What causes blight on tomatoes? - Plant Care Guide

If your tomatoes have blight, it refers to one of several devastating fungal diseases that cause rapid browning, wilting, and decay of foliage, stems, and fruits. The most common and impactful types are Early Blight, Late Blight, and Septoria Leaf Spot, each with distinct symptoms and environmental triggers. Identifying the specific type of blight affecting your tomatoes is crucial for effective treatment and prevention.

What is Blight on Tomatoes, and What Does it Look Like?

Blight on tomatoes is a general term encompassing several destructive fungal (or oomycete, a fungus-like organism) diseases that can rapidly damage and even kill tomato plants. The symptoms and conditions favoring each type vary, so careful observation is key to accurate diagnosis.

Early Blight (Alternaria solani)

What it is: A common fungal disease that typically affects tomato plants earlier in the growing season, often when plants are stressed or after periods of rain. It can also affect potatoes.
What it looks like (Symptoms):
- On leaves: Appears as dark brown spots with distinctive concentric rings (like a target pattern) on older, lower leaves first. A yellow halo may surround the spots. As spots enlarge and merge, the leaves turn yellow, then brown, and fall off.
- On stems: Dark, sunken lesions with concentric rings can form on stems, especially near the soil line.
- On fruits: Dark, leathery, sunken spots with concentric rings can appear on the stem end of fruits.
- Overall plant: Gradual defoliation from the bottom up.

Late Blight (Phytophthora infestans)

What it is: A highly aggressive and destructive oomycete disease (similar to a water mold) that is notoriously famous for causing the Irish Potato Famine. It thrives in cool, wet, and humid conditions and spreads rapidly, especially on tomatoes and potatoes. It can quickly wipe out an entire crop.
What it looks like (Symptoms):
- On leaves: Large, irregular, water-soaked, dark green to brown spots appear, often starting at the leaf tips or edges. These spots quickly enlarge.
- On the underside of leaves: In humid conditions, a characteristic fuzzy, whitish fungal growth may be visible around the edges of the lesions, especially in the morning.
- On stems: Dark, water-soaked lesions appear on stems and petioles (leaf stalks), causing them to collapse.
- On fruits: Irregular, greasy, gray-green to brown lesions develop rapidly on green or ripe fruits. The affected area becomes firm and leathery.
- Overall plant: Rapid, widespread wilting and collapse, often looking like the plant was hit by a frost.

Septoria Leaf Spot (Septoria lycopersici)

What it is: A common fungal disease that also affects tomato foliage, often mistaken for early blight but with key differences in symptom appearance.
What it looks like (Symptoms):
- On leaves: Numerous small, circular spots (about 1/8 to 1/4 inch in diameter) with dark brown borders and light gray or tan centers. Crucially, tiny black speck-like dots (fruiting bodies of the fungus) are visible within the light centers.
- Affected areas: Typically starts on older, lower leaves and works its way up.
- Overall plant: Yellowing, wilting, and premature defoliation, though usually less rapid than late blight. It rarely affects stems or fruits directly.

Identifying these distinct visual cues is essential for determining which type of blight is on your tomatoes and choosing the appropriate management strategy.

Why Does Humidity and Leaf Wetness Cause Blight on Tomatoes?

High humidity and prolonged leaf wetness are critical environmental conditions that directly cause blight on tomatoes, as virtually all fungal and oomycete pathogens, including those responsible for early blight, late blight, and Septoria leaf spot, require moisture to germinate spores and infect plant tissue.

