Question of the Week
Answer: First, make sure that you have identified Septoria Leaf spot as indeed the culprit of your tomato woes. Following are some ways of identifying Septoria leaf spot, which is a fungus, compared to many of the bacterial diseases that affect tomatoes.
Numerous, small, water-soaked spots, which are the first noticeable characteristic of Septoria leaf spot, appear on the lower leaves after fruit set. Spots enlarge to a uniform size of approximately 1/16 to 1/4 inch in diameter. They have dark brown borders and tan or light colored centers. Yellow haloes often surround the spots. Severely infected leaves die and drop off. Septoria leaf spot is sometimes confused with bacterial spot of tomato. Septoria leaf spot is easily distinguished from early blight/ bacterial spot, another foliar disease of tomato, by the uniform, small size of the spots (which if you look closely are spore receptacles) and the lack of concentric rings in the spots. The presence of fruiting bodies (the spores) of the fungus, visible as tiny black specks in the centers of the spots, confirms Septoria leaf spot.
Favorable weather permits infection to move up the stem, causing a progressive loss of foliage from the bottom of the plant upward. Plants appear to wither from the bottom up. Loss of foliage causes a decrease in the size of the fruits and exposes fruit to sunscald. Spotting of the stem and blossoms may also occur.
Once you have identified the disease it is helpful to know its life cycle in order to develop a pest management strategy.
Septoria lycopersici overwinters in old tomato debris and on wild solanaceous plants, such as ground cherry, nightshade, and jimsonweed. Seeds and transplants may also carry the fungus. The disease is favored by moderate temperatures and abundant rainfall. Spore production is abundant when temperatures are 60°-80°F (15.5°-28°C). Spores are easily spread by wind and rain. Infection occurs on lower leaves after the plants begin to set fruit.
Cultural control is one of the only ways to control/ prevent Septoria Leaf Spot.
• Dispose of crop refuse by plowing under deep or composting.
• Control weeds (particularly those in the Solanum genera) in and around the edge of the garden.
• Rotate tomatoes with cereals, corn, or legumes. A 4-year rotation is recommended where disease has been severe.
• Mulch acts as a barrier between soil and plant to prevent splashing spores onto plant.
• Prune lower leaves that are infected at first sign—will also improve air circulation.
• Stay out of growing areas when the foliage is wet.
• Water early in the day and, if possible, avoid wetting the foliage, through using drip irrigation or soaker hoses.
• Stake plants.
• Be sure plants have adequate nutrition.
Control via biorational, compost tea, etc.
Fungicides, organic or not, have shown limited results with Septoria leaf spot:
• Copper and sulfur are fungicides approved by the National Organic Program (NOP) Standards. Application of copper is a routine disease control practice in organic tomato production in the eastern United States. Copper functions both as a fungicide and bactericide and is labeled (under the NOP) for anthracnose, bacterial speck, bacterial spot, early and late blight, gray leaf mold, and septoria leaf spot.
Commercial products like Kocide 101™ are used in both conventional and organic tomato production for the control of Septoria leaf spot, bacterial spot, bacterial speck, anthracnose, and early blight. Applications are made on a 7-10 day schedule and the result may be 8-12 sprays per growing season. See the resource Eggplant, Pepper, and Tomato XXIV; Septoria Leaf Spot by Howard Schwartz and David H. Gent of High Plains IPM for information on applying specific copper fungicidal controls. Note that the pesticides listed in this publication are not all organic. Only some of the copper fungicides are permissible.
Under the NOP, The National List of Allowed and Prohibited Substances §205.600 are:
1) Coppers, fixed—copper hydroxide, copper oxide, copper oxychloride, includes products exempted from EPA tolerance, Provided, That, copper-based materials must be used in a manner that minimizes accumulation in the soil and shall not be used as herbicides.
(2) Copper sulfate—Substance must be used in a manner that minimizes accumulation of copper in the soil.
• The use of copper fungicides in organic production is somewhat controversial. It is directly toxic at applied rates to some beneficial organisms, particularly earthworms and some soil microbes such as blue-green algae—an important nitrogen-fixer in many soils. Excessive use can also result in the buildup to phytotoxic (crop damaging) levels of copper in the soil. Thus, organic growers often monitor soil copper levels through regular soil testing.
Research has shown that compost teas seem to work more for phytopthera (root rot) and Pythium (damping off) diseases in tomatoes. It might be beneficial to try spraying (fully cured) compost tea on just a few plants to see if it is effective for you. ATTRA offers a publication titled Notes on Compost Teas.
Unfortunately, there has not shown to be any effective biological control agent for Septoria Leaf Spot. Biological fungicides are a new and emerging field. F-Stop™, registered as a seed treatment for tomatoes, contains a biocontrol agent called Trichoderma viride sensu and may be effective for treating seed infected with Septoria leaf spot. There is no conclusive evidence on its efficacy with Septoria Leaf spot specifically, however.
The ATTRA publication titled Organic Tomato Production can help with general tomato disease control strategies as well as some specific information on other tomato diseases.
Hansen, Mary Ann, Extension Plant Pathologist, Department of Plant Pathology, Physiology and Weed Science, Virginia Tech. Adapted from a previous publication by R.C. Lambe. Septoria Leaf Spot of Tomato. Virginia State University Cooperative Extension. Publication Number: 450-711, Posted December 2000.
Delahaut, Karen and Walt Stevenson. Tomato Disorders: Early Leaf Blight and Septoria Leaf Spot. University of Wisconsin Extension. 2004. http://s142412519.onlinehome.us/uw/pdfs/A2606.PDF
Schwartz, Howard F. and David H. Gent. Eggplant, Pepper, and Tomato XXIV; Septoria Leaf Spot. High Plains IPM.
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