Tomato Diseases

Editor’s Note:  We have updated Cleve Campbell’s excellent 2015 article with new resources on tomato diseases.

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As the days grow longer and the summer heats up, I wait anxiously for that first ripe, sun-warmed, sweet tomato. For many of my gardening friends, myself included, summer really doesn’t start until that first summer-ripened tomato is plucked from the vine.

One thing I have learned about growing tomatoes over forty years of gardening is that many things can go wrong in the tomato patch; some can be fixed and some can’t be fixed, and no tomato crop is perfect. Growing tomatoes is not for the gardener seeking perfection. A certain amount of loss is normal, and just like the stock market, some years are more enjoyable than others.

Over the years I have found that there is little point in chasing perfection with chemicals. Many problems can be prevented if tomato varieties are carefully chosen and properly cared for; they are less susceptible to disease and pest problems. And one important lesson I have learned is that healthy plants don’t always start, stay, or end that way. Even in the most challenging year, I try to remember: problems in the tomato patch are an opportunity to learn and to prepare for next year. And the most important thing I’ve learned is that even in a bad year, those garden tomatoes sure taste a lot better than those from the supermarket.

This month in The Garden Shed we are going to take a look at some common tomato diseases that occur in Virginia. They include: early blight, septoria leafspot, verticillium and fusarium wilts, late blight, tobacco mosaic virus and bacterial spot. One common cultural or physiological disorder —  blossom end rot — will also be reviewed.

Early Blight

Early blight, which is caused by the fungus Alteraria solani, is common in Virginia. It occurs to some extent every year wherever tomatoes are grown. Don’t be confused by the name “early” as the disease may occur at any time during the growing season. Early blight causes irregular, brown leaf spots (lesions) that range in size up to ½ inch in diameter.

The most important diagnostic indicator of early blight is the formation of dark, concentric rings within the lesion, giving the spots a target-like or bulls eye appearance, and often causing the leaf to turn yellow, dry up, and fall off. The lesions initially appear on the lower, older leaves near the base of the plant and can progress rapidly up from the lower foliage to new growth during wet weather. Early blight may also produce symptoms on the plant stems and fruit. Dark, sunken, leathery lesions appear on the stem-end of the fruit. On older fruit, these lesions reach considerable size and the rot extends deep into the flesh of the fruit. Heavily infected fruits usually drop to the ground. Other vegetables in the garden that are susceptible to early blight include: potatoes, peppers and eggplants.

The fungi responsible for this disease can survive up to a year in the soil as well as in infected vine residue, seeds, and weeds left in the garden over the winter. In the spring and summer, spores of these fungi can be splashed or blown to tomato leaves.  Warm temperatures, abundant rainfall and high relative humidity favor disease development. The disease is more aggressive when plants are weakened or stressed by poor nutrition, drought or by the wounds of pests.

                                                                                                                                                                                                                            Septoria Leaf Spot

Septoria Leaf Spot is caused by the fungus, Septoria lycopersici, characterized by several small, gray, round leaf spots with dark borders. A few black, pinhead dots may be seen within the spots.  For a very helpful introduction, watch this video: Septoria Leaf Spot on Tomato/VATech Plant Disease Clinic

Like Early Blight, the spores survive in residues from diseased plants. Septoria leaf spot can occur anytime during the growing season. Septoria leaf spot diseases first develops on the older leaves nearest the ground and continues upward on new leaves as the growing season progresses. Heavily-infected leaves may scorch and wilt, giving the plant the appearance of a wilt disease. The fruits are rarely infected; however, the leaf loss reduces fruit yield and quality, and the exposed fruits are more susceptible.

 Control of Early Blight and Septoria Leafspot

 1. Remove all infected plant material (including infected fruit) from the garden and destroy it.  Never compost plant material suspected to be infected with Early blight or Septoria leaf spot.

2. Both early blight and septoria are soil-borne diseases, so whenever possible do not plant tomatoes in the same place year after year. If not possible to rotate to a different plot, rotate to a different section of the garden; if possible, avoid planting in areas where potatoes, peppers or eggplants were planted in the prior year.

3. Consider using stakes or wire cages to support tomato vines. By keeping the vines off the ground you can reduce the chance of diseases by reducing soil splash on the leaves and fruit. Caged plants are less prone to the spread of disease from plant-handling than staked plants.  Why?  Staked plants are handled more frequently than caged plants, and that handling results in more open wounds, which are a way in for diseases.

