Early blight is one of the most common and destructive diseases affecting tomato plants. This fungal disease can quickly defoliate tomato plants, leading to reduced yield and even plant death in severe cases. Knowing how to identify, prevent, and treat early tomato blight is key for any home gardener looking to grow tomatoes successfully.
What Causes Early Tomato Blight?
Early tomato blight is caused by the fungus Alternaria solani. This fungus thrives in warm, humid conditions and spreads rapidly under these ideal conditions. A. solani produces spores that can overwinter in plant debris and soil, providing an early source of inoculum in the spring. These spores infect tomato leaves and stems, producing the characteristic symptoms of early blight.
How to Identify Early Tomato Blight
Identifying early blight symptoms is crucial for effective treatment. Here are the key signs to look for:
Leaf Spots
Small, brown spots first appear on older tomato foliage near the ground. These lesions enlarge to form more distinct circular or angular brown spots up to 1/2 inch across. Centers of lesions may fall out, giving the leaf a shot hole appearance.
Stem Lesions
Sunken, dark brown to black lesions develop on tomatoes stems. These lesions may expand to girdle stems, causing wilting of foliage above. Lesions can also form on petioles, causing leaf drop.
Defoliation
As the disease progresses, tomato plants rapidly lose leaves beginning on the lower branches. Entire plants may be completely defoliated in advanced stages of infection.
Fruit Spots
Small, dark, leathery spots can develop on tomato fruit. These spots usually remain small and superficial but can become entry points for secondary fruit rot pathogens.
Early Tomato Blight vs. Late Tomato Blight
It’s important not to confuse early blight caused by A. solani with late blight caused by Phytophthora infestans. While both diseases affect tomato foliage and stems, there are a few key differences:
- Leaf lesions: Early blight lesions are typically brown, while late blight lesions are brownish-black. Late blight lesions are also more irregularly shaped.
- Leaf undersides: Early blight only infects upper leaf surfaces. Late blight will also grow on undersides of leaves.
- Stem lesions: Early blight stem lesions are dark brown to black. Late blight stem lesions are brownish-black with a greasy appearance.
- Weather conditions: Early blight favors warm, dry weather. Late blight thrives in cooler, wetter conditions.
- Fruit symptoms: Early blight causes small, dry spots. Late blight causes large, firm lesions.
How to Prevent Early Tomato Blight
Preventing early blight infection is much easier than treating an existing infection. Here are some tips:
Start with Resistant Varieties
Plant tomato varieties with resistance or tolerance to early blight. Look for designations like EB, EBT, or ABE on plant tags. Parthenocarpic varieties also show tolerance.
Crop Rotation
Avoid planting tomatoes in the same spot as last year. Rotate tomato locations on at least a 3-year cycle. This prevents buildup of blight fungal spores in the soil.
Stake and Prune Plants
Staking or caging tomato plants improves air circulation through the foliage. Prune plants to an open canopy by removing lower leaves and suckers. This keeps leaves dry.
Water at Base of Plants
Avoid overhead watering. Instead, use drip irrigation or soaker hoses to deliver water right to the soil. This prevents fungal spore spread via water splash.
Apply Preventative Fungicides
Apply broad spectrum fungicides like chlorothalonil or mancozeb on a regular schedule just before disease appears. Organic options include copper fungicides or Bacillus subtilis.
Sanitize Tools and Remove Debris
Disinfect tools between plants with a 10% bleach solution to avoid spreading spores. Remove and destroy all tomato debris after harvest.
How to Treat an Early Tomato Blight Infection
If preventative steps fail and early blight strikes, rapid treatment is needed to save plants:
Remove Infected Leaves
Pick off and destroy any leaves showing early spotting or lesions. This prevents spores from developing and spreading.
Apply Targeted Fungicides
Switch to fungicides effective against early blight like maneb, fenamidone, strobilurins, or biofungicides with Trichoderma harzianum. Always follow label directions.
Increase Air Circulation
Improve airflow by pruning affected plants. Remove branches with more than 50% defoliation or dead stems.
Water Carefully
Water at the base of plants and avoid overhead irrigation. Water early in the day so leaves dry out. Drip irrigation is ideal.
Use Compost Teas
Apply compost teas or extracts to coat leaves with beneficial microbes. These can inhibit growth of disease fungi. Reapply after rain.
Remove Heavily Infested Plants
If more than 75% of a plant is defoliated, it likely can’t be saved. Remove and destroy the plant to prevent further spore spread.
6 Key Questions about Early Tomato Blight
Early blight in tomatoes can be frustrating for home gardeners. Here are answers to some frequently asked questions:
1. When does early tomato blight usually appear?
Early blight infection typically starts after the first fruit sets on tomato plants. It intensifies through the summer, peaking in late summer to early fall. Hot, humid weather favors disease spread.
2. Can you eat tomatoes affected by early blight?
Tomatoes with early blight spots only on the leaves and stems are still safe to eat. Any fruit showing dark, leathery rot spots should be discarded. The fungus does not affect internal fruit flesh unless secondary rots take hold.
3. Does early blight affect all tomato varieties?
Nearly all tomatoes are susceptible to some degree. But indeterminate, beefsteak types often show more susceptibility than compact determinate varieties. Parthenocarpic and some heirlooms have better blight resistance.
4. Can early blight spread to other plants besides tomatoes?
The early blight fungus can infect any plant in the nightshade family, including potatoes, eggplants, and peppers. It also infects cucurbits like melons, cucumbers, and squash.
5. Can you save seeds from an early blight infected plant?
It’s best not to save seeds from diseased tomato plants. The early blight fungus can potentially be seed-borne, leading to infected seedlings the following season. Purchase new seed each year from reputable sources.
6. Does early blight affect tomatoes every year?
In most regions, early blight fungal spores persist year to year in soil and plant debris. But disease severity varies seasonally depending on weather conditions. Hot, humid summers see the worst outbreaks. Dry seasons limit spread.
Conclusion
Left unchecked, early tomato blight can quickly devastate backyard tomato patches and market farms alike. Being able to accurately identify early symptoms allows for timely treatment. Combining preventative measures like resistant varieties and fungicides with cultural practices like staking, pruning, and careful watering gives tomatoes the best chance of thriving, even in blight-prone regions. Catching infections before widespread defoliation occurs is key to saving plants. With vigilance and swift action, gardeners can still successfully grow delicious, bountiful tomatoes despite the threat of this common disease.