Amy Dabbs: Battling disease in the name of healthy tomatoes
Q Help! My tomato plants have a disease again this year. How can I grow fresh, healthy tomatoes in my garden?
A: Nothing is more disappointing than seeing the fruits of your labor literally disappear before your eyes. Tomato diseases affect nearly every would-be tomato grower, so it's a good idea to learn how to identify and outwit common diseases before they get between you and those tomato sandwiches.
Gardeners must know what the problem is before deciding on a course of action. Learning to recognize common symptoms can help identify tomato diseases.
Tell-tale signs such as wilting, leaf spots and yellowing are most often seen.
Here is a quick look at the top five diseases affecting tomatoes in our region, according to my colleagues Anthony Keinath, a vegetable pathologist at the Clemson Coastal Research and Education Center, and Roger Francis, Charleston County Clemson Extension Agent.
Tomato spotted wilt. A viral disease, TSW spreads through the feeding activity of tiny insects called thrips. Keinath says the only effective control is to grow resistant varieties such as BHN-1021, a round slicing tomato; BHN-968, a cherry tomato; or Granadero, a plum-type tomato. It is also important to maintain a weed-free garden as thrips overwinter on weeds. The early drought and warm winter temperatures are likely to blame for this year's early outbreak of the virus.
Southern bacterial wilt. Like something from a sci-fi movie, soil-dwelling bacteria enter plants through wounded roots. These bacteria quickly multiply, clogging water-conducting cells in plants with slimy ooze. The result is dramatic wilting that no wishful watering will cure. Francis says plants look fine in the morning but by afternoon are near death. There are no chemical controls and the only defense is to plant resistant varieties such as Neptune and Tropic Boy, or grow tomatoes in containers.
Bacterial spot. Look for small, dark, greasy spots on foliage and brown, scabby spots only on green fruits. This disease is spread by water splashing soil-containing bacteria onto leaves. Infected leaves should be removed quickly. Prevent spread by treating with a copper-based fungicide.
Southern blight. Often confused with other wilt diseases because of drooping leaves, this fungal disease exhibits a dry, brown rot on stems at the soil line. Girdling lesions on the stems ultimately cause the whole plant to wilt and die. This soil-dwelling fungus also rots fruit where it touches the soil. Aluminum foil “collars” around the base of plants may block the fungus from infecting the stems but is no guarantee. No chemical controls are available.
Early blight. A bull's eye pattern in the center of leaf spots characterizes this disease that affects older leaves near the soil first. A blast of heat and humidity spur growth of this fungal disease. Severe outbreaks can be controlled with fungicides that contain the active ingredients chlorothalonil, mancozeb or fixed copper. All provide effective early blight control when used according to label directions. The only tolerant variety is Mountain Fresh Plus, so prevention is critical.
What to do
So how does anyone ever grow a ripe tomato with all these diseases lurking like bad guys twirling their mustaches? By learning about disease life cycles and using these cultural controls, gardeners can foil any plots threatening their veggie patch.
Tomatoes should be grown in a spot where they will get six hours of sunlight each day.
Ensure plants receive 1 to 1˝ inches of water per week. Direct water at roots, not foliage. Use soaker hoses or drip irrigation.
Rotate tomatoes and their relatives — eggplant, peppers and Irish potatoes — with unrelated crops such as corn or beans.
Mulch with 2 to 3 inches of grass clippings, pine straw or leaves to keep soil from splashing onto foliage. Mulching maintains soil moisture and allows roots to take up calcium efficiently.
Avoid crowding plants. Follow spacing recommendations on labels or seed packets.
Do not over fertilize plants. Excessive amounts of nitrogen or potassium depress the uptake of calcium.
Test garden soil and maintain a pH between 6 and 6.5. Soil samples can be brought to the Clemson Extension office for testing. Call 722-5490.
Remove soil from tools, tomato stakes and containers before reusing.
Add organic matter to clay or sandy soil to increase plant uptake of water and calcium.
Find out more information at the Clemson University Home & Garden Information Center at www.clemson.edu/extension.
Want to learn more about gardening? Applications are now being accepted for the 2012 Master Gardener Class. See the calendar of Page D3 for more details.
Amy L. Dabbs is the urban horticulture extension agent and tri-county Master Gardener coordinator for the Clemson University Cooperative Extension. Send questions to email@example.com.