How to Identify, Control and Prevent Late Blight of Tomatoes (2024)

Tomatoes are a favorite crop for many gardeners, but they are susceptible to a wide range of diseases. Among these, late blight is one of the most infamous and destructive. Understanding how to identify and prevent this disease can save your tomato plants and ensure a bountiful harvest. This article will provide an in-depth guide on late blight, including its causes, symptoms, and effective prevention strategies.

Late blight is a fungal disease caused by the pathogen Phytophthora infestans, infamous for causing the Irish Potato Famine in the 1840s. While it affects both potatoes and tomatoes, its impact on tomato plants can be particularly devastating, rapidly killing plants and destroying fruit. The disease can spread quickly under favorable conditions, making it crucial for gardeners and farmers to be vigilant and take proactive measures to manage it effectively.

What is Late Blight?

Late blight is a disease that primarily affects plants in the Solanaceae family, which includes tomatoes and potatoes. The pathogen responsible, Phytophthora infestans, thrives in cool, moist environments and spreads through spores. These spores can travel long distances by wind and water, making the disease challenging to control once it takes hold in an area.

Historical Significance

The importance of late blight extends beyond the garden, as it has had significant historical impacts. Most notably, it was the cause of the Irish Potato Famine, leading to massive crop failures and widespread famine. Understanding this historical context underscores the need for effective management practices today.

Identification of Late Blight

Identifying late blight early is crucial for managing the disease and minimizing its impact. Here are some key symptoms to look out for:

Symptoms in Leaves

• Dark Lesions: Look for dark, water-soaked spots on the leaves. These spots often have a pale green or yellow halo around them.
• Blight Spots: As the disease progresses, these spots can enlarge and turn brown, often surrounded by a pale border.
• White Fungal Growth: In humid conditions, you might notice a white, fuzzy growth on the undersides of the leaves, which is the spore-producing part of the fungus.
• Wilting: The affected leaves may wilt and die, often remaining attached to the plant.

Symptoms in Stems

• Dark Streaks: Stems may develop dark brown or black streaks, particularly in wet conditions.
• Stem Lesions: These lesions can girdle the stem, causing the plant to collapse.

Symptoms in Fruit

• Dark, Greasy Spots: Look for large, dark, greasy-looking spots on the fruit.
• Rotting: Infected fruits often rot quickly, becoming mushy and inedible.
• White Mold: In severe cases, white mold may appear on the fruit surface.

Key Points for Identification

• Rapid Spread: Late blight can spread quickly, affecting multiple plants in a short period.
• Moisture Dependency: The disease is more prevalent in areas with frequent rain or high humidity.
• Environmental Conditions: Cool temperatures (60-70°F) and wet weather favor the development and spread of the disease.

Environmental Conditions Favoring Late Blight

Understanding the environmental conditions that favor late blight can help in implementing preventive measures.

Temperature

• Optimal Temperature: Late blight thrives at temperatures between 60°F (15°C) and 70°F (21°C).
• Temperature Fluctuations: Large fluctuations in temperature can stress plants, making them more susceptible to infection.

Humidity

• High Humidity: The pathogen requires high humidity to produce spores and spread. Humidity levels above 90% are particularly conducive to disease development.
• Prolonged Wetness: Leaves that remain wet for extended periods are more likely to develop late blight.

Rainfall

• Frequent Rain: Frequent or prolonged rain provides the necessary moisture for the spores to germinate and infect plants.
• Splashing Water: Rainwater can splash spores from the soil onto the plants, aiding in the spread of the disease.

Air Circulation

• Poor Air Circulation: Crowded plantings with poor air circulation are more susceptible to late blight due to increased humidity around the plants.
• Windborne Spores: While spores can be carried by wind, areas with good air circulation are less likely to have humidity buildup, reducing the risk of disease spread.