4.  Give your tomato plants space — at least 3 feet — to allow good air circulation, which will reduce the humidity around the plant.  Both early blight and septoria leaf spot are more aggressive in a humid environment.

5.  When pruning tomatoes, disinfect your pruning tools frequently to avoid spreading spores from plant to plant.

6.  Healthy plants tend to resist diseases better than plants stressed from lack of water or nutrients. Tomatoes planted in well-drained and properly fertilized soil, will be less prone to early blight and infection. As a general rule, at midseason, full-grown tomato plants require about 1 inch of water per week. Add water gradually, allowing the water to soak into the soil. Avoid overhead irrigation, which can lead to an increase in diseases. Watering early in the day will allow the plants to quickly and thoroughly dry. Do not allow the soil to become so dry that the plants  wilt. Avoid fluctuations of too much and then too little water. Adding a layer of organic mulch such as straw, leaves or grass can reduce water evaporation, help reduce weeds, and reduce soil splash when it rains. Avoid using grass clippings from a lawn recently treated with herbicides.

7.  Try planting early blight-resistant tomatoes. Tomato varieties suggested to be more tolerant of early blight include ‘Mountain Magic’, ‘Mountain Fresh Plus F1’, ‘Bush Celebrity Hybrid’, ‘Big Beef Hybrid’, ‘Celebrity Hybrid’, ‘Rutgers’, ‘Juliet F1’, ‘Tommy Toe’, ‘Old Brooks’, or ‘Cabernet F1’. When purchasing seeds or plants, look for the symbol “As” on the seed packet or plant label, which denotes resistance or tolerance to Alternaria solani or early blight. Remember, resistance or tolerance does not mean the variety is completely immune to those specific diseases. It suggests a specific variety has greater tolerance to a particular disease. Disease-resistant varieties may still be affected by the disease, but they typically have less damage than a non-resistant variety.

Late Blight

Late blight (VCE Pub. ANR-6) is caused by the fungus-like organism Phytophora infestans and is a very destructive disease in tomato and potato crops. The pathogen is best known for causing the devastating Irish potato famine in the 1840’s, resulting in the deaths of more than 1 million people and causing another million people to leave Ireland. For an excellent introduction, watch this video/VCE.

The late blight pathogen attacks all above-ground parts of the tomato plant. The first symptoms of late blight on tomato leaves are irregularly-shaped, water-soaked lesions, often with a light halo or ring around them. Unlike early blight and septoria leaf spot diseases, these lesions usually begin on the younger, more succulent leaves in the top portion of the plant canopy and then migrate down the plant to the lower leaves.

During periods of high humidity, white cotton growth may be visible on the underside of the leaf. Spots are visible on both sides of the leaves. As the disease progresses, lesions enlarge, causing leaves to brown, shrivel, and die. Late blight can also attack tomato fruit in all stages of development. Fungal garden spores are spread between plants and gardens by rain and wind. The ideal weather for spread is temperatures in the upper 70’s and high humidity. Complete defoliation can occur within 14 days under ideal conditions.

Unfortunately, there is no cure for late blight. Once you observe the symptoms, all infected plants should be removed from the garden. Never compost the plants. Instead, burn them or place them in a large plastic bag and place in the sun to bake for a few days before putting in the trash can.

 

Verticillium and Fusarium Wilts:

Verticillium wilt is a disease caused by a fungus — Verticillium albo-artum — that attacks over 200 plants, including tomatoes, potatoes, eggplants, strawberries, and raspberries. The fungus is soil-borne and can reside in the soil for many years after it is contaminated; therefore, rotating crops is essential to controlling this disease. The fungus enters the plant though the feeder roots and grows into the water-conducting vessels (xylem) in the stem. As the vessels become clogged and collapse, the water supply to the leaves is blocked. The first symptoms usually appear on the older bottom leaves. The leaves become yellow, dry up, and drop prematurely. The upper shoots may also wilt during mid-day. Leaf tips curl upward at the margin and defoliation may continue up the plant. At an advanced stage of infection, the internal portion of the stem at the base of the plant will appear dark and discolored. The disease may continue until the plant is wilted, stunned or dead.

