Controlling Tomato Diseases

Tomatoes are the one vegetable—or fruit, botanically speaking—that most of us look forward to when we plant our gardens in the spring, however; this year more than most it seems diseases are lurking around every corner threatening to destroy our harvest. There are dozens of diseases that can infect tomatoes, however, it is usually a handful that cause major problems.

tomato diseases

Early blight is one of the most common tomato diseases. It usually starts on the bottom leaves and works its way up until it eventually defoliates large portions of the plant. Leaves with late blight develop black spots with yellow areas around them, kind of like black spot on roses. The infection continues until the leaves drop off. Early blight is worse during warm wet weather and is made even worse by overhead (sprinkler) watering. Control by mulching tomatoes, this will keep soil from splashing on the leaves where the fungus overwinters and will reduce the amount of watering you will need to do. When watering tomatoes, never use sprinklers, it wets the leaf, which in turn doubles your chances of fungal diseases. Instead, use soaker hoses. Fungicides are also good to reduce the instance of disease and to slow it down. Fungicides such as mancozeb, maneb, and daconil (chlorothalonil) are effective against fungal diseases of tomatoes. When spraying fungicides follow all label directions and be sure to spray the undersides of the leaves as well as the tops.

Late blight appears as large water soaked areas on leaves that eventually turn brown and papery. Sometimes the lesions are surrounded by visible white rings of mold especially if leaves are wet. You can also have green to black irregular lesions on the stems. Again don’t overhead water, use mulch and fungicides as listed above.

Septoria leaf spot begins as water soaked lesions on older leaves. The spots eventually turn brown with gray centers and die. This disease makes a ‘frog eye’ appearance to the leaves. The entire leaf will drop if the infection is severe. To control septoria be sure to clean your garden up in the fall because it overwinters on old leaf and stem debris. Use fungicides listed previously if infection occurs.

Alternaria canker attacks leaves, fruit, and stems. Leaf areas between veins turn brown and are killed leading to leaf curl and eventual death of the entire leaf. Alternaria symptoms on fruit are dark brown spots of rot and on the stem it appears as a sunken dark brown area known as stem canker. To control alternaria clean debris from the fields in the fall, don’t wound or knick existing plants during cultivation, and don’t overhead irrigate. Fungicides sprays should be used to prevent infection if you have had it in the past.

Verticilium and Fusarium wilts are fungal diseases that originate in the soil. These bacterial diseases enter the plants through the roots and cause the leaves to yellow and the plant to wilt and die. They block the vascular system of the plants which blocks the flow of nutrients and water. Crop rotation is key to controlling this disease, especially if you grow heirloom varieties of tomatoes. Plants such as eggplant, pepper, tobacco, and potatoes are susceptible to these wilts and should not be rotated with tomatoes. The easiest way to avoid these diseases is to plant hybrid varieties which are rated as resistant such as better boy, big beef, beef master, and park’s whopper among many others. You can also rotate areas out susceptible plant species for 4-6 years to rid the soil of the fungus.

Submitted by Dennis Morgeson, Agent for Horticulture, Washington Co. Cooperative Extension Service