Identifying Late Blight vs. Early Blight

It’s important to distinguish between late blight and early blight, as they require different management strategies. Here’s a quick comparison:

Aspect	Late Blight	Early Blight
Causal Agent	Phytophthora infestans	Alternaria solani
Leaf Symptoms	Water-soaked spots, pale halo, white growth	Target-like spots, concentric rings
Fruit Symptoms	Greasy spots, rapid decay	Sunken spots, leathery texture
Stem Symptoms	Dark streaks, girdling	Brown streaks, but less severe than late blight
Weather Factors	Cool, wet conditions	Warm, humid conditions

Lifecycle of Late Blight Pathogen

Understanding the lifecycle of the late blight pathogen can provide insights into how it spreads and how to break the cycle.

Spore Production

• Sporulation: The pathogen produces sporangia (spore cases) on infected plant tissue, primarily on leaves and stems.
• Dispersal: Sporangia are dispersed by wind, rain, or water splashes, landing on new plant surfaces.

Germination

• Infection: Once on a susceptible plant, the sporangia germinate in the presence of moisture, forming zoospores that penetrate plant tissue.
• Moisture Dependency: Germination and infection require free water, making wet leaves and stems particularly vulnerable.

Colonization

• Tissue Invasion: The pathogen colonizes plant tissue, spreading through the plant and causing the characteristic symptoms of late blight.
• Rapid Spread: The disease can spread quickly under favorable conditions, often leading to total plant collapse.

Overwintering

• Survival: In regions with mild winters, the pathogen can overwinter in infected plant debris or tubers, becoming a source of inoculum for the following growing season.

Preventing Late Blight

Preventing late blight involves a combination of cultural practices, resistant varieties, and timely interventions. Here are some effective strategies:

Resistant Varieties

• Select Resistant Varieties: Choose tomato varieties that are resistant or tolerant to late blight. While no variety is completely immune, some have better resistance than others.
  • Examples of Resistant Varieties:
    • ‘Mountain Magic’
    • ‘Defiant PhR’
    • ‘Plum Regal’
    • ‘Legend’

Crop Rotation

• Implement Crop Rotation: Avoid planting tomatoes or potatoes in the same location for consecutive years. Rotate crops with non-host plants to reduce the buildup of the pathogen in the soil.

Proper Spacing and Pruning

• Space Plants Adequately: Provide enough space between plants to ensure good air circulation, which helps reduce humidity around the foliage.
• Prune for Airflow: Prune lower leaves and suckers to improve air circulation and reduce the time leaves remain wet after rain or watering.

Watering Practices

• Water at the Base: Avoid overhead watering, which wets the foliage. Instead, use drip irrigation or water at the base of the plants.
• Water Early in the Day: Water plants early in the morning to allow leaves to dry quickly, reducing the window of opportunity for infection.

Mulching

• Apply Mulch: Use organic mulch like straw or wood chips around the base of the plants to prevent soil splashes that can carry spores to the foliage.

Monitoring and Early Detection

• Regular Monitoring: Regularly inspect plants for early signs of late blight. Prompt removal of infected plant material can slow the spread of the disease.
• Use of Forecasting Tools: Utilize disease forecasting tools and local alerts to stay informed about late blight outbreaks in your area.

Controlling Late Blight

Despite best efforts, late blight can still appear. Here’s how to manage an outbreak effectively:

Chemical Control

• Fungicides: Use fungicides as a preventive measure, especially during high-risk periods. Fungicides containing chlorothalonil, mancozeb, or copper can be effective.
• Application Timing: Apply fungicides before symptoms appear and continue at regular intervals, following the manufacturer’s instructions.

Organic Control

• Biological Fungicides: Consider using organic fungicides containing Bacillus subtilis or other beneficial microbes to suppress late blight.
• Copper Sprays: Copper-based sprays can be used in organic systems but should be applied with caution to avoid environmental buildup.

Removal of Infected Material

• Prompt Removal: Remove and destroy infected plant material immediately to prevent the spread of spores.
• Disposal Methods: Do not compost infected material, as the pathogen can survive in the compost pile. Instead, bag and dispose of it in the trash or burn it if permitted.

Sanitation Practices

• Clean Tools: Regularly clean and sanitize gardening tools and equipment to prevent spreading the disease between plants.
• Avoid Working Wet Plants: Refrain from working with wet plants, as this can spread spores from infected to healthy plants.

What should be done if tomatoes have been infected by late blight?

