Potato Early Blight (Alternaria solani): Target Spot Treatment Guide (2026)
クイック回答
Remove affected lower leaves and apply chlorothalonil fungicide (2g/L) every 7–10 days. Start spraying preventively when plants reach 15–20cm height, especially in warm, humid weather. For organic growers, use copper hydroxide (2.5g/L) and maintain 3-year crop rotation. Hill soil around stems to protect tubers from spore splash. Ensure proper spacing (75×30cm) for airflow.
どのように見えますか？
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Early stage
Small (1–3mm), dark brown spots appear on the oldest, lowest leaves. Spots have faint concentric rings — the classic 'target spot' or 'bull's-eye' pattern. A yellow halo may surround each lesion. Often mistaken for natural aging.
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Advanced stage
Lesions enlarge to 1–2cm with prominent concentric rings. Multiple spots per leaf, causing extensive yellowing and leaf drop. Disease progresses upward from lower to upper canopy. Stems may show dark, elongated lesions near the soil line.
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Severe stage
Most lower and middle foliage is dead. Remaining leaves have multiple coalescing lesions. Premature defoliation reduces tuber size significantly — yield loss of 20–50%. Tubers may develop dark, sunken surface lesions with leathery texture.
治療方法
Apply copper hydroxide spray
Mix copper hydroxide 77% WP at 2.5g per liter of water. Spray on all foliage ensuring both upper and lower leaf surfaces are covered. Copper prevents spore germination on leaf surfaces.
タイミング: Every 7 days starting when plants are 15cm tall; increase to every 5 days during rainy periods
Remove infected lower leaves
Hand-pick or prune all leaves showing target-spot lesions. Bag and remove from the field — do not compost. This reduces spore production by up to 60% and improves air circulation around the canopy.
タイミング: Inspect weekly; remove immediately upon finding symptoms
Apply thick organic mulch
Lay 8–10cm of straw or grass mulch around plants. Mulch prevents soil-splash — the primary way spores reach lower leaves from the ground. Also conserves soil moisture, reducing plant stress.
タイミング: After hilling, maintain throughout the season
Compost tea foliar spray
Brew aerated compost tea for 24 hours (1kg mature compost per 10L water with an aquarium pump). The beneficial microorganisms compete with Alternaria on leaf surfaces.
タイミング: Every 10–14 days as a preventive supplement
最適：小規模農家、有機認証、家庭菜園
Chlorothalonil 75% WP
Mancozeb 75% WP
Azoxystrobin 25% SC (Amistar)
最適：大規模農業、深刻な発生
Use certified disease-free seed potatoes
Buy seed from certified suppliers who test for Alternaria. Infected seed tubers are a primary source of introduction into new fields. Even small surface lesions can carry enough inoculum to establish the disease.
Maintain 3-year crop rotation
Do not plant potatoes, tomatoes, eggplant, or peppers (all Solanaceae) in the same field for at least 3 years. Rotate with cereals (maize, wheat) or legumes (beans, peas) to break the disease cycle.
Proper hilling and spacing
Hill soil 15–20cm around stems at 4 and 8 weeks after planting. Space rows 75cm apart, plants 30cm within rows. Good hilling protects tubers from spore-laden rain splash; spacing improves air circulation.
Adequate and consistent irrigation
Drought-stressed plants are far more susceptible. Use drip irrigation to maintain consistent soil moisture without wetting foliage. Avoid overhead sprinklers, which spread spores and prolong leaf wetness.
最良の治療は予防です
この問題が発生するとき
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気温
24–29°C (optimal for Alternaria sporulation and infection)
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湿度
Alternating wet and dry conditions; needs 1–2 hours leaf wetness for infection
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降雨量
Rain splash moves spores from soil to lower leaves; prolonged wet weather accelerates spread
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季節
Mid to late season — disease worsens as plants mature and senesce
Potato農家向け
For potato growers: early blight is called 'early' because it appears earlier in the season than late blight, not because it's less serious. In warm climates, it can cause more total yield loss than late blight. Start your fungicide program preventively at 15–20cm plant height — don't wait for symptoms. The most cost-effective strategy is: certified seed + proper hilling + chlorothalonil/mancozeb rotation every 7–10 days. Varieties with partial resistance include 'Kennebec,' 'Elba,' and 'Kufri Jyoti' (India). Maintain adequate potassium and phosphorus — K-deficient plants are especially vulnerable.
農家もよく尋ねる質問
What's the difference between early blight and late blight on potatoes?
Early blight (Alternaria solani) causes small, dark spots with concentric target rings, starting on lower leaves, favoring warm weather (24–29°C). Late blight (Phytophthora infestans) causes large, water-soaked, rapidly expanding lesions with white fuzzy growth underneath, in cool wet weather (10–20°C). Late blight is faster and more destructive but early blight is more persistent.
Can I eat potatoes from plants infected with early blight?
Yes. Tubers with minor surface lesions can be eaten after peeling — the flesh inside is typically fine. However, tubers with deep, sunken lesions should be discarded as secondary rot organisms (bacteria) often invade through blight lesions. Always cure harvested tubers at 15°C for 10–14 days before storage.
Why does early blight always start on the bottom leaves?
Three reasons: (1) lower leaves are closest to the soil where Alternaria spores overwinter, and rain splashes spores upward; (2) lower leaves are oldest and weakest — the plant redirects nutrients to upper growth; (3) lower canopy has the poorest air circulation and stays wet longest after rain or dew.
How often should I spray fungicide for early blight?
In dry conditions: every 10–14 days. In wet or humid conditions: every 5–7 days. Always start before symptoms appear (at 15–20cm plant height). Alternate between at least 2 different fungicide groups (e.g., chlorothalonil and azoxystrobin) to prevent resistance. Reapply after >25mm of rainfall.
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