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Jane Westoby

Understanding & Preventing Tomato Blight


Tomato Blight

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Tomato blight is one of the most feared diseases among gardeners and commercial growers alike. It can decimate a crop in a short time and spread to other plants, making prevention and management essential.


What is Tomato Blight?

Tomato blight is a fungal disease which affects tomatoes and related plants, causing damage to the leaves, stems, and fruit. There are two primary types of blight which affect tomatoes:

  1. Early Blight (Alternaria solani): This disease typically affects older plants, leading to spots on leaves, weakened stems, and premature fruit drop. Early blight is common but not usually as devastating as late blight.

  2. Late Blight (Phytophthora infestans): Late blight is more aggressive and notorious for wiping out entire crops. It’s the same pathogen responsible for the Irish Potato Famine in the 19th century. Late blight thrives in warm, moist environments and spreads quickly once established.


Both types of blight can seriously hinder tomato production, reducing yields and the quality of the fruit. However, late blight is the more dangerous of the two, as it is highly contagious and difficult to manage once it takes hold.


What Else Does Tomato Blight Affect?

Tomato blight doesn’t only affect tomatoes. Plants in the nightshade family, particularly potatoes, are also highly susceptible to blight. Late blight is notorious for jumping between tomatoes and potatoes, which is why it’s critical for gardeners to be cautious if they’re growing both crops. Other nightshades like eggplants and peppers can also be affected to a lesser extent, but tomatoes and potatoes are by far the most vulnerable.


How Does Tomato Blight Occur?

Blight occurs when the fungal spores responsible for the disease find the right conditions to thrive. Both early and late blight are favored by warm, humid weather. The spores spread easily through the air, rainwater, or contaminated soil, making it a challenge to contain once it gets a foothold.

Here’s a breakdown of how blight typically establishes itself:

  1. Spores in the Environment: Blight spores can survive in plant debris or infected soil, overwintering until conditions become favorable. For late blight, wind-borne spores can travel several miles, making it hard to predict its spread.

  2. Warm, Wet Conditions: Blight loves warm temperatures (around 15-25°C) paired with high humidity. Heavy rains or watering plants from overhead can create the perfect environment for spores to germinate on wet leaves.

  3. Infected Seedlings or Transplants: Blight can also occur when growers unknowingly introduce infected plants or seedlings into their gardens. This is why starting with healthy, disease-free plants is crucial.

  4. Lack of Air Circulation: Dense plantings, poor air circulation, or failing to prune tomatoes can lead to wet, crowded conditions where blight can thrive. Air movement helps dry the plants and reduce the risk of fungal growth.


How to Prevent Tomato Blight

Prevention is by far the best approach to managing tomato blight, as curing or stopping the disease once it’s taken hold is extremely difficult. Here are several key strategies for preventing both early and late blight:


1. Choose Resistant Varieties

One of the best ways to avoid blight is by choosing blight-resistant tomato varieties. Many modern cultivars are bred specifically to resist early and late blight, providing a genetic shield against the disease.

2. Practice Crop Rotation

Since blight spores can persist in the soil, it’s crucial to avoid planting tomatoes or other nightshades in the same spot each year. Rotate your crops annually, ideally planting tomatoes in a different section of your garden for at least 3-4 years before returning to the same area. This reduces the risk of soil-borne spores infecting new plants.

3. Provide Good Air Circulation

Space your tomato plants adequately to ensure good airflow. Pruning the lower leaves and suckers on your plants can also help increase air circulation, preventing moisture buildup. Proper spacing reduces the chances of fungal spores settling on leaves and encourages faster drying after rain or watering.

4. Water the Right Way

To minimize blight, avoid overhead watering, which can splash fungal spores onto the leaves. Instead, use a soaker hose or drip irrigation to water your plants at the base, keeping the foliage dry. Water early in the day to allow plants time to dry out before evening.

5. Mulch to Prevent Soil Splash

Blight spores can splash onto the lower leaves from infected soil during watering or rain. Mulching around the base of your plants with straw, wood chips, or compost can act as a barrier, reducing the likelihood of spores making contact with the leaves.

6. Remove Infected Debris

At the end of the growing season, be diligent about removing any plant debris from your garden, especially if blight has been present. Infected plant material should be discarded (not composted), as the spores can survive the winter and infect next year’s plants.

7. Apply Fungicide (If Necessary)

In areas where late blight is a recurring problem, you may want to use a fungicide as a preventative measure. Organic options such as copper-based sprays or those containing potassium bicarbonate can be effective at slowing the spread of blight when applied early in the season.


What to Do if You Have Tomato Blight

If you suspect that blight has affected your tomatoes, it’s important to act quickly to prevent further spread. Here’s what you can do if your plants are showing signs of blight:

1. Identify the Disease

First, confirm that what you’re seeing is indeed blight. Early blight typically manifests as brown, concentric rings on older leaves, often with yellowing surrounding the lesions. Late blight appears as water-soaked spots that quickly turn brown or black, with fuzzy white growth often appearing on the undersides of the leaves in humid conditions.

2. Remove Affected Foliage

For early blight, you may be able to slow the disease by pruning away infected leaves as soon as you spot them. Be sure to disinfect your pruning tools between cuts to avoid spreading the disease to healthy plants.

3. Dispose of Infected Plants

If late blight has struck, the best course of action is often to remove and destroy the infected plants entirely to stop the disease from spreading. Do not compost these plants, as the spores can survive and re-infect future crops.

4. Protect Nearby Plants

Once blight is present, it’s important to protect nearby healthy plants. Apply a fungicide to healthy plants to slow the spread, and keep an eye on weather conditions that could promote further disease.

5. Plan for Next Year

If your garden has suffered from blight, it’s essential to plan ahead for future seasons. Rotate your crops, avoid growing tomatoes or potatoes in the same area, and consider using resistant varieties to reduce the likelihood of a recurrence.


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