by Brian Hudelson, Senior Outreach Specialist and Director University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic
If you have ever grown tomatoes, you may have had your plants dry up and lose their leaves beginning in mid-summer. You are left with the tomato equivalent of Charlie Brown’s Christmas tree, with a few tomato “ornaments” clinging to bare branches and stems. If this has happened to you, you have personal experience with one of the fungal tomato leaf blights.
Common tomato leaf blights include Septoria leaf spot and early blight. These diseases first appear on lower leaves. Septoria leaf spot begins as necrotic (i.e., dead) spots that are roughly ¼ inch in diameter and have a light-colored center, a dark edge, and sometimes a yellow halo. Early blight forms larger necrotic spots (up to approximately 1 inch in diameter), with concentric rings, giving the spots a target-like look. Spots of both Septoria leaf spot and early blight subsequently merge, causing leaves to collapse and plants to eventually defoliate.
Tomato leaf blights typically occur every summer but are more common during rainy seasons or in gardens where plants are watered with overhead sprinklers. Moisture provided by rain and overhead watering wets leaves and provides the perfect environment for infections to occur.
The best strategies for managing tomato leaf blights are preventative ones. Once symptoms are present, management options are limited. To help reduce the impact of tomato leaf blights:
• Remove old tomato debris Leaf blight fungi survive in dead tomato tissue (and potato tissue as well). Removing and destroying last year’s plants will reduce the amount of tomato blight fungi in your garden. Debris can be buried, burned, or composted. When composting, the pile temperature must reach 140°F or more, and the pile contents must be turned routinely so that the pile heats evenly. The combination of heat and decay of plant tissue helps eliminate disease-causing fungi.
• Decontaminate items used in your garden Bits of tomato debris harboring leaf blight fungi can cling to tomato cages, stakes or other items that you use when gardening. A thirty second or longer treatment of these items with 10 percent bleach or 70 percent alcohol can help kill these fungi. Spray disinfectants containing approximately 70 percent alcohol also can be used. Spray items until they drip and then allow them to air dry.
• Use leaf blight resistant tomatoes While resistant varieties are available and an option for management, such varieties are not common, and oftentimes have bland-tasting fruit.
Septoria leaf spot is a common leaf disease that can defoliate tomato plants.
• Space tomatoes plants appropriately Plants should be spaced so that at their mature size, leaves on adjacent plants do not overlap. Providing sufficient space allows good air flow between plants that can reduce drying time when leaves get wet.
• Mulch Use approximately 1 inch of a high-quality mulch over this year’s and last year’s tomato-growing areas. Mulch provides a physical barrier that prevents spores of leaf blight fungi from blowing or splashing from small bits of tomato debris in the soil onto this year’s tomato plants. When mulching, avoid using old tomato or potato debris, wood chip mulches of unknown composition, or grass clippings from herbicide-treated lawns, as these materials can lead to disease issues or problems with chemical toxicities.
• Thin plants Like proper plant spacing (see above), removing lower leaves (as well as branches here and there) as tomato plants grow can help promote better air flow and more rapid leaf drying, thus leading to a less favorable environment for infections to occur.
• Do not overhead water Use a soaker or drip hose instead. These hoses apply water directly to the soil and help keep leaves dry.
• Use preventative fungicide sprays Use this option only when you have had a leaf blight problem for many years and other control strategies have failed. In home gardens, fungicides labeled for use on tomatoes and containing copper are the products of choice. Applications must be started before symptoms are observed, and continued approximately every seven to fourteen days. Uniform coverage of upper and lower leaf surfaces and stems is critical for fungicides to be effective.
• Buy your tomatoes As much as you like growing tomatoes, sometimes when leaf blights are chronic and severe, your best option may be to simply forgo the frustration of battling disease, and find local fresh market growers whom you can support by buying their produce.
Having problems with tomato leaf blights (or any other plant diseases)? Consider submitting a sample to the UW-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) for a diagnosis. Details on sample submission are available through your county UW-Extension office, at http://pddc.wisc.edu, by calling 608-262-2863 or by emailing bdh@plantpath.wisc.edu.