“Tomato blight” is not one disease. It is a name gardeners use for three different problems — early blight, late blight, and Septoria leaf spot — and they need different responses. The single most important fact about all three is that no fungicide cures an established infection. Sprays protect healthy leaves; they do not heal spotted ones. Prevention, started before symptoms appear, is the whole game.
The three blights
- Early blight (Alternaria solani): a fungus that favours warm weather, around 82–86°F (28–30°C). It causes dark spots with concentric rings — a bull’s-eye pattern — on older, lower leaves first, then moves up the plant. Fruit can develop dark, leathery patches near the stem end. It is a mid- to late-summer disease.
- Late blight (Phytophthora infestans): the pathogen behind the Irish potato famine, and the most destructive of the three. It prefers cool, wet weather, around 60–70°F (15–21°C), with abundant moisture. Greasy-looking dark patches appear on leaves and stems, often with a white mouldy growth on the leaf undersides in humid weather, and fruit rots quickly. In favourable weather it can infect an entire planting within a few days.
- Septoria leaf spot: small, circular spots with grey centres and dark borders, mostly on lower leaves. It rarely kills a plant on its own but weakens it and is often found alongside early blight.
Symptoms appear roughly 5–7 days after infection, so by the time you see spots, the infection is already a week old. That is why waiting for symptoms before acting is a losing strategy.
Prevention first
- Space and air. Give plants room for air circulation and prune the lower leaves so foliage dries quickly after rain. Stake or cage plants so leaves stay off the soil.
- Water the soil, not the leaves. Drip irrigation or watering at the base keeps foliage dry. Water in the morning so any wet leaves dry during the day.
- Mulch. A thick mulch layer stops soil — and the spores in it — from splashing onto the lower leaves.
- Rotate crops. Do not plant tomatoes, potatoes, peppers, or eggplants in the same bed year after year. Early blight survives in soil and plant debris.
- Choose resistant varieties where available, and buy healthy transplants — check the undersides of leaves before you buy.
- Remove infected leaves promptly at the first sign, and bag them for the trash. Never compost diseased tomato plants: blight spores survive home composting and reinfect the next crop.
Fungicides: what the extension services say
North Carolina State University’s 2026 tomato fungicide guide sets out weekly spray programmes built around protectant fungicides. For home gardens, the practical options are:
- Chlorothalonil (e.g. Daconil): a broad-spectrum protectant, effective against late blight and leaf spots. Apply every 7–10 days as prevention, shortening to 5–7 days once symptoms appear in the area.
- Mancozeb: a protectant that disrupts multiple fungal processes, making resistance less likely. Strong against early blight and Septoria leaf spot, but with pre-harvest interval restrictions — check the label and stop applications well before harvest.
- Copper fungicides (fixed copper): the main organic option. Copper controls bacterial spots well and gives moderate fungal suppression, but University of Kentucky research cited in garden guides shows it provides only limited control of late blight, and NC State notes copper was dropped from its minimal programme because of resistant bacterial spot populations. It has a 0-day harvest interval, so it can be used up to picking time.
Baking soda sprays are often recommended online, but university research shows they provide only partial control and should not replace proven fungicides during an outbreak.
A few rules that apply to every spray:
- Start before symptoms appear, especially during humid or rainy weather, and reapply after heavy rain — protectant fungicides wash off.
- Always read the current product label before applying any chemical, and check whether the product is approved for home garden use on tomatoes in your country. Product availability and restrictions change.
- Observe pre-harvest intervals and wear the protective equipment the label requires.
- Rotate modes of action if you spray repeatedly; resistance to single-site fungicides such as strobilurins is known in the early blight pathogen.
When a plant is badly hit
If late blight is spreading fast through a plant, remove the plant entirely, bag it, and put it in the trash — do not compost it. A plant that is mostly defoliated will not produce a useful crop, and it is a source of spores for every other tomato and potato in the area. It is better to lose one plant than the whole bed.
Sources
- NC State Extension — 2026 Fungicide Spray Guide for Tomato in North Carolina: https://plantpathology.ces.ncsu.edu/documents/1105784/2026-NCSU-Fungicide-Spray-Guide-for-Tomato-in-North-Carolina-1.pdf
- Smart Garden Guru — Tomato Blight Treatment Guide: Control & Prevention: https://smartgardenguru.com/blog/tomato-blight-treatment-guide-control-prevention
- University of Minnesota Extension — Parasitic Diseases of Tomato: https://conservancy.umn.edu/server/api/core/bitstreams/c23fe550-e6e5-4118-ae0a-5c8df5742ba9/content