Published on Wednesday, 30 November -0001 00:00
By Janelle Kuechle,
U of M Extension
Tomato season is coming up quickly and with our warm humid weather, it is important to be watchful for diseases affecting tomatoes. Alternaria leaf spot (Alternaria solani) and Septoria leaf spot (Septoria lycopersici) are two fungal diseases commonly seen in tomatoes. Characteristic symptoms of these diseases start with the lower leaves of the plants turning yellow, then brown, and then drying up and falling from the plant.
Septoria leaf spot is the most common fungal disease of tomatoes and primarily infects leaves, although stem and fruit lesions also occur. This disease can affect plants at any stage of plant development. Round, yellow spots develop on the lower leaves, these spots enlarge and turn brown to gray. Tiny black fruiting bodies then form in the center of the leaf spots. These fruiting bodies produce spots which cause secondary infections, usually upwards throughout the plant. For this reason, the disease typically is seen spreading up the plant like a ladder. Infected leaves turn yellow then brown and fall from the plant. Defoliation can expose fruit to the sun which will cause sunscald damage. Under warm temperatures and wet conditions, splashing rain, insects and even hands and clothing of gardeners can spread the fungal spores. Septoria also overwinters in the debris of diseased plants, in the soil, and in weeds of the nightshade family. Sanitation is the first step in keeping this disease from spreading. It is important to destroy any infected plant material and to keep weeds at bay throughout the season and after harvest. Remove plant material from the garden and keep a crop rotation throughout your garden to keep lingering pests and diseases from affecting plants every year. If your plants have been affected with spots in the lower leaves, prune infected leaves off of tomato plants and dispose of them, however do not remove more than 1/3 of the plant. It is also important to water plants at the base of the plant early in the day and stake or cage plants to increase air circulation. This prevents the disease from spreading through water splashing on the leaves. The use of mulch around the base of plants will help to prevent water splash from carrying spores upward into the plant, as well as help to maintain an even supply of moisture.
It also may be helpful to apply fungicides, particularly when weather conditions are conducive to development of disease. Scout plants often and begin a fungicide spray as soon as symptoms appear. Fungicides will not cure affected plants; only prevent spread from getting too severe. Always read labels carefully and apply as directed. Removing the most severely infected foliage and applying the fungicide to foliage that does not show the symptoms will reduce the impact and the spread of the disease. The following fungicides as labeled for control of Septoria leaf spot. Apply one of the following at 7-10 day intervals starting at the first sign of the disease. Copper-containing
products (Bordeaux 8-8-100, Kocide, TopCop). These products are considered acceptable for organic production. Use sulfur-containing products carefully as it can burn the leaves if it is applied at high temperatures or at rates higher than the label recommendation. For conventional control, use products containing Chlorothalonil, Maneb or Mancozeb. Be sure that any product you select is labeled for application to tomatoes and follow all directions and precautions. When applying these products, it is important to apply it to the undersides of the leaves as well as the surface.