Tomatoes are one of the most popular vegetables grown in home gardens. However, tomato plants are susceptible to several diseases that can significantly reduce yield and fruit quality. Identifying the signs and symptoms of common tomato diseases is the first step in treating and preventing further spread. This guide covers the most common tomato plant diseases, their identification, organic and chemical treatment options, and prevention methods.
Common Tomato Plant Diseases
Tomato plants can be affected by over a dozen serious diseases. The most widespread and destructive tomato diseases include:
Early Blight
Early blight, caused by the fungus Alternaria solani, affects leaves, stems and fruits. It first appears as small brown spots on older leaflets and spreads to form dark brown concentric rings, leading to drying and defoliation. Lesions may also develop on stems and fruit.
Early blight thrives in warm, humid conditions with temperatures of 75-85°F. Overhead irrigation and crowded plants encourage disease spread. It overwinters in soil and plant debris.
Late Blight
The fungus Phytophthora infestans causes late blight, which affects all parts of the tomato plant. On leaves, irregularly-shaped brown to black lesions appear first on older growth. White fungal growth may develop on the underside in wet conditions. Stem lesions are brown to black with a greasy appearance. Fruit lesions start light brown and progress to black.
Cool, wet weather promotes late blight, which can spread rapidly. The disease survives in infected potato tubers and tomato plant debris.
Septoria Leaf Spot
Septoria leaf spot, caused by the fungus Septoria lycopersici, manifests as small water-soaked spots on older leaves. Spots become brown with a dark margin and tiny black fruiting bodies. Severe infection causes leaf yellowing and defoliation. Stem and fruit lesions are rare.
Septoria leaf spot favors wet foliage and temperatures between 60-85°F. It overwinters on crop debris and volunteer tomato plants.
Bacterial Spot
The bacterium Xanthomonas campestris pv. vesicatoria leads to bacterial spot. Small (1/16 to 1/8 inch), water-soaked, greasy spots first appear on leaves. Spots become brown to black with a yellow halo. Older leaves exhibit spotting and blighting along the edges first. Fruit spots start as raised blisters and turn scabby and black.
Warm, wet and windy conditions promote spread of bacterial spot. The bacteria survive between crops in plant debris, soil, stakes and trellises.
Bacterial Canker
Another bacterial disease, canker is caused by Clavibacter michiganensis. Margins of leaflets are wavy, have brown spots and turn upward. Stem lesions appear water-soaked and ooze bacteria. Cankers expand and crack, becoming orange-brown. Fruit spots are raised and brown initially, then become scabby.
Bacterial canker enters through wounds and natural openings. It persists on stakes, greenhouse structures and field equipment between seasons.
Fusarium Wilt
The soil-borne fungus Fusarium oxysporum f. sp. lycopersici leads to Fusarium wilt. Leaves wilt from the bottom up, yellow and brown. Vascular tissue of stems and roots appears brown. Plants often die.
The fungus enters through the roots and colonizes the vascular system. It persists in infested soil indefinitely.
Verticillium Wilt
Another soil-borne fungus, Verticillium dahliae, causes Verticillium wilt. Leaves yellow from the edge inward and dry up. Vascular tissue browns in stems and roots, and plants wilt and die.
The fungus enters through the roots. It can survive in soil for many years.
Southern Blight
The fungus Sclerotium rolfsii causes southern blight. Symptoms start as wilting of one or more branches, which eventually collapse and die. White fungal mycelium and tan spherical structures may cover infected lower stems.
Southern blight thrives in hot, humid weather with temperatures above 85°F. The fungus resides in soil and on plant debris.
Root-Knot Nematodes
Plant-parasitic roundworms called nematodes cause root-knot. Roots develop wart-like galls up to 1 inch large. Aboveground, plants are stunted and wilt in hot weather. Tomatoes are highly susceptible.
Nematodes enter roots as juveniles and induce gall formation. They persist in soil and plant material.
Identifying Tomato Diseases
Accurate diagnosis of tomato problems involves careful examination of all parts of the plant and consideration of weather conditions, plant history and pattern of symptoms.