How it causes blight:
1. Spore Germination: Blight spores (from the soil, infected debris, or airborne sources) cannot infect a dry leaf. They need a film of free water on the leaf surface to germinate and penetrate the plant's cuticle. This water can come from rain, heavy dew, splashing water from irrigation, or high humidity condensing on leaves. The longer the leaves remain wet, the higher the chance of successful infection.
2. Fungal Growth and Spread: Once the spores germinate and the pathogen enters the plant, high humidity in the air promotes the rapid growth and reproduction of the fungus or oomycete. This leads to the expansion of existing lesions and the production of new spores, accelerating the disease cycle.
3. Trapped Moisture: Dense foliage, lack of proper plant spacing, and poor air circulation create a humid, stagnant microclimate within the plant canopy. This traps moisture on and between leaves, significantly prolonging leaf wetness duration.
Symptoms:
- Rapid onset and spread of blight symptoms (spots, lesions, fuzzy growth) on leaves, especially after periods of rain or consistently humid weather.
- The disease often appears first or is most severe in the lower, denser parts of the plant where air circulation is naturally poorer.
- Plants that remain wet for long periods will show greater susceptibility.
Solution:
1. Water at the Base: Always water your tomato plants at the soil level using methods like drip irrigation, soaker hoses, or careful hand watering. Avoid overhead sprinklers, especially in the evening. A drip irrigation kit for tomatoes is an excellent investment.
2. Water in the Morning: If you must use overhead watering (not recommended), do so early in the morning. This allows the foliage to dry completely before nightfall, reducing the critical leaf wetness period.
3. Improve Air Circulation:
  - Proper Spacing: Plant tomatoes with adequate space between them to allow for good airflow.
  - Pruning: Regularly prune out suckers and the lower leaves that touch the soil or are yellowing. This opens up the plant's center, improves air circulation, and reduces splash-up from the soil.
  - Staking/Caging: Use strong tomato cages or stakes to keep foliage off the ground, further enhancing airflow and preventing contact with soil-borne spores.

By diligently managing humidity and leaf wetness, you can significantly reduce the conditions that promote blight on tomatoes and strengthen your plants' natural defenses.

Can Infected Plant Debris or Soil Carry Blight on Tomatoes?

Yes, infected plant debris and soil are major sources that carry blight on tomatoes from one season to the next, leading to recurrent infections. These materials harbor overwintering spores, which become the primary source of infection for new tomato plants.

For Early Blight (Alternaria solani) and Septoria Leaf Spot (Septoria lycopersici)

How it carries blight: Both Early Blight and Septoria Leaf Spot fungi can survive on infected tomato (or potato) plant debris left in the garden soil over winter. The spores can also live in the soil itself. When conditions become favorable (warmth and moisture) in the spring, these overwintered spores can splash up from the soil onto the lower leaves of new tomato plants, initiating the infection cycle.
Symptoms:
- Blight appearing on the lower leaves of tomato plants early in the season.
- Recurrent blight problems year after year in the same planting area.
Solution:
1. Thorough Fall Cleanup (Crucial!): At the end of the growing season, remove and destroy ALL tomato plant debris (leaves, stems, and any fallen fruit) from the garden bed. Do not compost diseased plant material, as most home compost piles do not reach high enough temperatures to kill the fungal spores. Bag it and dispose of it in the trash.
2. Deep Tilling: If blight has been severe, deep tilling the soil in the fall can help bury any remaining spores, reducing their ability to infect emerging plants.
3. Avoid Overhead Watering: As previously discussed, watering at the base of the plant prevents splashing soil-borne spores onto leaves.
4. Mulch: Applying a layer of clean organic mulch (like straw or shredded leaves) over the soil surface after planting creates a physical barrier, preventing soil-borne spores from splashing onto lower leaves.

For Late Blight (Phytophthora infestans)

How it carries blight: While Late Blight spores can sometimes survive in infected potato tubers left in the ground, they generally do NOT overwinter in garden soil or plant debris in most climates because the oomycete prefers living tissue. The primary way Late Blight appears in home gardens is via airborne spores blown in from distant infected tomato or potato fields, or from infected potato volunteer plants that sprouted from overlooked tubers.
Symptoms: Late Blight can appear suddenly and aggressively, sometimes seemingly out of nowhere, especially after cool, wet weather periods.
Solution:
1. Remove Volunteer Plants: Remove any volunteer potato or tomato plants that sprout from previous crops.
2. Source Healthy Plants: Ensure any new seedlings or potato tubers you plant are healthy and disease-free.
3. Regional Monitoring: Keep an eye on local university extension alerts for Late Blight outbreaks in your region, as it can spread rapidly over long distances.

Regardless of the type of blight, practicing excellent garden sanitation and appropriate crop rotation are foundational steps in preventing blight on tomatoes by breaking the disease cycle and reducing the initial source of infection.

Can Nutrient Imbalances Make Tomatoes More Susceptible to Blight?

