Ah, tomatoes — revered by many as the quintessential home garden crop. Every gardener who grows them hankers for a hearty yield, but many may not realize that tomato diseases, which can strike quickly and be difficult to recognize, pose one of the biggest potential roadblocks to good production. Follow these instructions on how to recognize the telltale signs of common tomato diseases, and discover proactive steps you can take to help prevent pathogens from infecting your prized plants.
The fungus Alternaria solani, which is present worldwide wherever tomatoes or potatoes are grown, causes early blight. Prevalent throughout the United States, the fungus overwinters on tomato plant debris and perennial tomato-family weeds, including hairy nightshade, horsenettle, jimsonweed, sacred thorn-apple and silverleaf nightshade. Spores can then spread to tomatoes in spring via wind or splashing rain, but they need wet leaf surfaces to germinate and grow. Low leaves that drip with dew each morning provide perfect conditions for early blight.
In early summer, when tomato plants begin to set fruit, brown spots will develop on the lowest leaves of infected plants. As the spots expand and become more numerous, leaf tissues between the spots may turn yellow before the leaf eventually withers. Use a magnifying glass to examine spots that are about a quarter-inch across. Early blight causes spots with outer rings around a bull’s-eye center. In most cases, early blight damage will be limited to the lower third of tomato plants (where conditions are more damp), and these plants may manage to produce a good crop. In rainy years, however, early blight can take down entire plants.
If early blight is a threat in your area, rotate plants to a new patch of ground each year, and keep tomato foliage dry. Make sure plants aren’t crowded (space them far enough apart to prevent plants from touching), use stakes or cages to hold the foliage off the ground, and avoid watering plants late in the day. Apply a thick mulch to prevent mud and water from splashing onto plants, as moisture on lower leaves can activate the disease. Control tomato-family weeds that grow close to your garden, and think twice before adopting volunteer tomato plants that appear early in the season — they may be loaded with an invisible cargo of early blight spores. As soon as you see the first early blight leaf spots, use pruning shears to clip off all leaves within 12 to 18 inches of the ground, removing no more than 20 percent of the plant’s total leaf mass.
Resistant varieties are new and few, but they’re worth growing in areas where early blight is a severe problem. Try ‘Defiant,’ ‘Jasper’ or ‘Mountain Magic.’ (See Find Disease-Resistant Tomato Varieties.) Copper is an organic fungicide that helps prevent early blight, but it’s not especially safe. It’s “allowed with restrictions” by organic-certification rules. With repeated use, it can build up to levels in soil that can be toxic to earthworms.
Look-alikes: Verticillium wilt, fusarium wilt, septoria leaf spot and nutritional stress can each cause older leaves to yellow and wither, but these other problems don’t show the rings or halos around irregularly shaped leaf spots that are the signature of early blight. Late blight spots are much larger and faster to develop compared with those caused by early blight, and they are rarely limited to the bottom section of the plant.
Fusarium oxysporum f. sp. lycopersici, a soil fungus present in warm to moderate climates worldwide, causes fusarium wilt. Where summers have many days above 80 degrees Fahrenheit, fusarium is the most widespread tomato disease. After entering a plant through its roots, the fungi multiply in the plant’s vascular system, often causing the leaves on individual stems to show symptoms first. Infected plants tend to grow normally until they begin to set fruit. At that point, leaves on some stems will start to yellow and wilt severely in midday sun. As the disease progresses, more stems and leaves will yellow and wilt, until the plants eventually collapse and die. No other common tomato disease causes such distinctive yellowing. If you pull up an infected plant and cut across a dying stem, you’ll see brown streaks within.
After this fungus is present in soil, it’s there to stay, as it doesn’t need a host plant. To prevent major fusarium problems, rotate crops so that tomatoes aren’t grown in the same soil more often than once every four years. Fortunately, many varieties are resistant, and you can grow susceptible varieties in large containers filled with packaged potting soil.
Look-alikes: Early blight and verticillium wilt cause leaf tips to turn yellow, but in these other diseases the yellow coloration is limited to older leaves close to the ground. With fusarium, yellowing is evident in the midsection of the plant. Root-knot nematodes often cause plants to grow slowly and wilt in hot weather, but plants do not show a clear yellow color. Severely struggling plants may have both problems, because root damage caused by nematodes can serve as entry points for fusarium fungi. Plants affected by septoria leaf spot often show yellow leaves with numerous small dark dots sprinkled over the yellowing leaves. Septoria leaf spot is triggered by periods of cool rain, however, which is not true of fusarium.
Phytophthora infestans, an algae-like microorganism that thrives in wet conditions and is often referred to as a “water mold,” gives plants this disease. Late blight, which requires a live plant host, is most prevalent in years in which summers are mild and rainy. Infections have become increasingly common in the Northeast and Mid-Atlantic states.
Also a problem for potatoes, late blight destroys plants’ leaves, thus depriving plants of their photosynthetic energy supply. Plants can become infected at any age, whenever leaves are wet for more than eight hours and temperatures are between 64 and 72 degrees. Growing in a hoop house isn’t a solution, because its high humidity level can encourage late blight.
Diseased leaves will show tan, wet patches and begin to droop. If wet weather persists, plants will shrivel and die within two to three days, as if they had melted. Infected plants may survive if the weather turns warm and dry, but any eventual fruits will likely have leathery patches. Late blight often overwinters on infected potato tubers and on weeds in the nightshade family, but it has also been found on commercial seedlings. Never accept tomato seedlings that show suspicious leaf spots of any kind. In high-risk areas, plant any of the recently developed late blight-resistant tomatoes, such as ‘Iron Lady.’