Fusarium Wilt

Like verticillium wilt, fusarium wilt is caused by a fungus that is soil-borne and passes into the feeder roots and moves upward in the xylem of the stem, blocking the water-conducting vessels and causing the wilting of the leaves. The first indication of the disease in small plants is the drooping and wilting of lower leaves, with a loss of green color, followed by wilting and death of the plant. Often leaves on only one side of the stem turn yellow at first; the yellow leaves gradually wilt and die. The stems of wilted plants show no sign of soft decay, but when the stem is cut lengthwise, the woody part next to the green outer cortex shows a dark brown discoloration of the water-conducting vessels.

Unfortunately, once a tomato plant shows symptoms of a wilt disease, it cannot be cured of the problem.  It should be pulled up and removed from the garden. Removing old and diseased plant debris during the growing season and at the end of the growing season won’t eliminate it the next year, but can help reduce the population of the diseases that overwinter in the soil over time. Because verticillium and fusarium wilt fungus all survive in the soil for several years, it will be hard to prevent the disease each year. Crop rotation can help but is often of limited value in the home garden because of limited space. If you experience wilt problems this year, if at all possible, avoid planting tomatoes in the same spot next year. Along with avoiding the infected area next year, you need to avoid planting plants in the same family, such as peppers, eggplant, and potatoes. These vegetables are all closely-related and can be infected by similar diseases. If you have wilt problems in a raised bed, one solution may be to remove the contaminated soil and replace it with new dirt.

Doing battle with verticillium and fusarium diseases takes some careful planning at the beginning of the growing season when selecting varieties and deciding where to plant in order to avoid infected areas. When selecting which tomato varieties to grow, select the ones that are disease-resistant.  How?  Well, when you look at a plant label or seed packet, look for the letters that serve as a code for which disease it is resistant to. Verticillium wilt resistance is represented by the letter “V”  and fusarium wilt resistance is  indicated by the letter “F.”  You will find that some varieties have resistance to more than one disease, while others may have no resistance at all.

The recent interest in growing heirloom tomatoes may result in an increased incidence of fusarium and verticillium wilts, as generally these plants are less disease-resistant than hybrid tomatoes. However, in recent years a movement has been underway to graft the tops of heirloom tomato plants to rootstock that is disease resistant. Learn more about tomato grafting in this Penn State Extension article.

Tobacco Mosaic

Tobacco Mosaic (virus): Symptoms are intermingled patches of normal and light green or yellowish colors on the leaves of infected plants. Tobacco Mosaic damages leaves, flowers, and fruit, causing stunting of the plant. Several strains of the virus are known to cause different symptoms. The virus is highly infectious and readily spreads by any means, even in a tiny amount of sap. The most common means of transmission is by handling contaminated plants. The virus may also be present in certain types of tobacco; therefore, use of tobacco may also be a source of the virus.

Presently, there are no known efficient chemical controls that eliminate viral infection from plant tissues once they do occur. Tobacco mosaic virus is the most persistent plant virus known. It has been known to survive up to 50 years in dried plant parts. Therefore, sanitation is the single most important practice in controlling tobacco mosaic virus.

Control of Tobacco Mosaic Virus:

1. Remove and destroy infected plants. Pull plants with mosaic symptoms immediately. Remove the debris from the garden area and destroy.
2. Keep your garden weed-free. Some weeds may be harboring the virus. These represent potential sources of the disease.
3. Always wash you hands thoroughly and disinfect tools. Before handling plants, wash with soap and water, especially if you use tobacco products.
4. Plant resistant varieties of tomatoes.  Varieties that are resistant to tobacco mosaic virus are labeled “T” resistant.

Bacterial Spot/Speck:

Bacterial Spot/Speck can involve several different species and strains of bacteria. Some bacteria attack both tomato and pepper while others only attack one crop or the other. Bacterial spot can lead to severe damage to tomato and pepper plants. The pathogen attacks all parts of the plant —  leaves, flowers, fruits and stems —  causing spots or blemishes on these plant parts; however, most damage occurs on the leaves. Outbreaks of bacterial spot can result in leaf drop and poor fruit-set in the garden. Defoliation due to leaf spotting can increase the incidence of sunscald on fruit. In addition to the poor appearance of the fruit, fruit injury allows entry of secondary fruit-rotting organisms, causing further damage to the fruit.  Some scientists are working to develop a variety of tomato that is resistant to bacterial spot.  See wusfnews.wusf.usf.edu.

