf roll in mid-season as the plant directs water to developing fruit. Again, no other symptoms, and no treatment needed.
Potato leafroll virus and a few other viruses cause permanent, year-round upward leaf curl with brittle, stiff texture. If leaves stay curled in cool morning temperatures and feel unusually stiff, virus is a possibility - but confirm by the other symptoms in section 7.
3. Leaves Curl Downward (Under)
Leaves that curl inward and down - the opposite direction from heat roll - warrant more attention.
Tomato yellow leaf curl virus (TYLCV): a begomovirus transmitted exclusively by silverleaf whitefly (Bemisia tabaci). Affected leaves curl downward and inward with yellowing at the margins, and the entire plant may be stunted. TYLCV is most prevalent in warm-season gardens in the southeastern US and California. The key confirmation: check the undersides of leaves for whitefly adults (tiny white insects that scatter when disturbed). If whiteflies are present and leaves are curling down with yellow edges, TYLCV is the likely diagnosis.
What to do: no cure for infected plants - remove and destroy. Control whitefly populations on remaining plants with insecticidal soap or yellow sticky traps to prevent further spread. TYLCV-resistant tomato varieties are available (look for “TYLCV” in the disease resistance code).
Herbicide drift: 2,4-D and other broadleaf herbicides cause distinctive twisting and downward curl of leaves, often with distorted growth. New growth is most affected - if new leaves are twisted, cupped, and curling while older leaves look normal, think herbicide. The source is usually a neighbor’s lawn treatment or a home application that drifted. No treatment; the plant often recovers over several weeks if not re-exposed.
4. Purple or Bronze Leaf Undersides
Leaves that look dark green or normal on top but show reddish-purple or bronze coloration on the undersides.
Phosphorus deficiency: the classic presentation. Young plants, especially in cold soil (below 55°F), cannot take up phosphorus efficiently, and the resulting accumulation of anthocyanins turns leaf undersides purple. This is extremely common in spring when gardeners transplant too early. The good news: once soil temperatures rise above 60°F, the coloration usually disappears on its own as phosphorus uptake normalizes. If purple undersides persist in warm weather, apply a phosphorus-containing fertilizer.
Cold soil mimics phosphorus deficiency - confirm by checking soil temperature at 4-inch depth. Below 55°F, phosphorus becomes unavailable even when present in adequate amounts. Wait for soil to warm before adding fertilizer.
Tomato spotted wilt virus (TSWV): causes bronze discoloration of leaf tissue, ring spots or lesions on leaves and fruit, and often a streaking or bronzing on the stems. TSWV is transmitted by western flower thrips (Frankliniella occidentalis) and is difficult to distinguish from cold-soil phosphorus deficiency early in infection. Key difference: TSWV symptoms persist and worsen as temperatures warm, while phosphorus deficiency improves. TSWV also produces distinctive ring spots on fruit - look for concentric yellow-green rings on ripening fruit if you suspect it.
5. Brown or Black Spots on Leaves
Two fungal diseases are most commonly responsible and can be difficult to distinguish.
Early blight (Alternaria solani): spots are 1/4 to 1/2 inch, with concentric rings inside (bullseye pattern), surrounded by a yellow halo. Starts on the oldest, lowest leaves. Grows larger over time.
Septoria leaf spot (Septoria lycopersici): spots are smaller (1/8 inch maximum), circular, with dark brown borders and lighter gray or tan centers. May contain tiny dark dots (pycnidia - the fungal fruiting bodies) visible with a hand lens. Septoria leaf spot typically produces more spots per leaf than early blight. Both diseases spread upward from the lowest leaves.
| Feature | Early Blight | Septoria Leaf Spot |
|---|---|---|
| Spot size | Large (1/4-1/2 in) | Small (1/8 in max) |
| Spot center | Brown with rings | Gray/tan with dark dots |
| Yellow halo | Usually present | Less prominent |
| Pattern | Bullseye/target | Circular, uniform |
Management is the same for both: remove affected leaves, copper fungicide, mulch to prevent spore splash, avoid overhead irrigation, improve airflow by staking and pruning lower branches.