Immediate Removal:

• Remove Infected Plants: Promptly remove and destroy any infected plants, including fallen leaves and fruit, to prevent the spread of the disease. Do not compost them, as the pathogen can survive and infect future crops.

Fungicide Application:

• Apply Fungicides: Use fungicides containing chlorothalonil or copper-based products as a preventative measure on remaining healthy plants. Ensure thorough coverage, especially on the undersides of leaves.

Improve Air Circulation:

• Prune and Space Plants: Improve airflow by pruning lower leaves and ensuring proper spacing between plants. This reduces humidity levels, making conditions less favorable for the pathogen.

Crop Rotation:

• Practice Crop Rotation: Avoid planting tomatoes or potatoes in the same area for at least two years to prevent soil-borne recurrence.

Are infected tomatoes safe to eat?

Tomatoes infected with late blight are generally not safe to eat. Here’s why they should be avoided:

1. Quality Concerns:
   • Texture and Taste: Infected tomatoes develop brown, sunken spots that can turn soft and slimy. The disease alters the texture and taste, making them unappealing and unfit for consumption.
2. Health Risks:
   • Secondary Infections: While late blight itself is not harmful to humans, the compromised integrity of the fruit can harbor harmful bacteria and mold, posing potential health risks if consumed.
3. Preventive Measures:
   • Inspection and Disposal: Always inspect tomatoes before use and dispose of any showing signs of late blight infection.

Frequently Asked Questions about Tomatoes Blight

How do you identify late blight in tomatoes?

1. Leaf Symptoms:
   • Dark, Water-Soaked Spots: Look for irregularly shaped dark green or brown lesions on leaves, often surrounded by a pale green or yellow halo. These spots quickly expand, turning the leaf tissue brown and causing it to shrivel. In humid conditions, a white, fuzzy mold may develop on the undersides of the leaves.
2. Stem and Fruit Symptoms:
   • Stems: Infected stems exhibit dark brown or black streaks, which can girdle the stem and lead to plant collapse.
   • Fruit: Tomatoes affected by late blight display brown, greasy spots that rapidly enlarge, making the fruit rot and become inedible.
3. Weather Conditions:
   • Humidity and Rain: Late blight thrives in cool, wet weather, with temperatures ranging from 60°F to 70°F (15°C to 21°C). Monitoring weather conditions can help anticipate potential outbreaks.

Early detection is essential. Regularly inspect plants, especially after rainy or humid periods, to prevent widespread infection and crop loss.

What is the best way to prevent tomato blight?

Choose Resistant Varieties:

• Plant Resistant Cultivars: Opt for tomato varieties bred for resistance to early and late blight, such as ‘Mountain Magic’ or ‘Defiant PhR’, which offer increased protection against these diseases.

Proper Spacing and Pruning:

• Ensure Adequate Airflow: Space plants properly and prune lower leaves to improve air circulation, reducing humidity around the foliage and lowering the risk of fungal growth.

Watering Practices:

• Water at the Base: Use drip irrigation or a soaker hose to water plants at the soil level, preventing moisture from lingering on the leaves and fostering blight conditions.

Mulching and Crop Rotation:

• Apply Mulch: Use mulch to prevent soil splash, which can carry blight spores. Rotate crops annually to prevent the buildup of pathogens in the soil.

Fungicide Application:

• Preventative Sprays: Apply fungicides containing chlorothalonil or copper-based solutions as a preventive measure, especially in regions prone to blight outbreaks. Follow label instructions for safe and effective use.

What is the difference between symptoms of early and late blight of tomato?

Early Blight (Alternaria solani):

• Leaf Symptoms: Early blight typically begins on older, lower leaves. It presents as small, dark brown spots with concentric rings, resembling a bullseye pattern. These spots can merge, causing the leaf to yellow and die.
• Stem and Fruit Symptoms: On stems, it appears as small, dark lesions. Fruit can develop sunken, leathery spots near the stem end.

