Tomatoes are a favorite garden crop, but they can be affected by a variety of diseases that reduce plant health and yield. Learning to recognize common symptoms and understanding how diseases spread are the first steps toward effective management and a successful harvest.
Disease Management Strategy | Septoria Blight | Early Blight | Late Blight | Anthracnose | White Mold | Vascular Wilts | Bacterial Spot | Bacterial Speck | Bacterial Canker | Viruses | More Information
Disease Management Strategy
For many home gardeners, diseases and disorders are the most common problems. There are several steps that the gardener can take to reduce problems before they even start, regardless of the disease or disorder.
To manage diseases and disorders, a combination of cultural tactics is needed.
- Select varieties with disease resistance, especially if you have a known field history of some soilborne diseases. A partial list of disease-resistant cultivars can be found from Cornell University: Disease Resistant Tomato Varieties.
- Some tomatoes have letters following the cultivar name to represent the disease(s) they are resistant to.
- V=Verticillium wilt, F=Fusarium wilt; FF=Fusarium wilt races 1 and 2; FFF= Fusarium wilt races 1, 2, and 3; N=Nematodes; T=Tobacco Mosaic Virus; A=Alternaria (early blight); TSWV=Tomato spotted wilt virus; CMV=Cucumber mosaic virus; L=Septoria leaf spot; St=Gray leaf spot; ASC=Alternaria stem canker.
- Water and fertilize to maintain plants in a vigorous condition. Healthy plants are more resilient against disease pressures.
- Avoid fluctuations of too much and too little water. Inconsistent watering stresses plants, making them more vulnerable to disease, and many physiological disorders are exacerbated by inconsistent watering.
- Keep foliage dry. This is accomplished by:
- Spacing plants far enough apart that they will not be crowded when fully grown. This allows the foliage to dry rapidly.
- Watering at the base of the plants and in the morning rather than the evening, to minimize the time the leaves are wet.
- Avoid working with plants when foliage is wet (from rains, irrigation, or heavy dew), causing the spread of disease-causing microorganisms (also known as pathogens).
- Eradicate weeds. Some weeds can serve as a reservoir for pathogens causing disease in tomatoes. Additionally, removing weeds can help improve light penetration and airflow, allowing foliage to dry quicker.
- Utilize mulch to reduce soil splashing. This reduces the likelihood of diseases present in the soil from splashing onto the leaves. It has the added benefit of reducing weeds. Mulch of any type could potentially be used, but grass clippings, shredded leaves, and straw are great options.
- Stake or trellis plants to reduce contact with the soil.
- Remove as much plant debris as possible in the fall. This helps reduce the overwintering of tomato pathogens.
- Plant disease-free transplants.
- Practice good crop rotation so that tomatoes (and closely related crops like potatoes, peppers, and eggplant) are grown in the same ground only every three or four years.
- Accurately identify the problem. You can only treat it appropriately if you know what the problem is.
- Using fungicides appropriately. Fungicides may be used to protect new plant tissue and prevent pathogen infection and spread. Fungicide applications should always be used in combination with other management tactics and used as the last option for control. Remember always to read and follow pesticide labels.
Diseases
Septoria Leaf Spot and Blight
Septoria leaf spot, caused by the fungus Septoria lycopersici, is the most common foliar disease of tomatoes in Iowa. It first appears as small, water-soaked spots that soon become circular spots about 1/8 inch in diameter. The lesions gradually develop grayish-white centers with dark edges. When conditions are favorable, fungal fruiting bodies appear as tiny black specks in the centers of the spots.
Lower leaves are infected first, and the disease progresses upward the plant. Spores are spread to new leaves by splashing rain or overhead watering. Heavily infected leaves turn yellow, wither, and eventually fall off. Defoliation can be severe, especially after periods of prolonged warm, wet weather. Infection can occur at any stage of plant development but appears most frequently after plants have begun to set fruit. The fungus survives the winter in tomato debris. The importance of leaves in the lower canopy of a tomato plant to fruit production diminishes as the season progresses. Some amount of lower canopy defoliation should be considered tolerable and not warrant immediate concern unless environmental conditions favor the progression of the disease in frequent rain events.