6. Wilting Despite Adequate Soil Moisture
A plant that wilts during the heat of the day and recovers by evening is probably showing normal heat stress. A plant that wilts and doesn’t recover overnight - in moist soil - has a vascular disease.
The stem cut test: cut the main stem 6-8 inches above soil level and examine the cut surface.
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Fusarium wilt (Fusarium oxysporum f.sp. lycopersici): the vascular tissue inside the stem shows a brown or reddish-brown ring just inside the outer stem layer. This discoloration is the diagnostic feature - it represents the fungus colonizing and blocking the xylem. The brown ring distinguishes Fusarium from normal tissue (cream/white vascular ring).
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Bacterial wilt (Ralstonia solanacearum): the stem cut looks normal. Now hold both cut ends close to each other in still air for 30 seconds, then slowly pull apart. Bacterial wilt causes fine, thread-like “threads” or “strings” of bacterial ooze to form between the cut surfaces as you separate them - visible evidence of bacterial colonies in the xylem. Alternatively, place a cut stem section in a glass of water: cloudy, milky strands floating out of the cut end within a minute indicate bacterial wilt.
Both wilt diseases are soilborne, persist in the soil for years, and cannot be cured once the plant is infected. Remove and destroy affected plants (do not compost). Bacterial wilt requires an insect vector (cucumber beetles) in some regions; weed control and beetle management reduces risk. Fusarium wilt is managed through resistant varieties (look for “F” in the disease resistance designation, “FF” for two races).
7. Mosaic or Mottled Leaf Pattern
Leaves that show irregular light and dark green patches, sometimes with yellowing, creating a mosaic effect.
Tobacco mosaic virus (TMV) and related tobamoviruses (Tomato mosaic virus, ToMV) cause this pattern. TMV is mechanically transmitted - through hands, tools, and clothing after touching infected tobacco products or infected plants. If you smoke and don’t wash your hands before gardening, TMV is a genuine risk. The mosaic pattern is often accompanied by leaf distortion, fern-like narrowing of leaf tissue, and in severe cases, dark brown streaking on stems.
Cucumber mosaic virus (CMV): similar mosaic pattern, transmitted by aphids. CMV typically causes more pronounced leaf distortion and may cause fruit to be mottled or deformed.
What to do: no treatment for virus-infected plants. Remove and destroy. Wash hands and disinfect tools with 10% bleach solution or 70% isopropyl alcohol between plants. Resistant tomato varieties (designated “TMV” or “ToMV” in resistance codes) are widely available.
TMV prevention: wash hands before working in the garden, especially after any contact with tobacco products. TMV survives on tobacco in cigarettes and cigars. This is not a theoretical risk - commercial tomato production operations have strict no-smoking policies near plantings for this reason.
8. Blossom Drop Without Fruit Set
Flowers appear, then drop without developing fruit - leaving no trace or leaving a tiny green nub that falls.
Temperature thresholds: tomatoes require pollen viability to set fruit. High temperatures above 85-90°F during the day (particularly night temperatures above 70°F) cause pollen sterility, and flowers abort (UC Davis Cooperative Extension, Tomato Production in California, 2020). Low temperatures below 55°F cause similar pollen failure. The result is flowers that open and drop without pollination.
This is a timing and location problem, not a disease. Solutions:
- Plant at the right time so fruit set occurs when temperatures are in the 65-85°F range
- Choose heat-set varieties (e.g., ‘Solar Fire’, ‘Heatmaster’, ‘Florida 91’) for consistently hot summers
- Provide afternoon shade in extreme heat zones (Zone 9+) to keep temperatures in range
- In short seasons, prioritize early-setting varieties
Night temperatures above 75°F are the most common cause of summer blossom drop. Day temperature is less important than overnight minimum - if nights stay above 75°F for more than a week, fruit set stalls. This is why tomatoes that produce well in June and July may stop setting fruit in August in hot climates.
Related reading: Blossom End Rot - the calcium delivery problem that’s separate from these leaf diseases; Integrated Pest Management - building a framework for managing pests and diseases
Related crops: Tomato - full growing guide including disease-resistant variety selection