The disease begins on older leaves at the base of the plant. Many small dark spots may first appear; the areas between the spots often turn yellow. Leaf spots often appear on both sides of the leaves. The spots quickly spread and kill the leaves. Dead leaves usually stay attached to the tomato plant.

Fruit blemishes begin on green fruit as small water-soaked spots that are 1/8 to 1/4 inch in diameter. Centers of these lesions become irregular and slightly sunken with large scabby surfaces. Often the disease extends into the seed cavity. Secondary decay organisms may invade the bacterial spot lesions, resulting in fruit decay. The disease only affects green fruit; once the fruit turns red and the acid content increases, the fruit is no longer susceptible to the disease.The bacterial spot infection often originates from contaminated seeds or transplants or plant debris remaining in the garden from previous diseased plants or on volunteer tomato or pepper plants. The bacteria can spread from plant to plant by wind, rain, overhead irrigation, tools, and humans.

Garden sanitation is an essential component to controlling bacterial infections:

1. Do not work in the garden when plants are wet; if you are moving wet leaves around, you may be spreading the disease with contaminated water from the leaves.
2. If you are pruning a plant and suspect that it may be infected, always disinfect your tools before moving to the next plant.
3. At the end of the gardening season, remove all plant material, including weeds, from the garden.
4. If you start your own plants from seeds, use a sterile mix.  If you are re-using pots or trays, sanitize them with a disinfectant such as a solution of 1 part bleach to 9 parts water.
5. Whenever possible, do not plant tomatoes in the same place year after year. If not possible to rotate to a different plot, rotate to a different section of the garden.  If possible, avoid planting in areas where potatoes, peppers or eggplants were planted in the last year.
6. Give your plants plenty of space. The longer leaves stay wet, the greater the risk of bacterial spot. Leaves will dry faster when there is good airflow;  this helps reduce the severity of bacterial spot.

 Blossom End Rot

Blossom End Rot (VEC Publication 450-703) is a troublesome cultural problem that many of my gardening friends and myself have experienced. Unlike the various other problems discussed, blossom end rot is not caused by a disease organism; rather, it is a physiological disorder that occurs when there is insufficient calcium available to the developing fruit.

Initial symptoms of blossom end rot generally appear as water-soaked areas near the blossom end of the fruit (the end opposite the stem). Initially small, the water-soaked spot enlarges and darkens rapidly as the fruit develops. The spot may enlarge until it covers as much as 1/3 to 1/2 of the entire fruit surface. As the spots grow, the tissue becomes shrunken and soon dries out, becoming flattened or concave. The infected area becomes black and leathery (See photo). The fruit does not soft rot unless the spots are invaded by a secondary fungi or bacteria, but that frequently happens. These secondary organisms are sometimes mistaken as the root cause of the problem.  But as the name of the disease implies, symptoms appear only on the blossom end of the fruit and no other parts of the plant.

While the occurrence of blossom end rot may indicate a calcium deficiency, in reality, the soil may have adequate calcium; however, for various reasons, the plant may not be able to absorb enough calcium to supply the rapidly-developing fruit. The cause of the problem may be one or a combination of the following factors:

1. Low soil pH
2. Low calcium levels in the soil
3. Extreme fluctuations in moisture
4. Damaged roots
5. Excess nitrogen fertilizers

Prior to planting, the main preventative measure is to have a soil test done to determine if adequate calcium is present in the soil. Also, it is recommended that a pH level of approximately 6.5 be maintained. The lower or more acid the pH, the less available the existing calcium is to the plant. If the results of your soil test indicate a low pH and low calcium levels and lime is recommended, it should be worked into the soil 2-3 months before planting to allow time for it to become effective.

Maintain a uniform supply of soil moisture by watering plants during periods of drought. The general rule of thumb is that tomato plants require about 1 inch of water per week. Mulching will often help to maintain even levels of moisture. Also, weeds will compete with the plants for moisture and should be removed.

Avoid cultivating closer that 1 foot to the plant to avoid damaging roots.

Do not over-fertilize, especially with high-nitrogen fertilizer, as it can cause problems with the uptake of calcium. Use nitrate forms of nitrogen or consider using an organic fertilizer. Also, avoid over-fertilization during fruiting.