Look for initial symptoms on oldest leaves and branches first. Check upper and lower leaf surfaces, stems, flowers and fruits. Compare affected plants with healthy ones. Consider the time of appearance and how rapidly symptoms spread.
Submit samples to diagnostic clinics for microscopic and lab analysis when the cause is unclear. Correct identification is key before attempting to treat diseases.
Some basic guidelines for diagnosing common tomato disorders:
- Circular leaf spots with concentric rings – Early blight
- Irregular leaf spots spreading upward – Late blight
- Tiny black dots on leaf spots – Septoria leaf spot
- Marginal leaf scorching and spotting – Bacterial spot
- Raised blisters on fruit – Bacterial spot
- Wilting branches with canker lesions – Bacterial canker
- Bottom-up leaf wilting and yellowing – Fusarium, Verticillium wilt
- White fungal growth on lower stems – Southern blight
- Gall formation on roots – Root-knot nematodes
Compare physical symptoms to descriptions and photographic guides to narrow down suspects. Consider factors like weather and plant history as well. Submitting samples to diagnostic labs can provide confirmation.
Organic Tomato Disease Treatments
Organic treatments can effectively manage many tomato fungal diseases without the use of synthetic chemicals. Integrated management using multiple tactics generally provides the best results.
Sanitation
Removal of diseased plant material is key. Collect and destroy debris after harvest. Rotate tomatoes to different beds each year. Sterilize tools and equipment. Weed thoroughly. These steps reduce overwintering inoculum.
Growing Conditions
Avoid overhead watering and provide good air circulation. Use drip irrigation and give plants adequate spacing. Stake, cage or trellis plants to prevent soil contact. This minimizes conditions favorable for disease.
Resistant Varieties
Choose tomato varieties with resistance, or tolerance, to common diseases like Fusarium and Verticillium wilt and nematodes. Ask seed company or catalog for resistant cultivars best suited to your climate.
Biological Treatments
Apply beneficial microbes like Bacillus subtilis and Streptomyces lydicus to protect against foliar diseases. Introduce nematode-destroying fungi into soil. Boost overall plant health with compost teas.
Organic Fungicides
For active infection, treat with OMRI-listed fungicides containing sulfur, copper, neem oil or potassium bicarbonate. Always follow label directions. Begin applications preventively before diseases are present.
Crop Rotation
Rotate tomatoes with unrelated vegetable families like beans, onions or lettuce to reduce soil-borne disease carryover. Wait at least 3 years before replanting tomatoes in the same spot.
Combining good cultural practices, resistant varieties, and targeted applications of organic protectant and biological products provides effective control of most tomato diseases without harmful chemicals.
Conventional Tomato Disease Treatments
While organic methods are preferred for managing tomato diseases, conventional fungicide and bactericide applications play an important role in protecting tomato crops for commercial growers.
Foliar Diseases
Several conventional fungicides are labeled for control of foliar diseases like early blight, late blight and leaf spot. These include chlorothalonil, mancozeb, copper compounds and strobilurin fungicides. Begin applications preventatively and repeat per label directions.
Bacterial spot and canker are best managed by integrating preventive copper sprays with targeted antibiotics like streptomycin. Avoid overhead irrigation. Hot water seed treatment helps prevent bacterial canker.
Soil-borne Diseases
For Fusarium, Verticillium wilts and southern blight, preplant fumigation with metam sodium or chloropicrin provides effective control. Grafting onto resistant rootstock is another option.
Nematicides such as fluopyram and fosthiazate combat damaging root knot nematodes. Apply preplant in accordance with label guidelines and resistance management programs.
While conventional fungicides and fumigants combat tomato diseases effectively, resistance and environmental impacts are concerns. Integrated treatment and prevention programs are recommended over sole reliance on chemicals. Consult local agricultural experts for specific recommendations in your region. Follow all safety, application and resistance management directions.
Preventing Tomato Plant Diseases
Prevention of problems is always preferable to treatment. Use integrated cultural, biological and targeted chemical control methods to keep tomato diseases in check.
Buy Healthy Transplants
Inspect plants closely and reject those with spots, canker lesions or galls on roots. Purchase certified disease-free plants from reputable nurseries. Avoid saving your own seed where diseases occurred.