Late Blight (Phytophthora infestans):

• Leaf Symptoms: Late blight is more aggressive, appearing as large, irregular, water-soaked lesions with a grayish-green hue. These lesions quickly turn brown and may have a white, fuzzy mold on the undersides in humid conditions.
• Stem and Fruit Symptoms: Stems exhibit dark, blackened areas, while fruits show greasy, brown, rapidly expanding spots that result in rot.

How do you check for blight?

Inspect Leaves:

• Early Blight: Look for small, brown lesions on older, lower leaves. These spots often have concentric rings, resembling a bullseye pattern, which causes the leaves to yellow and die.
• Late Blight: Check for large, irregular, water-soaked lesions on leaves, turning them dark brown or gray. Look for a white, fuzzy mold on the undersides of leaves in humid conditions.

Examine Stems:

• Early Blight: Observe for dark spots and streaks on the stems, which may cause weakening and breakage.
• Late Blight: Look for black, elongated lesions that can girdle stems and lead to plant collapse.

Inspect Fruits:

• Early Blight: Look for sunken, leathery spots near the stem end.
• Late Blight: Identify greasy, brown lesions on fruits that quickly rot.

Assess Environmental Conditions:

• Favorable Conditions: Monitor weather conditions; blights thrive in warm, wet environments. Early blight prefers 75°F to 85°F (24°C to 29°C), while late blight favors cooler temperatures of 60°F to 70°F (15°C to 21°C) with high humidity.

Is it safe to eat tomatoes with late blight?

While the blight itself is not toxic to humans, these lesions significantly compromise the fruit’s quality, making it unappealing and potentially unsafe. Here’s why:

1. Compromised Integrity:
   • Texture and Taste: Blighted tomatoes often become mushy and develop an off-flavor due to tissue breakdown, rendering them unpalatable.
2. Bacterial and Mold Growth:
   • Secondary Infections: The damaged areas are prone to secondary infections from bacteria and mold, which can pose health risks if consumed. These pathogens can produce harmful toxins that are unsafe to ingest.
3. Food Safety Concerns:
   • Increased Spoilage: Even if you cut away infected parts, the remaining fruit may still harbor invisible pathogens that accelerate spoilage and compromise food safety.

Does baking soda stop tomato blight?

Baking soda is often used as a homemade remedy to combat tomato blight, but its effectiveness is limited. While baking soda, or sodium bicarbonate, can create an alkaline environment on the leaf surface that may inhibit fungal growth, it does not cure existing infections or completely prevent new ones. Here’s how it works:

1. Alkaline Barrier:
   • Inhibits Fungal Growth: By raising the pH on leaf surfaces, baking soda can make conditions less favorable for blight-causing fungi, such as Alternaria solani (early blight) and Phytophthora infestans (late blight).
2. Application Method:
   • Homemade Spray: Mix 1 tablespoon of baking soda with 1 tablespoon of horticultural oil or dish soap in a gallon of water. Spray the solution on tomato plants weekly as a preventative measure.
3. Limitations:
   • Preventative, Not Curative: Baking soda is most effective as a preventative measure rather than a cure. For severe blight outbreaks, consider using fungicides or other professional treatments alongside cultural practices like proper pruning and spacing.

What is the best fungicide for tomato late blight?

1. Chlorothalonil:
   • Broad-Spectrum Protection: Chlorothalonil is a widely used fungicide that provides effective control against late blight by inhibiting spore germination. It’s suitable for preventive and early-stage treatments.
   • Application: Apply every 7-10 days, especially during wet, humid conditions that favor blight development. Follow label instructions for safe use.
2. Copper-Based Fungicides:
   • Organic Option: Copper fungicides, such as copper sulfate or copper oxychloride, offer a more organic approach to blight control. They work by disrupting fungal cell enzymes.
   • Usage: Apply as a protective barrier before symptoms appear, and repeat every 7-10 days during the growing season.
3. Mancozeb:
   • Preventive Control: Mancozeb is a preventive fungicide that coats leaves and inhibits spore development, effectively stopping blight before it takes hold.
   • Timing: Regular applications during the growing season can provide excellent protection against blight.

Recommendation: For best results, integrate these fungicides into a comprehensive disease management program that includes crop rotation, proper spacing, and regular monitoring. Always adhere to product guidelines and safety measures when applying fungicides to ensure effective and safe blight management.

