Septoria leaf spot is a very common disease of tomato plants, caused by the fungus Septoria lycopersici. It appears as small, circular spots on older tomato leaves, usually starting on the lower leaves and moving up the plant. The spots have gray centers with dark edges, and tiny black dots may be visible in the centers. Left untreated, septoria leaf spot can cause significant defoliation and reduce yields. However, with proper treatment and prevention methods, its impact can be minimized.
What Causes Septoria Leaf Spot on Tomatoes?
Septoria leaf spot is caused by the fungus Septoria lycopersici. This fungus overwinters in crop debris and soil. In the spring, airborne spores are spread to tomato plants via wind and rain splash. Once the spores land on tomato foliage, they germinate under cool, wet conditions.
The fungus grows optimally in temperatures between 60-80°F and requires prolonged leaf wetness from rain, irrigation, or high humidity to infect plants. Overhead irrigation and crowded tomato plants promote disease development.
Several factors contribute to septoria leaf spot infection:
- Infected tomato debris from previous seasons
- Cool, wet weather
- Overhead watering
- High humidity
- Close plant spacing
Once a plant is infected, the fungus produces more spores on the underside of spots which then spread the disease to additional leaves. Septoria leaf spot spreads rapidly under ideal conditions.
Symptoms of Septoria Leaf Spot
The initial symptoms of septoria leaf spot include:
- Small (1/8 to 1/4 inch), circular spots on older tomato leaves near the bottom of the plant. Spots may merge and form larger lesions.
- Lesions start as pale greenish-yellow areas that turn brown to black as they mature.
- Lesions have tan, gray or white centers with dark borders.
- Tiny black fungal fruiting bodies may be visible in the centers of spots when viewed with a hand lens. These structures produce spores.
- Heavily infected leaves turn yellow, wither and fall off. Defoliation typically starts on lower leaves.
- Fruit remains uninfected but sunscald and yield loss may occur due to defoliation.
How to Treat and Control Septoria Leaf Spot
If septoria leaf spot is detected early, treatment measures can help reduce disease severity and prevent major crop losses. Integrated management using cultural practices and fungicides is recommended.
Cultural Control Methods
- Remove and destroy all tomato debris after harvest to eliminate overwintering sites for the fungus.
- Rotate tomato crops to a different area of the garden each year to prevent buildup of Septoria fungi in the soil. Avoid planting tomatoes in the same spot 2-3 years in a row.
- Space and stake tomato plants to improve air circulation and reduce foliage moisture.
- Water at the base of plants instead of overhead watering to keep foliage dry. Avoid wetting leaves.
- Remove and destroy infected lower leaves immediately to limit spore production.
- Avoid working among wet plants to limit spread of fungal spores.
- Apply mulch around plants to prevent soil splash onto lower leaves.
Chemical Treatment
Fungicides can be applied preventatively or once symptoms are noticed:
- Chlorothalonil is an effective fungicide for septoria leaf spot control. It can be used every 7-10 days prior to disease onset.
- Copper-based fungicides such as copper oxychloride applied every 7-14 days can help protect plants. They have low toxicity to humans.
- Mancozeb is a broad spectrum fungicide labeled for septoria leaf spot. It requires reapplication every 5-7 days.
- Azoxystrobin can be used preventatively or at first sign of disease. It provides 7-14 days of protection.
Be sure to cover the undersides of leaves thoroughly with any fungicide spray. Follow label application rates and intervals. Combine fungicides with cultural practices for best results. Organic options include neem oil or bacillus subtilis. Resistant tomato varieties can also help reduce septoria leaf spot damage.
How to Identify Septoria Leaf Spot vs. Other Tomato Diseases
Septoria leaf spot may be confused with early blight, bacterial spot, or other fungal diseases at first glance. Here are tips for identifying septoria:
Compared to early blight:
- Septoria lesions are smaller in size, about 1/8-1/4 inch. Early blight spots can expand to 1/2 inch.
- Septoria centers are lighter in color – grayish or tannish. Early blight centers are dark brown to black.
