📈 Diseases of Tomato — Early Blight, Late Blight, Wilt

Tomato (Solanum lycopersicum) is one of the most widely grown vegetable crops in India. It is susceptible to a wide range of fungal, bacterial, and viral diseases that cause significant economic losses.

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Early Blight

Causal Organism: Alternaria solani Sorauer

Symptoms

• Dark brown to black, circular lesions with concentric rings (target-board pattern) on older leaves
• Lesions often surrounded by a yellow chlorotic halo
• Lower leaves are affected first, and the disease progresses upward
• On fruits, dark, sunken, leathery spots develop near the stem end (fruit rot)
• Severe defoliation reduces fruit size and yield

Disease Cycle

The fungus survives on infected crop debris in the soil and on volunteer tomato plants. Conidia are wind-borne and splash-dispersed. Warm temperatures (25-30 degrees C), high humidity, and heavy dew favour infection.

Management

• Practice crop rotation with non-solanaceous crops for 2-3 years
• Remove and destroy infected plant debris after harvest
• Spray Mancozeb (0.25%) or Chlorothalonil (0.2%) at first symptom appearance
• Use resistant or tolerant varieties where available
• Mulching reduces soil splash and lowers disease pressure

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Late Blight

Causal Organism: Phytophthora infestans (Mont.) de Bary — an Oomycete

Late blight is the same pathogen responsible for the Great Irish Potato Famine (1845-1849).

Symptoms

• Large, irregular, water-soaked, pale green to dark brown lesions on leaves
• White, downy sporulation visible on the lower leaf surface under humid conditions
• Stems show dark brown to black lesions, often girdling the plant
• Fruits develop firm, brown, greasy-looking patches

Favourable Conditions

Factor	Optimal Range
Temperature	15-22 degrees C
Humidity	Above 90%
Rainfall	Prolonged wet periods
Dew	Heavy, persistent dew

Management

• Spray Metalaxyl + Mancozeb (Ridomil MZ, 0.25%) or Cymoxanil + Mancozeb preventively
• Destroy volunteer plants and crop debris
• Avoid overhead irrigation; use drip irrigation instead
• Use resistant varieties such as Arka Rakshak and Arka Samrat
• Monitor weather forecasts and initiate sprays before predicted wet spells

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Fusarium Wilt

Causal Organism: Fusarium oxysporum f. sp. lycopersici (Sacc.) Snyd. & Hans.

Symptoms

• Yellowing and wilting of leaves on one side of the plant initially (unilateral wilting)
• Upward curling of leaf margins
• Brown discolouration of vascular bundles visible when the stem is cut longitudinally
• Progressive wilting and death of the entire plant

Management

• Use resistant varieties carrying the I-1, I-2, or I-3 resistance genes
• Soil solarisation during summer months (April-May) with transparent polyethylene sheets
• Apply Trichoderma viride (4 g/kg seed) as seed treatment and soil application
• Maintain soil pH above 6.5 (liming reduces disease severity)
• Practice crop rotation for at least 4-5 years

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Bacterial Wilt

Causal Organism: Ralstonia solanacearum

• Rapid, sudden wilting of the entire plant without yellowing
• Cut stems placed in water show milky bacterial ooze streaming from the vascular tissue
• Soil drenching with Bleaching powder (10 g/L) and use of resistant rootstocks are key management measures

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Summary Cheat Sheet

Disease	Causal Organism	Hallmark Symptom	Core Management
Early blight	Alternaria solani	Target-board leaf lesions on older foliage	Crop rotation + protective fungicides
Late blight	Phytophthora infestans	Water-soaked lesions with white downy growth	Preventive anti-oomycete spray schedule
Fusarium wilt	Fusarium oxysporum f. sp. lycopersici	Unilateral wilt with vascular browning	Resistant varieties + soil sanitation
Bacterial wilt	Ralstonia solanacearum	Sudden wilt with bacterial ooze	Field hygiene + resistant rootstocks

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

CABI Crop Protection Compendium

Website