Tomato Diseases in the Home Garden

Tomatoes are a favorite summer vegetable for many gardeners. Here in Missouri there are a lot of disease problems that can ruin the chance of a crop, shorten the harvest season and reduce our yield, or reduce the quality of the fruit. In fact, the health of the plant can even have an impact on the flavor of the tomato. There are several common problems we frequently see on tomatoes in the home garden. Most frequently, we see Septoria leaf spot and early blight, two common foliar diseases. Occasionally we will see bacterial spot and speck, foliar diseases which can also damage the fruit. Problems that affect the entire plant include Fusarium wilt and herbicide injuries. Damage to the fruit frequently includes blossom end rot, sunscald and anthracnose. When you can identify the symptoms of each of these common problems you can take the proper control measures and avoid them in the future.

Foliar Diseases: Septoria leaf spot (Septoria lycopersici) and early blight (Alternaria solani) are fungi that infect leaves; early blight can also affect the fruit. Virtually anyone who raises tomatoes in Missouri has likely seen these diseases on their plants. These diseases infect the lower leaves first and gradually work their way up the plant. For Septoria, the symptoms are small circular brown to tan lesions with a light tan center. If you look closely with a hand lens, you will see little black dots in the center, which are the fruiting bodies of the fungus. At first glance, leaves infected with early blight look similar to Septoria, however the spots are larger than Septoria lesions, there is usually more chlorosis (yellowing) around the lesion area, and concentric rings can give the spots a target-like appearance. Bacterial spot and speck (Xanthomonas axonopodis pv. vesicatoria; Pseudomonas syringae pv. tomato) can also produce lesions on leaves, however the lesions are usually dark, water soaked, and greasy in appearance, and mature lesions are surrounded by a yellow halo. While the spots are initially small, they can come together and affect larger portions of the plant. Small dark, crusty spots also often disfigure the fruit. With any of these foliar diseases, infected leaves will eventually turn brown and fall from the plant.

Foliar Disease Control Measures: One of the most important things you can do is purchase vigorous, healthy seedlings that show no signs of disease, or you will battle these diseases all season. Vigorous, modern tomato varieties often develop fewer symptoms than older heirloom varieties, although very little resistance exists in common tomato varieties. It is also very important to keep the plants healthy and vigorous by waiting to plant in a sunny location until after the soil is warm and to use good horticultural growing practices throughout the season. Splashing water spreads these diseases and disease development is likely during periods of heavy dew and extended leaf wetness. Minimize leaf wetness by watering from below and staking or space plants farther apart to improve air circulation within the canopy. Reduce leaf contact with soil by mulching and avoid walking or working in the tomato patch while foliage is wet. While you can not cure plants once they have become infected, plants with these diseases can produce well despite being infected, especially when disease develops later in the season. Because all these diseases can survive between crops on infected tomato debris, remove and destroy debris soon after the plants stop producing. If Septoria and early blight are severe year after year and limit production, fungicide applications that contain chlorothalonil, mancozeb or copper applied every 7 to 10 days can also help in managment. Fungicides should be used in addition to the other management tactics, and according to the label.

Diseases that affect the entire plant: Another fungal disease occasionally seen in the home garden is Fusarium wilt (Fusarium oxysporum f. sp. lycopersici), which causes a general wilt and yellowing of the leaves. Recently infected plants may first wilt during the heat of the day, but seemingly recover the next morning. The wilting gradually becomes more extensive until the plant dies. When you cut into a stem of a diseased plant the vascular tissue is brown and is most noticeable just above the soil line. When this fungus is present in the soil it can survive for several years.

As no fungicides are available for control of Fusarium, management relies on resistant cultivars, crop rotation and maintenance of plant health. An “F” behind the cultivar name or in the description will indicate resistance, however, since there are several different strains of the fungus and not all cultivars have resistance to all strains it is possible to have Fusarium wilt despite purchasing resistant plants. Long rotations (6 to 7 years) between solanaceous crops can help. In small areas solarization (using plastic to heat the soil to high temperature) may be beneficial.

Tomatoes damaged by herbicide applications are frequently submitted to the plant diagnostic clinic. Tomatoes are very sensitive to drift from broadleaf herbicide applications to lawns. Even minimal exposure can cause abnormal leaf growth. New leaves develop puckered, cupped or curled growth. Sometimes, a single, brief exposure will not produce long lasting symptoms, however, direct contact or extended exposure that occurs when using herbicide-treated grass as mulch around tomatoes will cause moderate to severe stunting, fruit deformation and poor fruit development. Some of the leaf distortion and cupping is confused with aphid damage.

Fruit diseases and disorders: Frequently, tomato fruit develop a tan, sunken lesion at the blossom end or side of the fruit called blossom end rot. This is a physiological problem caused by a calcium deficiency, often due to unfavorable growing conditions, especially irregular moisture levels, which can make it difficult to take up sufficient calcium from the soil, even when calcium levels are high. This problem can be managed by maintaining sufficient moisture levels. Mulching can help maintain moisture, as will planting in a deep soil containing some organic matter.

Sunscald occurs on sides of fruit exposed to the sun. The affected areas are sunken, light brown or white. Keeping the foliage healthy and disease free will usually prevent exposure of the fruit to direct sun and protect them from developing the disorder.

Anthracnose (Colletotrichum coccodes) causes sunken lesions on ripe fruit. The lesions are small and circular, often with tiny black bumps in the middle.

Soft rots frequently occur when secondary organisms infect the lesion areas. Green fruit can become infected during periods when free moisture is present, but show no symptoms until ripening occurs. Control measures described for foliar diseases will also help manage anthracnose.

We often simply accept the early demise of our tomato plants or reach for the pesticides when we see a problem. By becoming familiar with the common problems and their symptoms, we can make more effective management decisions and prevent these problems from destroying our tomato crops and ruining our hard work every summer. When you have problems with your tomatoes, you are always welcome to submit a sample to the plant diagnostic clinic. Please see the February issue of Missouri Environment and Garden, or our website at http://soilplantlab.missouri.edu/plant/index.htm for more information on submitting samples.

Simeon Wright Plant Diagnostic Clinic
573-882-3019