Management of this disease should start with cultural tactics, in particular those disease management strategies described above. Some cultivars have some notable resistance to Septoria leaf spot and may be denoted with an "S" after the cultivar name on the seed envelope. Chemical options are also available, though chemical control is often an uphill battle once the disease is present in the home garden for a season. More information on the control of this disease can be found in these articles: How Can I Prevent Blight on my Tomatoes? and Septoria Leaf Spot.
Early Blight (Alternaria solani)
Early blight, caused by the fungus Alternaria solani, is also known as Alternaria leaf spot or target spot. It is a common disease in Iowa gardens and may attack plants alongside Septoria leaf spot.
Brown to black spots, 1⁄4 to 1⁄2 inch in diameter with dark edges, appear on lower leaves first. Spots frequently merge, forming irregular blotches. Dark, concentric rings often appear in leaf spots, resulting in the “target” appearance. Leaves turn yellow, dry up, and prematurely drop when only a few spots are present. The fungus occasionally attacks fruit, causing large, sunken areas with concentric rings and a black, velvety appearance. Warm, wet weather favors the rapid spread of early blight.
Management of this disease starts with cultural tactics, in particular those disease management strategies described above. Crop rotation is important, especially since potatoes, peppers, and eggplants can also serve as a host for this disease. Harvest all ripe fruit at every picking to avoid fruit infection. Resistant cultivars of tomatoes provide only partial resistance to early blight. These cultivars may be denoted with an "A" or "ASC" after the cultivar name. Chemical options are available as well. However, once symptoms appear, managing this disease is difficult to impossible, even with chemicals. More information on the control of this disease can be found in this article: How Can I Prevent Blight on my Tomatoes?
Late Blight (Phytophthora infestans)
Late blight, caused by the watermold Phytophthora infestans, rarely occurs in Iowa but can devastate tomato plantings during cool, rainy weather. Late blight may infect young (upper) or old (lower) leaves. It first appears as water-soaked areas that enlarge rapidly, forming irregular,
greenish-black blotches, giving the plant a frost-damaged appearance. The undersides of the leaves often show a downy white growth in moist weather.
Infection of green or ripe fruit produces large, irregularly shaped brown blotches. Infected fruit rapidly deteriorates into foul-smelling masses. Late blight usually appears in mid or late August during persistent cool, wet weather or when cool night temperatures cause frequent heavy dews.
Management of this disease starts with cultural tactics, in particular those disease management strategies described above. Crop rotation is important, especially since potatoes can also serve as a host for this disease. More information on control of this disease can be found in this article: How Can I Prevent Blight on my Tomatoes?
Anthracnose Fruit Rot
Anthracnose, caused by the two species of Colletotrichum, Colletotrichum coccodes and C. acutatum, is probably the most common fruit-attacking disease of tomato in Iowa. Symptoms first become visible on ripe or ripening fruit as small, sunken, circular spots in the skin. As these spots expand, they develop dark centers or
concentric rings of dark specks, which are the spore-producing bodies of the fungus. In moist weather, these bodies exude large numbers of spores, leaving diseased areas a cream to salmon-pink color. By this stage, decay has penetrated deeply into the tomato flesh. Spotted fruits may rot completely because of an attack by secondary fungi through anthracnose spots.
Anthracnose appears most commonly on ripe or overripe fruits. The fungus survives the winter on diseased tomato vines, in the soil, and in seeds. Spores are spread mainly by rain splash. Warm, wet weather causes the disease to spread and symptoms to develop. While insects or other wounds facilitate infection, tomatoes can also become infected in the absence of wounds.
Management of anthracnose starts with cultural tactics, particularly those disease management strategies described above. Harvest fruit regularly, picking all ripe fruit at each harvest and disposing of infected fruit.
White Mold (Timber Rot)
White mold is a fungal disease that can infect more than a hundred different plant species, including field crops, garden vegetables, herbaceous ornamental plants, and a number of common weeds.
White mold, caused by the fungus Sclerotinia sclerotiorum, is a serious disease that often results in the death of plants. Infection by the fungus is favored by cool, moist conditions. Diseased tissue initially shows a water-soaked appearance. White mold can be seen on the infected plant tissue when conditions are humid, and is usually most evident on stems. Small black sclerotia (round, hard black fungal survival structures), about the size of sunflower seeds, can be seen on or in the diseased tissue, often embedded in the cottony, white strands of the fungus. The dying tissue tends to show a bleached, dried appearance.
