Early blight on potatoes

Early blight on potatoes is so common that most gardeners accept some degree of it as a normal part of the growing season. The challenge is that it begins on the oldest lower leaves in midsummer and progresses upward precisely as tubers are sizing -- reduced photosynthesis during this period.

—- title: "Early blight on potatoes" slug: early-blight-on-potatoes hub: problems category: "Disease-by-host" description: "Early blight on potatoes reduces yield by killing leaves prematurely. Identify the target spots, distinguish from late blight, and manage with fungicide timing and variety selection." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Early blight on potatoes is so common that most gardeners accept some degree of it as a normal part of the growing season. The challenge is that it begins on the oldest lower leaves in midsummer and progresses upward precisely as tubers are sizing — reduced photosynthesis during this period directly reduces tuber yields. Managing early blight is therefore a yield management question, not just an aesthetic one.

I don't grow potatoes at my Long Island property, so this guide draws on University Extension research.

The pathogen

Early blight of potato is caused by Alternaria solani, the same species that causes early blight of tomato. Per Penn State Extension, the fungus overwinters in infected plant debris in soil and on infected tubers; seed potatoes infected with the pathogen can introduce it to new garden sites. Conidia (spores) are splash-dispersed from soil and infected leaves to healthy foliage.

A related species, Alternaria alternata, causes similar but less severe lesions on potato under some conditions.

Identification

Leaf symptoms

Per Cornell Cooperative Extension, early blight on potatoes produces:

1. Target-spot lesions — the classic bull's-eye pattern of concentric brown rings, 0.25–0.5 inch in diameter, on lower leaves first
2. Yellow halo — a ring of chlorotic tissue around the lesion
3. Lower leaf preference — oldest, lowermost leaves are infected first; the disease progresses upward
4. Premature leaf death — infected leaves yellow completely and drop, creating the progressive "bottom-up" defoliation pattern

Distinguishing early blight from late blight

Per Penn State Extension, the two diseases are frequently confused. Key differences:

In most gardens, early blight is far more common; late blight requires cool, wet conditions and is epidemic only in specific years. See Late blight on potatoes for full management.

Tuber symptoms

Per Clemson HGIC, early blight can infect tubers, producing irregular, dark, slightly sunken lesions on the skin. Infected tissue beneath the skin is brown and dry. Tuber early blight is less common than foliage disease but can reduce storability.

Conditions that favor disease

Per Penn State Extension:

• Warm temperatures (75–85°F, 24–29°C)
• Wet periods alternating with dry periods (which produce the "binge-purge" stress cycle that weakens plants)
• Nutrient-stressed plants, especially those deficient in nitrogen
• Soil previously planted with tomatoes or potatoes

Yield impact

Per Cornell Cooperative Extension, the timing of defoliation relative to tuber development determines yield impact:

• Early blight that removes significant foliage before tubers reach full size (before tuber skin-set in late summer) directly reduces yield
• Early blight that progresses after skin-set and natural vine dieback has minimal additional yield impact

The practical implication is that early blight appearing in July is more consequential than the same disease appearing in late August on a crop that has largely finished tuber sizing.

Management

Mulching

Per Penn State Extension, 3–4 inches of straw or other organic mulch between potato rows reduces soil splash of spores onto lower leaves. Apply mulch after plant emergence and hill-up.

Crop rotation

Per Cornell Cooperative Extension, rotate potatoes and tomatoes to different garden sections on a 2–3 year cycle. Both crops are susceptible to A. solani, and growing them in succession in the same location builds soil inoculum levels.

Seed potato selection

Per Penn State Extension, use certified disease-free seed potatoes rather than saved tubers from previous plantings. Infected seed potatoes introduce the pathogen directly to the new planting site.

Fertility management

Per Cornell Cooperative Extension, nitrogen-deficient plants are significantly more susceptible to early blight. Ensure adequate nitrogen from transplant through tuber bulk-up. Side-dress with a nitrogen source when plants are 6–8 inches tall if soil test indicates deficiency.

Fungicides

Per Penn State Extension, preventive fungicide applications reduce disease severity when begun at plant canopy closure (before disease appears) or at first symptom. Apply every 7–10 days during warm, wet periods. Registered active ingredients:

• Chlorothalonil — effective protectant; widely available for home use
• Mancozeb — effective; observe pre-harvest intervals (check label)
• Copper-based fungicides — organic option
• Azoxystrobin — systemic; effective; rotate with contact fungicides

Pre-harvest intervals on fungicide labels must be observed; do not apply within the restricted period before harvest.

Variety selection

Per NC State Extension, some potato varieties show better early blight tolerance than others. Russet Burbank, the standard commercial variety, is relatively susceptible. Varieties including Allegany, Kennebec, and Red Pontiac show better field tolerance in comparative trials. Consult current regional variety trial data from Cornell or Penn State for current recommendations.

Common problems table

Frequently Asked Questions

Can I use tomatoes and potatoes in the same rotation beds?

Per Penn State Extension, no. Both are susceptible to Alternaria solani. Effective rotation requires separating all solanaceous crops (tomatoes, potatoes, eggplant, peppers) from each other for 2–3 years in a given bed.

Are cut seed potatoes from last year's crop safe to plant?

Per Cornell Cooperative Extension, saved tubers can carry A. solani into new beds. Certified seed potatoes from reputable suppliers are screened for major pathogens and are the lower-risk option, particularly in gardens with early blight history.

Does early blight on potatoes spread to my tomatoes?

Per Penn State Extension, yes. Alternaria solani infects both tomatoes and potatoes. Keeping these crops well-separated in the garden and rotating them together (not in sequence in the same bed) reduces cross-contamination.

When should I stop applying fungicide before potato harvest?

Per Penn State Extension, check the label of the specific product for pre-harvest intervals. Chlorothalonil labels typically specify 0–7 days before harvest depending on formulation; mancozeb labels specify 7 days. Observe these restrictions.

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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 — Early Blight of Potato
2. Cornell Cooperative Extension — Potato Disease Management
3. Clemson HGIC — Potatoes
4. NC State Extension — Potato Variety Trials