Late blight on tomatoes

Late blight is the disease that caused the Irish Potato Famine of the 1840s. The same pathogen -- Phytophthora infestans -- attacks tomatoes as well as potatoes, and in favorable years it can destroy a healthy tomato planting within days to two weeks. The 2009 epidemic in the northeastern US,.

—- title: "Late blight on tomatoes" slug: late-blight-on-tomatoes hub: problems category: "Disease-by-host" description: "Late blight can destroy tomato plants within days. Identify the water-soaked lesions, understand why 2009-type epidemics happen, and apply fungicides before the disease arrives in your region." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Late blight is the disease that caused the Irish Potato Famine of the 1840s. The same pathogen — Phytophthora infestans — attacks tomatoes as well as potatoes, and in favorable years it can destroy a healthy tomato planting within days to two weeks. The 2009 epidemic in the northeastern US, spread through infected transplants sold at big-box retailers, demonstrated that the disease can arrive in suburban gardens unexpectedly and spread rapidly through an entire region.

I don't grow tomatoes at my Long Island property, so this guide is sourced entirely from University Extension and USDA publications.

The pathogen

Late blight is caused by Phytophthora infestans, an oomycete (not a true fungus) in the same pathogen group as downy mildews. Per Cornell Cooperative Extension, in much of the northern US, P. infestans does not survive in outdoor soil over winter — it overwinters primarily in infected potato tubers (cull piles, unharvested potatoes, seed tubers). New strains introduced from nursery-level infected transplants, as in 2009, can initiate epidemics in seasons and regions where the disease would not normally appear.

Per Penn State Extension, the pathogen produces airborne sporangia that are dispersed by wind over long distances — a single infected plant can spread the disease to gardens and farms miles away.

Identification

Leaf symptoms

Per Penn State Extension, late blight on tomato produces:

1. Large, water-soaked, irregular lesions — initially appearing as oily, water-soaked patches on leaves; unlike early blight's defined target spots, late blight lesions are large (0.5 inch to several inches) with irregular, poorly defined margins
2. Tan-brown necrosis — water-soaked areas rapidly turn brown and dry in dry weather, or remain water-soaked and collapse in wet weather
3. White sporulation on leaf undersides — under humid conditions (especially overnight), the underside of lesions develops a white to pale gray cottony sporulation — P. infestans producing massive numbers of sporangia
4. Rapid progression — in cool, wet conditions (60–70°F, 15–21°C, with extended leaf wetness), the disease can destroy entire leaves overnight and kill plants within 3–7 days

Stem and fruit symptoms

Per Cornell Cooperative Extension, infected stems show dark, greasy, water-soaked lesions that girdle and kill the tissue above. Fruit develop firm, brown-black lesions that are greasy in texture and often have an uneven, corrugated surface. Fruit lesions extend deep into the flesh, unlike the surface lesions of early blight or anthracnose.

Distinguishing from early blight

Conditions that favor late blight

Per Penn State Extension, late blight requires:

• Temperatures of 60–70°F (15–21°C) — most severe in cool, overcast weather
• Prolonged leaf wetness (10+ hours from dew, rain, or fog)
• High relative humidity (90%+)

Late blight is typically worst in late summer when cool fronts arrive, in foggy coastal regions, and during wet La Niña years. Hot, dry summers suppress the disease significantly.

Disease monitoring resources

Per Cornell Cooperative Extension, the USAblight.org network tracks confirmed late blight reports across the US and provides forecast maps showing when the disease is moving toward specific regions. Per Penn State Extension, growers who monitor this network and initiate preventive fungicide programs before the disease arrives in their region are far better protected than those who respond to visible symptoms.

Management

Fungicides — begin before arrival

Per Penn State Extension, preventive fungicide applications must be in place before the disease arrives. Once late blight is visible, the plant is already extensively colonized. Effective registered active ingredients (note: oomycete chemistry differs from standard fungicides):

• Chlorothalonil — effective protectant; apply every 5–7 days during high-risk periods
• Copper-based fungicides — OMRI-listed; apply every 5–7 days during wet periods; the traditional organic option
• Mancozeb — effective protectant
• Cymoxanil + mancozeb combinations — systemic activity; most effective when disease is imminent
• Phosphonates — systemic oomycide; effective as preventive
• Fluopicolide + propamocarb — systemic; highly effective; labeled for home use in some formulations

Do not use standard fungicides (thiophanate-methyl, azoxystrobin alone, captan) as the primary defense against late blight — these have little efficacy against oomycetes.

Remove and destroy infected plants immediately

Per Cornell Cooperative Extension, a plant showing late blight symptoms is a massive spore source for neighbors' gardens and nearby farms. Remove it promptly, place it in a plastic bag without shaking, and dispose of it in the trash — not compost. Alternatively, per Penn State Extension, kill all foliage with a contact herbicide (glyphosate at twice normal rate) before removal to prevent spore release during handling.

Avoid overhead irrigation

Per Penn State Extension, drip irrigation eliminates the overnight leaf wetness that enables late blight infection and sporulation. If overhead irrigation is used, apply only in the morning so foliage dries before nightfall.

Resistant varieties

Per Cornell Cooperative Extension, resistance to late blight has been bred into some tomato varieties. 'Plum Regal', 'Mountain Magic', 'Defiant PhR', and 'Jasper' (cherry) show meaningful late blight resistance in Cornell and NC State trials. Resistance is partial — resistant varieties require less intense fungicide programs but may still show disease under severe epidemic conditions.

Common problems table

Frequently Asked Questions

Is late blight the same disease that caused the Irish Famine?

Per Cornell Cooperative Extension, yes — Phytophthora infestans caused the Irish potato famine of 1845–1849. The pathogen has evolved since then; current strains in North America include new genotypes that may differ in aggressiveness and fungicide sensitivity from historical strains, but the biology is the same.

Can I eat tomatoes from a plant with late blight?

Per Penn State Extension, fruit without visible lesions from a plant with foliar late blight can be consumed, though the shelf life will be short. Fruit with late blight lesions should be discarded — the affected flesh extends beyond the visible lesion boundary.

Why did late blight appear in my garden when it never has before?

Per Penn State Extension, late blight can arrive from distant sources via wind-blown sporangia. Following the 2009 epidemic pattern, infected transplants can introduce the disease to a garden. The disease is not endemic in northern soils the way early blight is — it needs to arrive each season, but arrival is not predictable.

Do I need to treat my soil after late blight?

Per Cornell Cooperative Extension, P. infestans does not persist in soil as a dormant structure. Infected potato tubers are the main overwintering host; the soil itself does not need treatment. Removing all plant debris and any unharvested potato tubers eliminates the overwinter reservoir.

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Recommended gear: Best tomato varieties for the home garden — determinate vs indeterminate — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Sources

1. Penn State Extension — Late Blight of Tomato and Potato
2. Cornell Cooperative Extension — Late Blight Management
3. USAblight.org — Late Blight Forecasting Network