- Septoria spots are mostly circular while early blight lesions are more irregularly shaped.
- Septoria is more common on younger leaves while early blight starts on oldest leaves near the ground.
Compared to bacterial spot:
- Bacterial spot lesions are more irregular while septoria spots are round.
- There are no dark fungal fruiting bodies in bacterial spot lesions like those sometimes seen in septoria spots.
- Bacterial spot centers dry out and holes form, unlike septoria.
- Bacterial spot doesn’t cause as much defoliation as septoria.
Compared to other fungal leaf spots:
- Septoria lesions are small and numerous. Leaf mold lesions are large, dark colored and fewer in number.
- Leaf mold and other fungal spots lack the tiny black fungal fruiting structures sometimes seen in septoria spots.
- Septoria causes more rapid and severe defoliation than other fungal leaf diseases.
Carefully examining the size, color, shape and distribution pattern of spots along with observing other symptoms can help accurately differentiate septoria leaf spot from other problems. Proper identification guides treatment and management.
Tomato Varieties that Resist Septoria Leaf Spot
Choosing resistant tomato varieties is an excellent cultural method to prevent septoria leaf spot damage. Many modern hybrids have enhanced disease resistance bred in.
Some tomato varieties reported to have improved tolerance or resistance to septoria leaf spot include:
- Ace 55 VFN
- Bella Rosa
- Biltmore
- Biltstar
- Celebrity
- Champion
- Jet Star
- Pink Girl
- Pony Express
- Scarlet Red
- Sun Leaper
- Defiant PHR
- Mountain Merit
- Scarlet Red
When purchasing tomato transplants, look for descriptors like F or N, which indicate resistance to fungal diseases or nematodes. The letters V and T indicate resistance to fungal wilts like verticillium or fusarium.
Heirloom varieties tend to be more susceptible to disease, while hybrids offer stronger septoria leaf spot resistance. Consult your local extension service or seed company for the best regional variety recommendations.
Can You Eat Tomatoes with Septoria Leaf Spot?
Septoria leaf spot only affects the foliage of tomato plants and does not spread to the fruit. Tomatoes infected with septoria are perfectly safe to eat once the fruit is washed well. Leaf spot diseases do not impact tomato quality or flavor.
However, unchecked septoria leaf spot can lower yields due to severe defoliation. Loss of foliage may also leave fruit more prone to sunscald. Controlling septoria allows for optimal fruit production and ripeness. The fruits themselves will not become diseased from this fungus, so tomatoes can be consumed even when septoria is present.
What is the Life Cycle of Septoria Leaf Spot?
The life cycle of septoria leaf spot has the following stages:
Overwintering – The septoria fungus survives the winter as structures called pseudothecia on infected plant debris in soil.
Spring spore release – In spring when temperatures reach 60 F, airborne spores called ascospores are shot from pseudothecia into the air. Rains and wind spread ascospores to tomato plants.
Leaf infection – Given sufficient moisture, spores that land on tomato leaves germinate and penetrate the leaf tissue through stomata. Initial infection occurs in as little as 6-12 hours under wet conditions.
Lesion development – Visible lesions become apparent in 5-14 days after infection. Gray spots with darkened edges form on leaves.
Sporulation – Tiny black fungal fruiting bodies called pycnidia form in lesion centers within 2-3 weeks. These structures generate more infectious spores called conidia.
Secondary spread – Conidia are dispersed by splashing water to cause new infections on leaves higher up the tomato plant. This repeating cycle amplifies disease severity.
Overwintering restart – As infected tomato plants decline, pseudothecia develop to survive winter and restart the cycle next spring.
The disease progresses most rapidly during cool, wet weather at 59-77°F. Hot summer conditions slow the fungus. Septoria leaf spot can complete several life cycles each growing season.
Key Takeaways on Septoria Leaf Spot in Tomatoes
- Septoria leaf spot is caused by the fungal pathogen Septoria lycopersici. Circular leaf spots with dark edges appear on older leaves and spread upward.
