Photo credit CSU Extension.
Tomato blight is a general term for several fungal-like diseases that attack tomato plants, causing leaf spots, stem damage, and fruit rot.
The two most common types are early blight and late blight. While they have different causes, both can weaken plants, reduce yields, and spread quickly under cool, wet, or humid conditions.
Symptoms typically appear on lower leaves as tiny dark brown spots on lower leaves soon after fruit set. Spots enlarge to >0.5” in diameter and develop a grayish-white center with a darker border. As the spots enlarge, they develop concentric, target-like rings. Spots may also develop on fruit and stems.
Fortunately, good gardening practices can significantly reduce the risk of blight.
Early Blight typically appears first on the lower leaves as small brown spots with distinctive concentric rings, giving them a "target-like" appearance. As the disease progresses, leaves turn yellow, wither, and drop, leaving plants stressed and fruit exposed to sunscald. Early blight often survives in infected plant debris and soil, making garden sanitation especially important.
Late Blight is more aggressive and can destroy a tomato crop in a matter of days under favorable conditions. Symptoms include large, dark, water-soaked lesions on leaves and stems, followed by white, fuzzy growth on the undersides of leaves during humid weather. Fruit develops firm, brown patches that continue to rot after harvest.
How to Prevent Tomato Blight
1. Plant Resistant Varieties
Choose tomato varieties with resistance or tolerance to common diseases whenever possible. There are many varieties of resistant tomatoes which will be described in seed catalogues. Resistance does not guarantee that early blight will not occur, but damage will be less severe.
2. Give Plants Plenty of Space
Space tomatoes 24–36 inches apart to improve air circulation and help foliage dry quickly after rain or watering.
3. Water at the Base
Avoid wetting the leaves. Drip irrigation or soaker hoses are ideal because they keep foliage dry and reduce disease spread.
4. Mulch Around Plants
Apply 2–3 inches of straw, shredded leaves, or compost around the base of plants to prevent soil from splashing onto leaves during rain or watering.
5. Prune Lower Leaves
Remove the bottom 6–12 inches of foliage once plants are established to improve airflow and reduce contact with soil.
6. Rotate Crops
Avoid planting tomatoes, peppers, potatoes, or eggplants in the same location for at least three years, as these crops share many of the same diseases.
7. Remove Infected Leaves Promptly
At the first sign of disease, prune affected leaves and dispose of them in the trash—don't compost heavily infected material.
8. Keep Plants Healthy
Consistent watering, proper fertilization, and sturdy support systems help tomatoes better withstand disease pressure.
Colorado Considerations
Colorado's dry climate naturally reduces blight compared to more humid regions, but summer thunderstorms and overhead irrigation can create conditions that favor disease. Good spacing, mulching, and watering at the soil level go a long way toward keeping plants healthy.
Organic Management
If blight has been a recurring problem, preventive fungicides containing copper or biological products containing Bacillus subtilis can help protect healthy foliage when applied before disease becomes widespread. Always follow the product label and reapply as directed, especially after periods of frequent rain.
CSU Extension article about tomato blight is here.
Symptoms typically appear on lower leaves as tiny dark brown spots on lower leaves soon after fruit set. Spots enlarge to >0.5” in diameter and develop a grayish-white center with a darker border. As the spots enlarge, they develop concentric, target-like rings. Spots may also develop on fruit and stems.
Fortunately, good gardening practices can significantly reduce the risk of blight.
Early Blight typically appears first on the lower leaves as small brown spots with distinctive concentric rings, giving them a "target-like" appearance. As the disease progresses, leaves turn yellow, wither, and drop, leaving plants stressed and fruit exposed to sunscald. Early blight often survives in infected plant debris and soil, making garden sanitation especially important.
Late Blight is more aggressive and can destroy a tomato crop in a matter of days under favorable conditions. Symptoms include large, dark, water-soaked lesions on leaves and stems, followed by white, fuzzy growth on the undersides of leaves during humid weather. Fruit develops firm, brown patches that continue to rot after harvest.
How to Prevent Tomato Blight
1. Plant Resistant Varieties
Choose tomato varieties with resistance or tolerance to common diseases whenever possible. There are many varieties of resistant tomatoes which will be described in seed catalogues. Resistance does not guarantee that early blight will not occur, but damage will be less severe.
2. Give Plants Plenty of Space
Space tomatoes 24–36 inches apart to improve air circulation and help foliage dry quickly after rain or watering.
3. Water at the Base
Avoid wetting the leaves. Drip irrigation or soaker hoses are ideal because they keep foliage dry and reduce disease spread.
4. Mulch Around Plants
Apply 2–3 inches of straw, shredded leaves, or compost around the base of plants to prevent soil from splashing onto leaves during rain or watering.
5. Prune Lower Leaves
Remove the bottom 6–12 inches of foliage once plants are established to improve airflow and reduce contact with soil.
6. Rotate Crops
Avoid planting tomatoes, peppers, potatoes, or eggplants in the same location for at least three years, as these crops share many of the same diseases.
7. Remove Infected Leaves Promptly
At the first sign of disease, prune affected leaves and dispose of them in the trash—don't compost heavily infected material.
8. Keep Plants Healthy
Consistent watering, proper fertilization, and sturdy support systems help tomatoes better withstand disease pressure.
Colorado Considerations
Colorado's dry climate naturally reduces blight compared to more humid regions, but summer thunderstorms and overhead irrigation can create conditions that favor disease. Good spacing, mulching, and watering at the soil level go a long way toward keeping plants healthy.
Organic Management
If blight has been a recurring problem, preventive fungicides containing copper or biological products containing Bacillus subtilis can help protect healthy foliage when applied before disease becomes widespread. Always follow the product label and reapply as directed, especially after periods of frequent rain.
CSU Extension article about tomato blight is here.
Tomato blight is much easier to prevent than to cure. By combining resistant varieties, proper spacing, mulching, careful watering, crop rotation, and regular inspection, you can greatly reduce the risk of disease and enjoy a productive harvest of healthy tomatoes.