Reasons Why Tomato Leaves Are Turning Brown and How To Fix It

In the intricate world of gardening, few sights are as disheartening as watching the vibrant green leaves of tomato plants slowly fade to a worrisome brown.

While tomatoes, often deemed the crown jewels of the summer garden, promise juicy fruits and rich flavors, they also present a plethora of challenges that can stump even seasoned gardeners.

Published: August 21, 2023.

Whether you're nurturing a single plant on a balcony or tending to an expansive garden patch, understanding the reasons behind this color transition can be the key to ensuring a healthy harvest.

Tomato leaves can turn brown for various reasons, ranging from environmental stressors to pest and disease issues. Here's a list of the most common causes:

Fungal Diseases

Various fungal diseases may cause tomato leaves to turn brown. Some of them include:

• Early Blight (Alternaria solani): Brown spots with concentric circles. Often starts on lower leaves and moves up.
• Late Blight (Phytophthora infestans): Irregular-shaped brown spots, sometimes with a white fungal growth on the underside during humid conditions.
• Septoria Leaf Spot: Small brown spots with light gray centers and dark edges.
• Fusarium Wilt and Verticillium Wilt: Browning of the leaves is often accompanied by wilting. The vascular system in the stem may show browning.

Preventing and managing fungal diseases in tomatoes is a combination of proactive measures and timely interventions. Here's a comprehensive guide to address both scenarios:

Prevention of Fungal Diseases

• Choose Resistant Varieties: Many modern tomato varieties are bred for resistance to certain fungal diseases. Check seed packets or plant tags for disease resistance codes.
• Crop Rotation: Avoid planting tomatoes in the same location year after year. Rotate with crops that are not in the Solanaceae family (like peppers, eggplants) to prevent soil-borne diseases from building up.
• Proper Spacing: Ensure plants are spaced adequately to allow for good air circulation, which can help keep foliage dry.
• Water at the Base: Use soaker hoses, drip irrigation, or water at the soil level to keep foliage as dry as possible. Wet leaves can create a conducive environment for fungi.
• Stake and Prune: Stake plants or use cages to keep them off the ground. Prune lower leaves, especially if they touch the soil, to minimize infection chances.
• Mulch: Use straw, plastic, or organic mulch around the base of the plant. This will prevent soil from splashing onto leaves during rain, which can spread soil-borne fungi.
• Clean Tools: Sanitize gardening tools, especially if they've come in contact with infected plants.
• Plant Healthy Seedlings: Only plant tomato seedlings that look healthy and free from any signs of disease.
• Avoid Working Wet Plants: Try not to handle or work with tomato plants when they are wet, as this can spread diseases.

Treatment After Onset of Fungal Diseases

• Identify the Disease: Properly diagnosing the issue is crucial for effective treatment. For instance, treatments for early blight won't be effective against late blight, and vice versa.
• Remove Infected Foliage: At the first sign of disease, remove affected leaves and dispose of them away from the garden (not in a compost pile).
• Fungicides: Consider using fungicides. Organic options like copper-based fungicides or products containing Bacillus subtilis can be effective against several tomato fungal diseases. For non-organic gardens, there are numerous chemical fungicides available.
• Consistent Treatment: If using fungicides, apply them consistently and according to the manufacturer's recommendations. Preventative sprays can also be applied if conditions are conducive to disease but before visible symptoms appear.
• Maintain Vigor: A healthy plant can better resist and recover from diseases. Ensure your tomatoes receive proper nutrition and are not stressed by drought or other environmental factors.
• Remove Severely Infected Plants: If a plant is heavily infected, it's often best to remove and dispose of the entire plant to prevent the disease from spreading to neighboring plants.

Remember, with most plant diseases, an ounce of prevention is worth a pound of cure. Regularly scouting and checking your tomato plants can help catch diseases early, making management easier and more effective.

Bacterial Diseases

• Bacterial Wilt (Ralstonia solanacearum): Rapid wilting of the plant without yellowing of leaves. The inner stem may ooze a milky liquid.
• Bacterial Spot (Xanthomonas campestris pv. vesicatoria): Dark brown spots that may be raised or scab-like.

Bacterial diseases are prevented by applying the same measures when preventing fungal diseases, but also by avoiding excess nitrogen and via biocontrol - some beneficial bacteria, like Bacillus species, can be applied to plants as a form of biological control against bacterial pathogens.

Managing bacterial diseases can be challenging due to the fast spread and resilience of many bacterial pathogens.

Integrated management, combining cultural practices, biological controls, and, when necessary, chemical interventions, is often the most effective approach.

Viral Diseases

Viral diseases such as Tomato Spotted Wilt Virus and Tomato Yellow Leaf Curl Virus can lead to browning of the leaves, among other symptoms.

Viral diseases in tomatoes can be particularly challenging because there are no direct treatments for viruses in plants. Instead, management revolves around prevention and mitigation.

Prevention of Viral Diseases

• Resistant Varieties: Opt for tomato varieties that are bred to be resistant to certain viral diseases.
• Vector Control: Many viral diseases are transmitted by insect vectors, especially aphids, whiteflies, and thrips. Implement practices to manage these pests:

- Use reflective mulches to deter aphids and whiteflies.

- Introduce beneficial insects, such as ladybugs and lacewings, which feed on aphids.

- Apply insecticidal soaps or neem oil to control these pests.

• Healthy Seedlings: Only plant healthy, disease-free seedlings. Purchase seeds from reputable suppliers. Some viruses can be seed-transmitted, so starting with clean seeds is crucial.