Sclerotia can survive in the soil for many years, making it a challenge to manage this disease. Because the white mold fungus produces these sclerotia (survival structures) and can infect so many plant species, rotation is often of limited usefulness. Since infection is favored by abundant moisture, practices that improve air circulation, such as wide spacing at planting, staking of plants, and preventing water splash on leaves and stems, can help reduce conditions favorable for disease development. While some plant species offer cultivars with varying levels of resistance to white mold, no tomato cultivars offer resistance. Fungicides are labeled to help control the disease in certain crops, but are not usually practical for use in the home garden. Sanitation, the careful removal of infected plants, is the most important management practice for home gardeners. Since sclerotia can survive in the soil for several years, removal of infested soil may be feasible only if the area affected is small. Many weeds are susceptible to white mold, so control of weeds in and around the garden is helpful in managing the disease.
Vascular Wilts
The initial symptoms of vascular wilts are wilting of the plant foliage during the heat of the day. Affected plants often recover in the evening or overnight. However, the wilting gradually worsens, and often, plants eventually die.
Vascular wilts in tomatoes are caused by two primary types of fungus, Fusarium (Fusarium oxysporum f. sp. lycopersici) and Verticillium (Verticillium alboatrum and Verticillium dahliae). These soil-borne fungi invade tomato plants through injured roots. The fungi spread into the stem's water-conducting tissue (xylem) and block water flow to the foliage. The foliage of affected plants turns yellow from the base of the stem upwards, then wilts and dies.
With Fusarium wilt, often only one side of a leaf midrib, one branch, or one side of a plant may be affected at first, showing chlorosis or yellowing, But eventually, the entire plant eventually wilts and succumbs. Affected plants die early and produce few, if any, fruit. With Verticillium wilt, symptoms do not progress along one side of a leaflet, branch, or plant. Infected plants may survive through the growing season but are stunted, and yield is reduced.
For both pathogens, a cut through the lower stem of a dead plant often reveals a brownish discoloration of the vascular tissue. Plants are susceptible at all stages of development, but symptoms are most obvious at or soon after flowering. Because symptoms can be so similar, a lab test is recommended to determine whether the problem is Fusarium or Verticillium.
Nothing can be done for plants that have Verticillium or Fusarium wilt. Plants that die should be removed and destroyed. Crop rotation is of limited value as the vascular wilt fungi may survive in the soil for several years. Using resistant cultivars is the most practical way for home gardeners to prevent losses due to vascular wilts. Resistant cultivars are available in seed catalogs and at garden centers. The letters V and F following the cultivar name denote cultivars resistant to Verticillium and Fusarium wilts. Resistant varieties may become infected, but the disease will not be as severe as with susceptible varieties, and a reasonable yield should still be obtained.
Bacterial Spot
Bacterial spot is caused by at least four different species of the bacterial genus Xanthomonas (X. vesicatoria, X. euvesicatoria, X. gardneri, and X. perforans), which cause disease on both tomato and pepper. The infection starts as small circular to irregularly shaped spots that appear greasy or watersoaked. As the spots enlarge, they often become surrounded by a yellow halo. Initially, they can look similar to spots caused by Septoria, but they will not develop gray-brown centers. The disease affects leaves, fruit, and stems. Spots on fruit will first appear as raised black spots that will later become large sunken scab-like areas.
The bacterium overwinters on the surface of seeds, in infected debris, and in soil. It is commonly brought into fields on infected transplants. Warm, rainy weather favors the rapid spread of bacterial spot.
In addition to general disease management tactics, starting with disease-free transplants and seed is important. Crop rotation is important, especially since peppers can also serve as a host for this disease. Avoid handling plants (pruning and tying, for example) any more than is necessary because wounds caused by handling allow bacteria to enter plants. Sprays of a fixed copper product can reduce the spread of the disease if applications begin when the first symptoms appear and are rotated with chlorothalonil or sulfur products. More information on the control of this disease can be found in this article: Bacterial Spot on Pepper and Tomatoes.
Bacterial Speck
Bacterial speck is caused by Pseudomonas syringae pv. tomato. This disease does not affect peppers. The disease affects leaves, fruit, and stems. In contrast to bacterial spot, this disease will cause tiny black specks (1/16-inch diameter) surrounded by yellow halos. The specks that develop on young green fruit are slightly raised, 1⁄32 to 1⁄16 inch in diameter, and have well-defined margins. The specks are considerably smaller than the spots caused by bacterial spot and do not penetrate the fruit. Although bacterial speck seldom reduces yields significantly, it can harm fruit quality. Infection is favored by cool (less than 70°F), wet conditions.