Practice Crop Rotation
Rotate tomato location yearly to avoid buildup of soil pests like nematodes and fungi. Do not follow tomatoes with potatoes or eggplant.
Control Weeds and Volunteers
Remove weeds, which harbor viruses like tobacco mosaic. Also pull volunteer tomato plants, as they may allow diseases to overwinter.
Disinfect Tools and Equipment
Clean and disinfect stakes, cages, trellises, pots and tools with a 10% bleach solution between seasons. Sterilize greenhouse surfaces.
Promote Air Circulation
Allow adequate spacing between plants for air movement. Use cages or trellising. Prune selectively to open up dense foliage. These steps reduce humidity levels.
Water at Base of Plants
Use soaker hoses or drip irrigation. Avoid wetting foliage, as it encourages bacterial and fungal diseases. Water early in the day so leaves can dry out.
Scout Regularly
Inspect plants frequently for early symptoms of problems. This allows early treatment before diseases spread. Destroy diseased plants promptly.
Apply Preventive Treatments
Use organic or conventional protective fungicides and bactericides prior to onset of favorable disease conditions. Focus on early blight, late blight, leaf mold and bacterial spot.
With vigilance and a combination of cultural, biological and targeted chemical controls, tomato plant diseases can be kept to a minimum, allowing plants to reach their full healthy potential.
Frequently Asked Questions About Tomato Plant Diseases
Here are answers to some common questions about identification and treatment of tomato plant diseases:
What causes yellow spots with black centers on tomato leaves?
These symptoms are characteristic of septoria leaf spot, caused by the fungus Septoria lycopersici. It is often confused with early blight. Carefully inspect leaf spots to distinguish small dark fungal fruiting bodies of septoria.
How can I tell the difference between late and early blight?
Late blight symptoms begin as irregular greasy lesions on upper leaves while early blight starts as small circular spots on lower leaves. Late blight advances very rapidly while early blight moves slower. Cool, wet conditions favor late blight.
Why do my tomato leaves turn yellow and wilt even when I water and fertilize regularly?
Wilting and yellowing leaves accompanied by vascular browning are classic signs of Fusarium or Verticillium wilt. Have a diagnostic lab confirm which soil-borne fungus is responsible. Check for resistance when selecting future varieties.
What causes the raised brown blisters on my tomato fruits?
Raised blister-like lesions on tomato fruits are characteristic of bacterial spot. Check leaf margins for brown greasy spotting also associated with this disease. Copper sprays can help manage bacterial spot.
How can I tell if root-knot nematodes are damaging my tomatoes?
Carefully dig up suspect plants and wash the roots. Round galls up to 1 inch across on roots confirm root-knot nematode attack. Rotate out of tomatoes for 3+ years. Plant resistant varieties in future.
What causes tomato plant collapse with white fungal growth on stems?
Southern blight leads to sudden wilting and death, often with visible white mycelium on lower stems. It thrives in hot, humid weather. Rotate crops and avoid overhead irrigation to reduce Southern blight issues.
Why do my tomato seedlings fall over at soil line and die?
These are classic damping-off symptoms, caused by several soil-borne fungi. Use sterile soilless media and avoid overwatering. Treat seedlings preventively with biofungicides. Allow good air circulation.
Proper diagnosis guides treatment and prevention of many tomato problems. Consult extension resources, diagnostic clinics or crop experts when uncertain. Quick action can protect tomato crops and avoid further spread.
Conclusion
Growing healthy, productive tomato plants requires vigilance against a myriad of potential diseases. The most serious threats include fungal problems like early and late blight, bacterial spot and canker, wilt diseases, and nematode pests. Identifying diseases accurately is critical in order to treat effectively. Organic options such as sanitation, plant resistance, beneficial microbes and organic sprays provide environmentally-friendly control. Conventional fungicides and soil fumigants also play a role where diseases threaten commercial crops. Preventing issues through cultural practices and monitoring is most important. An integrated disease management plan tailored to local conditions provides the best protection and avoids over reliance on any single method. With proper identification and treatment of diseases, vibrant tomato plants can thrive and bear bountiful harvests.