What is a natural spray for blight on tomatoes?

Baking Soda Solution:

• Ingredients: Mix 1 tablespoon of baking soda, 1 tablespoon of vegetable or horticultural oil, and a few drops of dish soap in 1 gallon of water.
• How It Works: Baking soda raises the leaf surface pH, creating an alkaline environment that hinders fungal growth. Apply every 7 to 10 days as a preventive measure.

Neem Oil:

• Benefits: Neem oil is a natural antifungal and antibacterial agent. It disrupts fungal cell growth and prevents spore germination.
• Application: Mix 1 teaspoon of neem oil with 1 quart of water and spray every 7-14 days.

Garlic Spray:

• Recipe: Blend 2-3 cloves of garlic with water, strain, and mix with 1 tablespoon of oil and a few drops of dish soap.
• Effectiveness: Garlic contains sulfur compounds with natural antifungal properties. Spray on affected plants to help control blight spread.

Can you treat late blight?

Immediate Removal:

• Remove Infected Plants: Quickly remove and destroy any infected plants and plant debris to prevent the spread of spores. Do not compost these materials.

Fungicide Application:

• Effective Fungicides: Use fungicides specifically designed for late blight, such as chlorothalonil or copper-based products. Apply these treatments every 7-10 days, particularly during humid, wet conditions that favor fungal growth.

Cultural Practices:

• Improve Air Circulation: Space plants properly and prune lower leaves to enhance airflow and reduce humidity around the foliage.
• Watering Techniques: Water at the base of plants to keep foliage dry and minimize fungal spread.

Disease Resistance:

• Resistant Varieties: In future plantings, consider using tomato varieties resistant to late blight.

What tomatoes are resistant to late blight?

‘Defiant PhR’:

• Attributes: A popular choice for its robust resistance to late blight and high yield. It produces medium-sized, round, red tomatoes with excellent flavor.

‘Mountain Magic’:

• Attributes: Known for its resistance to both late and early blight. This variety offers high productivity and flavorful, round red tomatoes. It’s also suited for various climates.

‘Brandywine Pink’:

• Attributes: A heirloom variety that combines late blight resistance with exceptional taste. It produces large, pink, beefsteak-type tomatoes.

‘Iron Lady’:

• Attributes: Developed specifically for late blight resistance, it produces round, red fruits with good flavor and texture.

‘Juliet’:

• Attributes: This cherry tomato variety is resistant to late blight and offers a sweet, tangy flavor with prolific fruit production.

When to apply fungicide to tomato plants?

Preventive Application:

• Before Symptoms Appear: Apply fungicide as a preventive measure before symptoms of diseases like late blight or early blight develop. This is particularly important during periods of high humidity or frequent rainfall, which increase the risk of fungal infections.

Regular Scheduling:

• Routine Applications: Follow a regular schedule for fungicide application, typically every 7-10 days, depending on the product and weather conditions. Consistency helps maintain protection throughout the growing season.

After Rain or Irrigation:

• Reapply as Needed: Reapply fungicide after heavy rain or irrigation that may wash away the treatment, reducing its effectiveness.

Inspect Plants:

• Check for Symptoms: Regularly inspect plants for early signs of disease and apply fungicide promptly if any symptoms are observed.

Can tomatoes recover from blight?

Recovering tomatoes from blight (Phytophthora infestans) is challenging, but with prompt and effective management, you can mitigate damage and improve plant health. Here’s how to approach recovery:

1. Immediate Action:
   • Remove Infected Plants: Quickly remove and destroy infected plants and plant debris to prevent the spread of the disease. Do not compost these materials.
2. Fungicide Treatment:
   • Apply Fungicides: Use effective fungicides, such as those containing chlorothalonil or copper, to manage ongoing infections and prevent further spread. Follow the application guidelines carefully.
3. Improve Plant Conditions:
   • Enhance Airflow: Prune affected plants to improve air circulation and reduce humidity, which helps limit fungal growth. Space plants appropriately to ensure good ventilation.
4. Crop Rotation:
   • Avoid Replanting in the Same Soil: Practice crop rotation to reduce the risk of recurrence in future seasons.