Yes, nutrient imbalances can make tomatoes significantly more susceptible to blight by weakening the plant's overall health and compromising its natural defenses. A plant that is stressed due to a lack or excess of nutrients is simply less resilient to disease pressure.

How nutrient imbalances contribute to susceptibility:
1. Under-fertilization / General Deficiencies:
  - Weakened Immune System: A chronic lack of essential macronutrients (Nitrogen, Phosphorus, Potassium) or micronutrients means the tomato plant cannot build strong cell walls, produce enough energy for growth, or mount an effective immune response to fungal attacks.
  - Stunted Growth and Poor Vigor: A consistently underfed tomato plant will be smaller, less vigorous, and show signs of general nutrient stress (e.g., pale leaves), making it an easier target for blight pathogens to infect and spread rapidly within.
  - Symptoms: Overall pale or yellowing leaves, stunted growth, reduced fruiting, and a general unhealthy appearance, often preceding or accompanying blight symptoms.
  - Solution: Conduct a soil test kit to identify specific deficiencies. Amend the soil with compost (which provides a slow-release, balanced nutrition) or apply a balanced tomato-specific fertilizer (e.g., NPK like 5-10-10 or 8-16-16) at appropriate times during the growing season. A tomato fertilizer ensures proper nutrient balance.
2. Over-fertilization / Nutrient Excesses:
  - Salt Burn: Applying too much synthetic fertilizer can lead to a buildup of soluble salts in the soil. This can "burn" the delicate roots, inhibiting water and nutrient absorption. The stressed, dehydrated plant (even in wet soil) becomes extremely susceptible to various issues, including root rot and subsequently, foliage diseases like blight.
  - Excess Nitrogen (especially early season): While nitrogen promotes leafy growth, too much nitrogen can lead to overly lush, soft, sappy foliage. This type of growth is more succulent and easier for fungal spores to penetrate and proliferate within. It also can reduce flowering and fruiting, directing all energy to vulnerable foliage.
  - Symptoms: Brown or crispy leaf edges, wilting despite wet soil, stunted growth (due to root damage), and often an excessively bushy, dark green appearance followed by increased disease susceptibility.
  - Solution: If suspected, flush the soil thoroughly with plain water to leach out excess salts. Reduce fertilizer application rates or frequency. Use slow-release or organic options that release nutrients gradually.

Maintaining optimal soil health and a balanced nutrient supply for your tomatoes is a crucial preventative measure against blight. A strong, well-fed plant possesses a more robust "immune system" to resist infection and recover from minor stressors.

What Role Does Overwatering or Poor Drainage Play in Blight on Tomatoes?

Overwatering and poor drainage are significant contributors to blight on tomatoes, primarily by creating the consistently wet and humid conditions that all major blight pathogens (Early Blight, Late Blight, Septoria Leaf Spot) thrive in. While these issues don't directly cause blight, they create the ideal breeding ground for infection and spread.

How they contribute to blight:
1. Prolonged Leaf Wetness:
  - Overwatering that involves overhead irrigation (sprinklers) directly wets the foliage. If done frequently or in the evening, leaves stay wet for extended periods, providing the critical moisture needed for blight spores to germinate and infect.
  - Poor drainage can lead to higher humidity at the soil surface and around the lower plant canopy, contributing to slower drying of lower leaves after rain or dew.
2. Root Stress and Disease Susceptibility:
  - Constantly soggy soil from overwatering or poor drainage deprives tomato roots of oxygen. This leads to root stress or root rot, which weakens the entire plant. A stressed plant has a compromised defense system, making it much more vulnerable to blight infections.
  - Gummy Stem Blight (which can affect tomatoes, similar to cucumbers) directly causes root and stem rots that thrive in wet conditions.
3. Pathogen Survival and Splash Dispersal:
  - Many blight pathogens (like Early Blight and Septoria Leaf Spot) survive in infected plant debris or in the soil. Waterlogged conditions can help these pathogens persist.
  - Splashing water (from rain or overhead irrigation) from constantly wet soil can easily transfer spores from the soil onto the lower leaves, initiating the infection.
Symptoms on the plant:
- Rapid development and spread of blight symptoms after periods of excessive moisture.
- Wilting, yellowing, and general decline (which can be due to root rot caused by the overwatering, before blight fully takes over).
- Lower leaves showing symptoms first due to contact with wet soil and poorer airflow.
Solution:
1. Water at the Base: The most effective strategy is to water tomatoes directly at the base of the plant, delivering water to the roots without wetting the foliage. Use drip irrigation, soaker hoses, or careful hand watering.
2. Water in the Morning: If you must use overhead irrigation, do so early in the morning so the leaves have ample time to dry completely before nightfall.
3. Improve Soil Drainage: Amend heavy clay soils with abundant organic matter (like compost) before planting to improve structure and drainage. Avoid planting in low spots where water collects.
4. Use Raised Beds: If your garden has persistently poor drainage, consider planting tomatoes in raised garden beds filled with well-draining soil.
5. Mulch: Apply a layer of clean organic mulch around the base of the plants to reduce soil splash-up onto lower leaves and help maintain consistent soil moisture.
6. Prune Lower Leaves: Remove the lowest leaves that are touching the soil to prevent direct contact with soil-borne spores.