• Barrier Methods: Use row covers to protect young plants from insect vectors. However, remember to remove them once the plants flower to allow for pollination.
• Sanitation: Always disinfect tools, pots, and greenhouse structures. Viruses can be mechanically transmitted, so sanitation is paramount.
• Crop Rotation: Practice crop rotation, especially if a viral outbreak has occurred in the past.
• Weed Control: Some weeds can act as reservoirs for viral diseases. Regularly eliminate weeds both inside and around the garden area.
• Avoid Tobacco: If you use tobacco, wash hands thoroughly before handling tomatoes. Tobacco can carry the Tobacco mosaic virus (TMV), which can infect tomatoes.

Management After Onset of Viral Diseases

• Early Detection: Familiarize yourself with the symptoms of common tomato viruses. These might include mottling, yellowing, leaf distortion, stunted growth, or fruit streaks.
• Rogueing: If you identify a plant with a viral disease, remove it immediately to prevent the spread to other plants. Dispose of it away from the garden and never compost it.
• Continue Vector Control: If a viral outbreak is associated with a specific insect vector, intensify measures to control that insect.
• Avoid Mechanical Transmission: Some viruses can spread through touch. After handling an infected plant, wash your hands and disinfect tools.
• Protect Neighboring Plants: If one plant gets infected, consider increasing preventative measures for neighboring plants, such as applying insecticides or increasing the frequency of monitoring.
• Support Plant Health: While it won't cure the virus, keeping a plant healthy can sometimes help it endure the infection better. Ensure it's receiving the right nutrients, water, and care.

Unlike with fungal or bacterial diseases, there's no direct treatment for viral infections in plants making prevention and early detection the best long term strategies.

Pest Damage

Some pests, like spider mites and thrips, suck the sap from the leaves, causing stippling, and in severe cases, the leaves may turn brown and die.

Pests can be a significant factor behind tomato leaves turning brown, either directly due to feeding or indirectly by transmitting diseases. Here's how to prevent damage from these pests:

Regular Monitoring

• Scout regularly: Examine your tomato plants several times a week for signs of pests or damage.
• Check undersides of leaves: Many pests like to hide on the underside of leaves.

Physical Barriers

• Row covers: Lightweight fabric can be used to cover young tomato plants, protecting them from pests like aphids and flea beetles.
• Collars: Placing a collar around the base of young tomato plants can help deter cutworms.

Garden Hygiene

• Remove plant debris: Many pests overwinter in plant debris. Clean up the garden at the end of each growing season.
• Mulch: Organic mulches like straw or hay can deter soil-borne pests and can also reduce the splashing of soil onto leaves, which can carry pests.

Attract Beneficial Insects

• Plant flowers: Flowers attract beneficial insects such as ladybugs, lacewings, and parasitic wasps that feed on harmful pests.
• Avoid broad-spectrum insecticides: These can kill beneficial insects along with harmful ones.

Biological Controls

• Beneficial nematodes: These microscopic worms can be applied to soil to kill various soil-borne pests.
• Bacillus thuringiensis (Bt): A natural soil bacterium that can be used as a safe insecticide against many caterpillar pests.

Insecticidal Soaps and Oils

• Neem oil: A natural pesticide that can deter a variety of pests.
• Insecticidal soap: Effective against soft-bodied pests like aphids and spider mites.

Proper Watering

Avoid overhead watering: Wet foliage can attract pests and promote disease. Water at the base of the plant instead.

Companion Planting

Marigolds, basil, and nasturtiums: These can deter certain tomato pests when planted nearby.

Trap Crops

Plant a crop that's more attractive to pests to draw them away from your tomatoes. For example, nasturtiums can act as a trap crop for aphids.

Pheromone Traps

These traps attract male pests, preventing them from breeding.

Diatomaceous Earth

This is a natural insect killer made from the fossilized remains of tiny aquatic organisms. It works mechanically, piercing the exoskeleton of pests like cutworms.

Hand Picking

For larger pests like tomato hornworms, hand-picking them off the plants can be effective.

Rotate Crops

Don't plant tomatoes in the same spot every year. This can help break the lifecycle of many pests.

When dealing with pests, it's crucial to identify the specific culprit so that the most effective control method can be applied. Always try non-chemical methods first, resorting to chemical pesticides only when necessary.

When using pesticides, always follow the label instructions, and choose products that target the specific pest without harming beneficial insects.

These are the most common causes that may turn tomato leaves brown. Other causes include:

Environmental Stress

• Sunscald: Direct exposure to intense sunlight can scorch the leaves.
• Temperature Stress: Extreme temperatures, either too hot or too cold, can cause leaf browning.
• Water Stress: Both over-watering and under-watering can lead to browning of the leaf edges or tips.
• Poor Drainage: Soggy soil can cause root rot, leading to brown leaves.

Nutrient Deficiencies or Imbalances

Nutrient deficiencies and/or imbalances, especially calcium (leading to Blossom End Rot), magnesium, potassium, and others.

Chemical Damage

Exposure to herbicides or even an overdose of certain pesticides or foliar sprays.

Salt Damage

High salt levels in the soil or water can lead to leaf burn.

Ozone Injury

In areas with high air pollution, ozone can cause a stippling or bronzing appearance on the upper leaf surface.

Transplant Shock

Newly transplanted tomatoes may have some leaf browning if they weren't gradually acclimated to the sun (hardened off).

To determine the exact cause of browning on your tomato leaves, you'll need to observe the entire plant, its environment, and the specific symptoms presented. Properly diagnosing the issue is crucial for implementing effective remedies.

A properly grown tomato plant is a resilient, vigorous plant that can withstand some pests and diseases, making ensuring proper growing conditions a paramount task for every gardener.

And when applying chemicals, always read the instructions; sometimes they can make more bad than good!