While severe blight infections often lead to significant damage or plant loss, these steps can help manage the disease and protect unaffected plants.

Can I reuse soil from tomatoes with blight?

Reusing soil from tomatoes affected by blight (Phytophthora infestans) requires caution to prevent the recurrence of the disease. Here’s what you need to consider:

1. Pathogen Persistence:
   • Disease Residue: Blight pathogens can persist in soil and on plant debris. Reusing soil without treatment may lead to reinfection of new plants.
2. Soil Treatment:
   • Solarization: To kill pathogens, you can solarize the soil by covering it with clear plastic for 4-6 weeks during sunny weather. This process raises the soil temperature, which helps eliminate many pathogens.
   • Fumigation: For more severe cases, consider using soil fumigants designed to eradicate soil-borne pathogens, following all safety guidelines.
3. Crop Rotation:
   • Change Location: Practice crop rotation by planting tomatoes or related crops in different soil to minimize the risk of disease buildup.

To ensure a healthy growing environment, it’s best to treat or replace soil that has previously hosted blight.

How long does tomato blight last in soil?

Tomato blight, caused by Phytophthora infestans, can persist in soil and plant debris for varying lengths of time, depending on environmental conditions and soil management practices. Here’s an overview:

Persistence Duration:

• Up to Several Months: The blight pathogen can survive in soil for several months or even years if conditions are favorable, such as in cool, moist environments. It remains viable on infected plant debris and in the soil, potentially causing reinfection in future crop

Can I compost tomatoes with blight?

Composting tomatoes infected with blight (Phytophthora infestans) is generally not recommended due to the risk of spreading the disease. Here’s why and what you should do instead:

1. Disease Spread Risk:
   • Pathogen Persistence: The blight pathogen can persist in compost, especially in home composting systems that may not reach high enough temperatures to kill the spores. This can lead to contamination of compost and subsequent infection of new plants.
2. Safe Disposal Practices:
   • Remove and Dispose: It is safer to remove and destroy infected tomato plants and fruit. Bag and dispose of these materials in the trash or municipal green waste collection to prevent disease spread.
3. Composting Alternatives:
   • Use Healthy Materials: Only add healthy plant materials to your compost. For blight-prone areas, consider using hot composting methods that reach temperatures of 130-160°F (54-71°C) to ensure pathogen destruction.

Proper disposal and avoiding the inclusion of infected materials in compost are crucial to prevent the spread of blight and maintain healthy garden practices.

Can you save tomatoes from blight?

Saving tomatoes from blight (Phytophthora infestans) requires prompt action and effective management strategies. While completely eradicating blight from infected plants can be challenging, here’s how you can mitigate the damage and protect remaining plants:

1. Immediate Removal:
   • Remove Infected Parts: Quickly remove and destroy infected plant parts, including leaves and fruits showing blight symptoms. This helps prevent the spread of the disease to healthy plants.
2. Fungicide Application:
   • Use Effective Treatments: Apply fungicides containing chlorothalonil, copper-based products, or Mancozeb as per the label instructions. Regular applications, especially during wet or humid conditions, can help manage the disease.
3. Improve Plant Conditions:
   • Enhance Airflow: Space plants properly and prune lower leaves to improve air circulation and reduce humidity, which limits fungal growth.
4. Crop Rotation:
   • Prevent Future Outbreaks: Practice crop rotation to reduce the risk of blight reoccurrence in future growing seasons.

While saving severely infected plants may not be feasible, these measures can help manage and reduce the impact of blight on your tomato crop.

Conclusion

Late blight is a formidable adversary for tomato growers, but with vigilant monitoring, proper cultural practices, and timely interventions, it can be managed effectively. By understanding the disease’s symptoms, environmental preferences, and lifecycle, you can implement strategies to protect your tomato plants and enjoy a healthy, productive growing season. Remember, the key to managing late blight lies in prevention and early detection, so stay informed and proactive in your gardening efforts.

By staying informed and vigilant, you can protect your tomato plants from late blight and ensure a healthy and bountiful harvest.