- Cool, wet conditions favor this disease which spreads by airborne spores. Overhead irrigation promotes epidemics.
- Remove lower infected leaves immediately and space plants for improved airflow to manage septoria. Fungicides also protect healthy tissue when applied early.
- Look for tomato varieties labeled as septoria or leaf spot resistant. Many hybrids have enhanced genetic resistance.
- Fruit can be eaten from infected tomato plants since the fungus only affects foliage. Controlling septoria maximizes yields.
- The fungus overwinters on debris before releasing spores in spring that initiate new infections. Stopping this cycle is key.
Careful attention to cultural practices, fungicide application, and selecting less susceptible tomatoes can help mitigate septoria leaf spot damage in the home garden. Consistent prevention from year to year is essential for effective management.
Frequently Asked Questions About Septoria Leaf Spot in Tomatoes
What does septoria leaf spot look like on tomatoes?
Septoria leaf spot appears as numerous small (1/8 to 1/4 inch) circular lesions on tomato leaves. The spots have light gray, tan or white centers with dark brown or black borders. Tiny black fungal fruiting bodies may be visible in the centers of lesions when viewed closely. Heavily infected leaves turn yellow and drop off the plant.
What causes septoria leaf spot on tomato plants?
The fungus Septoria lycopersici causes septoria leaf spot. It overwinters on infected debris and releases spores in spring that are spread to tomato plants by splashing water or wind. Prolonged leaf moisture allows the spores to infect leaves. Overhead irrigation, rain, and crowded plants promote disease development.
What is the best fungicide for septoria leaf spot on tomatoes?
Effective fungicides for septoria leaf spot include chlorothalonil, mancozeb, copper compounds, and azoxystrobin. They should be applied preventatively before disease onset or immediately at the first signs of infection. Complete coverage of the undersides of leaves is necessary for good control. Always follow label directions.
Should you remove leaves with septoria leaf spot from tomato plants?
Yes, it is recommended to immediately remove and destroy any lower leaves with septoria lesions to prevent spores from being produced on that tissue. This helps slow disease spread to upper leaves. Be sure to disinfect pruning shears between each cut to avoid spreading spores.
Can you still eat tomatoes with septoria leaf spot?
Tomatoes with septoria leaf spot are completely safe to eat. The fungal disease only affects the foliage and does not spread to the fruit. Simply wash tomatoes thoroughly before consumption. Controlling septoria allows for the best harvests.
How do you avoid septoria leaf spot on tomatoes?
Practices to help avoid septoria leaf spot include crop rotation, staking/pruning to improve air flow, drip irrigation, removing diseased leaves promptly, applying fungicides early, planting resistant varieties, keeping the garden free of debris, and avoiding working among wet plants.
How is septoria leaf spot different from early blight?
Septoria spots are small (1/8 inch), numerous and circular with light gray centers while early blight lesions are larger (up to 1/2 inch), irregularly shaped with dark brown to black centers. Septoria affects lower leaves first while early blight starts on the oldest leaves.
Can septoria leaf spot spread from tomatoes to other plants?
The species of septoria that infects tomatoes only affects solanaceous plants like tomatoes, potatoes, eggplants and nightshades. It does not spread to unrelated vegetable families. However, there are many different septoria fungal species that attack other types of plants.
Does septoria leaf spot affect tomato stems or fruit?
Septoria lycopersici only infects tomato foliage and does not spread to stems, petioles, branches or tomato fruit. However, severe leaf loss from septoria can expose fruits to sunscald. The fungus itself does not directly infect anything but the leaves.
Conclusion
Septoria leaf spot is a common but manageable tomato disease. Paying close attention to plant spacing, watering, and other cultural practices can help prevent initial infection. At the first sign of spots, fungicides and the removal of affected leaves is warranted to protect remaining foliage and limit spore production. Choosing less susceptible tomato varieties also mitigates septoria leaf spot damage. Diligent preventive care provides the best chance of a healthy, productive tomato crop. Though septoria leaf spot can be frustrating for gardeners, following integrated pest management principles allows for effective control with minimal effort.