Look-alikes: Early blight, fusarium and a few other diseases cause leaves to discolor, but not in the large patches seen with late blight. There are no spots present within late blight lesions. Plants damaged by herbicides may show strangely withered leaves, but remaining growth may become stiff and stunted, with strange twists and curls, not soft and fast-rotting as with late blight.
Root-knot nematodes (Meloidogyne species) and less-common sting nematodes (Belonolaimus longicaudatus) are microscopic, parasitic roundworms that infect tomatoes. They flourish in sandy soils in warm climates.
Plants that have nematodes living on their roots will grow more slowly and will wilt on hot days more than uninfected plants would. Foliage will often turn light green to yellow. Nematodes may infect several plants growing close together, while plants only 10 feet away may be fine. Symptoms will usually worsen as summer progresses and nematode populations increase.
Nematodes feed on tomato roots and cause the formation of knobby galls and swollen sections, which deprive aboveground plant parts of moisture and nutrients. Dig up an affected plant to make a clear diagnosis. Root-knot nematodes form galls of various sizes, which will look like tumors on the roots. Sting nematodes cause plant roots to grow into a tight, swollen mass close to the base of the plant.
Rotating crops and growing resistant varieties can help prevent problems. Keep beds weeded, because many weeds serve as nematode hosts. In high-risk areas, such as the warm coastal plains, some gardeners move their vegetable beds every few years to stay ahead of nematodes. Certain French marigold varieties, such as ‘Tangerine,’ ‘Petite Harmony’ and ‘Nema-Gone,’ reduce nematode numbers if used as a densely planted cover crop (plant your tomatoes in the marigold area the following spring). Solarizing infested soil under clear plastic — especially during late summer, when nematode populations are at their highest — may also help.
Look-alikes: The wounds made to roots by root-knot nematodes often serve as entry points for fusarium fungi, so tomato plants can suffer from both problems at the same time. Verticillium wilt also causes slow growth, but it is not common in sandy soils in warm climates.
Tomato Spotted Wilt
A plant virus of the Tospovirus family, this disease is spread by tiny sucking insects called thrips. It’s most common in warm climates, including the South, but has shown up across North America. More than 150 plants play host to this virus, and it can spread via infected vegetable plants, weeds, tobacco plants and many flowers, including dahlias and zinnias.
At least nine species of thrips, including common Western flower thrips, can transmit the virus to tomato plants. Thrips, which are less than an eighth-inch long, can fly or simply catch a ride on strong winds. After the virus enters tomato plants via thrips’ saliva, it multiplies and interferes with the plants’ cellular signaling pathways.
Soon after infection, the plants will become stunted, and the young leaves at the tops of the plants will develop numerous small spots that cause the leaves to appear slightly bronzed. The edges of affected leaves will curl and become distorted and stiff. Plants will begin to droop and wilt, and new growth will start to die back. Sometimes part of the plant may live longer, creating a one-sided appearance. Any fruits produced by infected plants will show numerous yellow to brown rings, and they’ll usually rot before they can ripen.
If tomato spotted wilt virus shows up in your garden, grow resistant varieties the following season. Controlling thrips is nearly impossible, but reflective film mulches may reduce their numbers in your tomato patch. Try growing plants to attract minute pirate bugs, which are natural predators of thrips. In greenhouses, monitor thrips using yellow or blue sticky traps.
Look-alikes: Tobacco mosaic virus causes tomato leaves to crinkle, harden and develop puckered yellow patches, but no spots appear on the leaves. Early blight and most other tomato leafspot diseases infect older foliage first and rarely appear on new growth at the top of a plant.
Verticillium albo-atrum and several closely related fungal species cause this disease. Present in soils worldwide, verticillium thrives in cool, moist soils and is most common in the Northeast.
After entering a susceptible tomato plant through its roots, the fungi multiply in the plant’s vascular system, causing it to struggle or fail. Leaf tips on lower leaves will turn yellow in a distinctive V-shaped pattern, and they’ll then become brown.
As the plants grow, many leaves will show one-sided yellowing on either side of the main leaf vein. Later in the season, deterioration of the plant’s vascular system will make it difficult for roots and stems to deliver water and nutrients to leaves and fruits, so the plant will wilt. Supplemental water won’t perk up an infected plant, and a few warm, sunny days will cause it to shrivel and die, often with some leaves still green.
If you pull up an infected plant and cut across the main stem at a point that was within 12 inches of the ground, you will see light-brown discoloration within the stem. Fruits from infected plants are edible, but they usually don’t ripen properly.
To combat verticillium wilt, grow any of the widely available resistant varieties, and rotate plantings so you’re not growing nightshades in the same soil more often than once every four years. Plants that show only mild symptoms can sometimes be nursed through the season with mulch and regular watering.
Look-alikes: Early blight causes leaf tips to turn yellow and wither, but circular early blight lesions have a bull’s-eye pattern; such spots are not present with verticillium wilt. Walnut wilt, caused by tomato roots coming into contact with the roots of walnut trees, is another look-alike, but it’s slower to develop and is not triggered by rainy spells.
Find Disease-Resistant Tomato Varieties
Seed catalog descriptions often feature a string of letters denoting a variety’s resistance to diseases. For the six tomato diseases discussed in this article, look for these abbreviations in seed catalogs.
Early blight: EB
Fusarium wilt: F, F1, F2 or F3
Late blight: LB
Tomato spotted wilt virus: TSWV, TSW or SWV
Verticillium wilt: V
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Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website, BarbaraPleasant.com, or finding her on Google+.