Be sure to read about three diseases that could become problems in our area: Southern Blight, Bacterial Wilt, and Tomato Spotted Wilt Virus, all of which are discussed by Dr. Steve Rideout of Virginia Tech in Tomato Disease Update – 2022.  As Dr. Rideout explains:

In summary: the diseases we chatted about in this article may seem a bit daunting to the new tomato-grower; however, there are many proactive things that the gardener can do to prevent the diseases.  Best of all, the preventive measures work on all the diseases discussed!  Don’t be discouraged!

The first proactive step that a gardener can do is to provide the conditions the tomato plants need to be happy. Healthy and happy tomato plants are able to fend off diseases far better than stressed or weak plants.  So fear not.  Just be sure to follow the disease-preventive measures:

• Select a site with full sun (at least 8 hours of sun).
• Provide good soil, amended with organic mater, nutrients, and a proper pH —  around 6.5.
• Select disease-free plants and varieties that are disease-resistant.
• Practice crop rotation and good sanitation in order to limit the spread of the disease.
• Provide constant levels of water, though drip irrigation and by mulching.

But if a problem develops, diagnose it. If you are not sure what disease you’re dealing with, take a sample of your diseased plant (in a bag) to your local Extension Office. They will not only help with diagnosing the problem, but also offer recommendations and possible solutions. There are several diagnostic tools online that also may be of help. Texas A&M has a very user-friendly tool that may be of help. The link to that web site is: http://aggie-horticulture.tamu.edu/vegetable/problem-solvers/tomato-problem-solver/

For fungicide solutions, refer to the 2023 Pest Management Guide/Va Tech/456-018/ENTO-523, Tables 2.3 and 2.4 (pages 2-28 and 2-31). If you elect to treat with fungicide, be sure that it will control the disease your plants have, that it’s not too late to be effective (some fungicides work only as preventatives), and that it is safe to use on tomatoes. Use the rate indicated on the label and follow the application instructions. Remember that the label is the law.  The diseases and fungicide treatments listed in the VCE Pest Management Guide are set forth below:

 

Happy gardening and thanks for joining us in The Garden Shed.  We hope to see you again next month.

SOURCES:

Virginia Cooperative Extension, Publication 426-418, “Tomatoes”,  Va. Tech.Pub. “Tomatoes”

Tomato Disease Update – 2022, VA Tech/wise.ext.vt.edu (Steve Rideout, Ph.D.
Professor and Extension Specialist of Plant Pathology Virginia Tech – School of Plant and Environmental Sciences)

“Tomato Diseases and Disorders in the Home Garden,” Penn State Extension

“Early Blight of Tomato,” NC State/ncsu.edu/early-blight-of-tomato

“Late Blight of Tomato and Potato”,  Va Coop.Ext/ANR-6/ANR-6

Video:  Late Blight on Tomato/VCE Master Gardeners

“Verticillium Wilt of Tomato and Eggplant,” NC State/ncsu.edu

“Fusarium Wilt of Tomato,” NC State/ncsu.edu

“Fusarium Wilt of Tomatoes in a Home Garden,” University of Maryland Extension

“To Graft or Not To Graft,”  Penn State Extension

“Tobacco Mosaic Virus of Tomato and Pepper”, Missouri Botanical Garden/Pests & Problems

“Bacterial Spot of Pepper and Tomato,” NC State/ncsu.edu

“Bacterial Diseases of Tomato,” University of Maryland Extension

 

Other Sources:

Missouri Botanical Garden, “Tomato Fruit Problems”, http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/visual-guides/tomato-fruit-problems.aspx

Texas A&M Extension, Vegetable Problem Solver”, “A Guide to the identification of Common Tomato Problems”, http://aggie-horticulture.tamu.edu/vegetable/problem-solvers/tomato-problem-solver/http://extension.missouri.edu/sare/documents/VegetableGrafting.pdfhttp://aggie-horticulture.tamu.edu/vegetable/problem-solvers/tomato-problem-solver/

Clemson Cooperative Extension, Publication HGIC 2217, “Tomato Diseases and Disorders”, http://www.clemson.edu/extension/hgic/pests/plant_pests/veg_fruit/hgic2217.html

Texas A&M Extension, Texas Plant Disease Hand Book, “Tomatoes”, http://plantdiseasehandbook.tamu.edu/food-crops/vegetable-crops/tomato/