Alternaria solani (Early Blight): Symptoms, Identification and Control
Alternaria solani (Early Blight): Symptoms, ID and Control Meta Description: How to identify early blight on potato and tomato, tell it apart from late blight, and control it with rotation, nutrition and fungicide programmes.
Quick identification
| Early blight (Alternaria solani) | |
|---|---|
| First appears on | Oldest, lowest leaves |
| Lesion shape | Angular to circular, limited by leaf veins |
| Key marker | Concentric rings inside the lesion — the “target spot” |
| Halo | Narrow yellow border |
| Weather | Warm days, alternating wet and dry periods |
| Tubers | Dark, sunken lesions with a raised, well-defined margin |
If the lesion has visible rings and sits on a lower leaf of an older plant, early blight is the first thing to rule in.
The video shows a potato leaf affected by early blight (Alternaria solani). The older lower leaves have characteristic dark brown lesions with distinct concentric rings, creating a “target-like” pattern. Yellowing around the spots, drying leaf edges, and progressive tissue damage are clearly visible.
The healthy green upper canopy contrasts with the infected lower leaves, illustrating the typical development of early blight on a potato plant.
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What Alternaria solani is
Alternaria solani is a necrotrophic fungus that causes early blight on potato, tomato and other solanaceous crops. Unlike a biotrophic pathogen, it kills plant tissue first and feeds on the dead cells, which is why it targets ageing and stressed foliage before healthy young growth.
The fungus survives between seasons in infected crop residue, in the soil, on volunteer plants, on solanaceous weeds, and on seed tubers. Spores are carried by wind, splashing rain and irrigation water, so the disease usually moves upward through the canopy from the lowest leaves.
Development is favoured by warm temperatures and repeated cycles of leaf wetness followed by drying. Continuous rain is less important than the alternation itself — dew, overhead irrigation and brief showers on warm days are ideal conditions.
Symptoms on potato
Leaves. Small dark brown to black spots appear on the lowest, oldest leaves. As they expand they become angular, bounded by the leaf veins, and develop the concentric rings that give the disease its “target spot” name. A narrow chlorotic halo often surrounds the lesion. Heavily infected leaves yellow, dry out and drop, and defoliation progresses upward.
Stems and petioles. Elongated dark lesions may form, sometimes with the same ringed pattern.
Tubers. Lesions are dark, slightly sunken and roughly circular, with a distinct raised margin separating diseased from healthy tissue. Beneath the surface the flesh shows a dry, corky, brown rot. Infection usually occurs during harvest when spores on the soil surface contact wounded tubers.
Yield impact. Early defoliation shortens the bulking period. Losses are driven less by the individual lesions than by how many weeks of green canopy the crop loses before senescence.
Symptoms on tomato
Seedlings. Dark lesions can girdle the stem at soil level — collar rot — killing young transplants.
Leaves. Same pattern as potato: dark spots with concentric rings on the lower leaves first, yellowing, then defoliation. Loss of canopy exposes fruit to sunscald.
Stems. Dark, sunken, ringed lesions.
Fruit. Infection enters at the stem end or through growth cracks and wounds. Lesions are dark, leathery and sunken, often with concentric rings and a velvety black spore layer in humid weather.
Early blight vs late blight: how to tell them apart
This is the distinction that matters most in the field, because the two diseases demand different responses and different chemistry.
| Early blight (Alternaria solani) | Late blight (Phytophthora infestans) | |
|---|---|---|
| Lesion edge | Sharp, angular, vein-limited | Irregular, water-soaked, diffuse |
| Pattern | Concentric rings | No rings |
| Colour | Dark brown, dry | Grey-green turning brown-black, greasy |
| Underside | No white growth | White sporulation ring in humid conditions |
| Starts on | Oldest lower leaves | Any leaf, often upper canopy |
| Weather | Warm, alternating wet/dry | Cool and persistently wet |
| Speed | Gradual, over weeks | Can destroy a crop in days |
| Smell | None | Distinct musty odour in wet rot |
Also consider brown spot (Alternaria alternata): lesions are smaller, more numerous and generally lack the strong concentric ringing. The two are frequently confused, and they differ in fungicide sensitivity, so laboratory confirmation is worthwhile when management decisions depend on it.
Why some fields get hit harder
Early blight is strongly tied to plant stress. The same variety in two fields can behave very differently depending on:
- Nitrogen status. Nitrogen-deficient crops senesce earlier and are markedly more susceptible.
- Crop maturity. Early-maturing varieties are hit sooner and harder than late-maturing ones.
- Other stresses. Drought, nematode pressure, potassium deficiency and viral infection all accelerate the disease.
- Rotation history. Short rotations leave infected residue in the field.
- Irrigation method. Overhead irrigation late in the day extends leaf wetness overnight.
Cultural control
Rotate out of solanaceous crops for at least two to three years. Rotation reduces the inoculum carried in residue, though airborne spores from neighbouring fields limit how much any single grower can achieve alone.
Manage residue. Deep incorporation or removal of infected haulm speeds decomposition and reduces overwintering inoculum.
Choose varieties carefully. Full resistance is not available, but tolerance differs substantially and is often linked to maturity. Later-maturing varieties generally hold their canopy longer.
Keep nitrogen adequate through bulking. Maintaining canopy vigour is one of the most effective and least expensive controls available.
Irrigate to shorten leaf wetness. Water early in the day; consider drip where practical.
Control solanaceous weeds and volunteers, which act as a green bridge between seasons.
Handle tubers gently at harvest. Allow skins to set, avoid bruising, and skip harvesting from wet soil where spore contact is greatest.
Chemical control and resistance management
Protectant fungicides applied before symptoms appear are far more effective than curative treatment once lesions are established.
Multi-site protectants — chlorothalonil (FRAC M5) and mancozeb (FRAC M3) — carry a low resistance risk and form the backbone of most programmes.
Single-site products are more active but vulnerable to resistance:
- QoI / strobilurins (FRAC 11) — azoxystrobin, pyraclostrobin. Resistance associated with the F129L mutation is now widespread in A. solani populations across many production regions, and field performance has declined accordingly.
- SDHI (FRAC 7) — boscalid, fluopyram, penthiopyrad. Mutations in the sdh genes have been documented, and sensitivity varies by region.
- DMI / triazoles (FRAC 3) — difenoconazole and others.
Resistance management basics: alternate FRAC groups, never apply a single-site product alone, limit the number of applications per group per season, and follow local label restrictions. Always verify product registration and rates for your country and crop.
Timing. Forecasting models help place the first spray. The P-Day model is used to time the initial potato application based on accumulated physiological days after emergence. TOM-CAST accumulates disease severity values from leaf wetness and temperature data and is widely used in tomato. Both outperform calendar spraying by targeting the periods when infection is actually likely.
Frequently asked questions
Does early blight affect the tubers I have already stored? Tuber lesions do not usually expand much in storage, but the damaged tissue is an entry point for soft rot bacteria. Cure the crop properly and keep storage dry and well ventilated.
Can I eat potatoes with early blight lesions? The affected tissue should be cut away. The concern is secondary rot rather than the fungus itself.
Will fungicides cure existing lesions? No. Fungicides protect healthy tissue. Existing lesions remain, which is why programmes begin before symptoms are visible.
Is early blight seed-borne? The fungus can be carried on seed tubers, so certified seed and rotation both matter.
Why does it always start on the bottom leaves? Older leaves have lower physiological resistance, and they sit closest to the inoculum reservoir in soil and residue.
Have you dealt with early blight on your farm this season? Share your experience and photos with us — we publish field reports from growers worldwide.