By diligently managing watering practices and ensuring excellent soil drainage, you create an environment that is far less hospitable to the development and spread of blight on tomatoes.

What About Cucumber Varieties Resistant to Blight?

Choosing tomato varieties resistant to blight is one of the most effective and sustainable strategies for preventing and managing these devastating diseases. Plant breeders have developed many cultivars with built-in genetic resistance that can significantly reduce your chances of facing a severe blight outbreak.

How resistance works: These varieties possess specific genes that enable them to recognize and defend against particular blight pathogens (e.g., Alternaria solani for Early Blight, Phytophthora infestans for Late Blight, or Septoria lycopersici for Septoria Leaf Spot). This resistance doesn't mean they are entirely immune, but it significantly reduces the likelihood of infection, or if they do get infected, the disease progression is often much slower and less severe, giving you more time to manage it and harvest fruit.
Symptoms on Resistant Varieties (if any): While highly resistant, some varieties might still show minor, very localized symptoms under extreme disease pressure, but they typically won't experience widespread defoliation, stem cankers, or plant collapse.
Identifying Resistant Varieties:
- Seed Packet Labels: Look for specific codes or terms on seed packets or plant tags that indicate disease resistance. Common abbreviations for blight resistance in tomatoes include:
  - EB for Early Blight Resistance
  - LB for Late Blight Resistance
  - SEPT or SLS for Septoria Leaf Spot Resistance
  - Other common resistances you might see include F (Fusarium Wilt), V (Verticillium Wilt), N (Nematodes), TMV (Tobacco Mosaic Virus), etc. A good seed catalog will often list a string of resistance codes.
- Seed Catalog Descriptions: Reputable seed catalogs and online retailers will clearly state the disease resistance of their tomato varieties in their descriptions.
- University Extension Recommendations: Your local university extension office often publishes lists of tomato varieties that perform well and are resistant to common diseases in your specific region, which can be invaluable.
Examples of Blight-Resistant Tomato Varieties (always check local availability and suitability, as new varieties emerge):
- For Early Blight Resistance: 'Defiant PhR', 'Mountain Magic', 'Iron Lady', 'Celebrity', 'Big Beef'.
- For Late Blight Resistance: 'Defiant PhR', 'Mountain Magic', 'Plum Regal', 'Legend', 'Matt's Wild Cherry' (some cherry tomatoes show natural resistance).
- For Septoria Leaf Spot Resistance: 'Defiant PhR', 'Mountain Magic', 'Celebrity', 'Big Beef'. (Many varieties offer resistance to multiple common diseases).

Benefits of Choosing Resistant Varieties:

Reduced Need for Fungicides: You'll likely need fewer, if any, fungicide applications.
Increased Yield: Healthier plants stay productive longer, leading to a better harvest.
Less Work: Less time and effort spent battling disease.
Environmental Benefit: Reduces the need for chemical interventions in the garden.

Even with resistant varieties, it's still crucial to implement good cultural practices (proper watering, air circulation, sanitation, crop rotation) to ensure the best possible growth and to further reduce overall disease pressure. Selecting tomato varieties resistant to blight is your best front-line defense against these challenging diseases.