Management of bacterial speck is the same as those used for bacterial spot.
Bacterial Canker
Bacterial canker is caused by Clavibacter michiganensis subsp. michiganensis, and can be one of the most destructive and difficult-to-manage diseases of tomatoes. The first noticeable symptom is wilting of plants and browning on the edges of the leaves. Sometimes stems can turn brown; later, they can split open, revealing a brown color. The pith (mushy stuff in the middle of the stem) may be completely discolored and 'mealy.' Fruit may also show small spots (1/8 inch across) with a raised brown center and a whitish margin or halo, often called bird's-eye spots.
Bacterial diseases that cause wilting can be distinguished from fungal diseases caused by Verticillium or Fusarium by checking for bacterial streaming. Bacterial streaming consists of bacterial masses 'oozing' out of vascular tissues, and this may be observed by slicing stems and placing them in water. If whitish or yellowish strands are observed after a couple of minutes, the wilting is most likely caused by bacteria.
Bacterial canker can survive the winter in plant debris, weed hosts, and wooden stakes. It may also occur from contaminated seed sources and transplants and be spread mechanically by pruning and de-suckering. Secondary spread may also occur by splashing water and by handling plants. Once the bacteria enter the vascular tissue, the disease progresses rapidly. Plants infected late in the season may show little reduction in yield; however, plants infected early may die and set no fruit.
The most effective way to control bacterial wilt is by ensuring you have clean seed and a certified disease-free transplant. After harvest, remove infested debris and sterilize all supports and equipment. When handling or pruning plants, tools should be disinfected between cuts unnecessary wounding should be avoided. Crop rotation can also help.
Viruses
The most common virus disease in Iowa is tomato spotted wilt virus (TSWV), but others can occur, including Tobacco Mosaic Virus (TMV), Cucumber Mosaic Virus (CMV), and Impatiens Necrotic Spot Virus (INSV).
Symptoms for viral diseases can vary and are notoriously difficult to pinpoint. In general, plants often become stunted in growth with reduced yields. Leaves can develop leaf curling, yellow or green mosaic patterns, or a bronzing appearance. Fruit are also affected with mosaic patterns, streaking, or mottled areas. Some plants may be infected and show no visible symptoms. TSWV causes distinctive yellow ringspots on mature fruit. Foliage can also be affected; plants are usually stunted, and tip leaves show a purplish discoloration.
Viruses are typically spread by thrips, aphids, or leafhoppers (thrips frequently spread TSWV). Some viruses can be transmitted through seed, so buying certified virus-free seed or transplants is important to preventing virus diseases in your garden.
There is no cure for infected plants. The first step in control is to discard diseased plants. Many viruses that infect tomatoes also infect peppers and potatoes, so avoid planting these crops next to each other. Additionally, many perennial weeds can serve as a reservoir for some viruses causing disease in tomatoes, so weed management is important, too. Insect management may also be beneficial in the transplant and early-season phases.
More Information
- Managing Tomato Diseases, Disorders, and Pests
- Tomato Diseases and Disorders (publication)
- Growing Tomatoes in the Home Garden
- Tomatoes (publication)
- Crop Rotation in the Vegetable Garden
Photo Credits: 1: Chelsea Harbach; 2: Nancy Gregory, University of Delaware, Bugwood.org; 3: Nancy Gregory, University of Delaware, Bugwood.org; 4: Edward Sikora, Auburn University, Bugwood.org; 5: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 6: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 7: Edward Sikora, Auburn University, Bugwood.org; 8: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org; 9: Nancy Gregory, University of Delaware, Bugwood.org; 10: Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org; 11: Rebecca A. Melanson, Mississippi State University Extension, Bugwood.org; 12: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 13: Edward Sikora, Auburn University, Bugwood.org; 14: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 15: Clemson University - USDA Cooperative Extension Slide Series , Bugwood.org; 16: Edward Sikora, Auburn University, Bugwood.org; 17: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 18: R.W. Samson, Purdue University, Bugwood.org; 19: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org; 20: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 21: Chelsea Harbach