Big beefsteaks or little cherry tomatoes; red, orange, purple, or striped tomatoes; grown in the ground, in containers, even hanging upside down from containers -- we American gardeners love them all! Tomatoes are the most popular home garden crop in the country, but unfortunately we're not the only ones who have a taste for tomatoes. We may find ourselves in competition with a variety of insects and disease-causing organisms for our precious tomato harvest.
The insects and diseases that are potential troublemakers for your tomato plants will vary depending on where in the country you're gardening and the specific conditions in your garden. In this article, we'll highlight some of the most common and widespread tomato insects and diseases. For more advice on the problems you're most likely to encounter in your area, check out the information on your state Extension service website.
INSECTS
Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org
Tomato Hornworms
Found across the country, and one of the most spectacular pests you're likely to encounter on your tomatoes, these huge, green caterpillars chomp on the leaves and fruits of peppers and eggplants as well as tomatoes. Because they are so large, even a few hornworms can consume a lot of plant material, so it's important to control them if they turn up in your tomato patch.
If they weren't such avid consumers of our veggies, we might better appreciate their rather impressive appearance. Up to 5 inches long, they are bright green with diagonal white stripes and black spots along their sides and a ferocious looking ″horn″ projecting from the rear. But don't worry -- the horn is just for show. These caterpillars neither bite nor sting.
The adult moth into which these caterpillars change is also eye-catching. Grayish-brown with yellow and white markings or orange spots and a 4-5 inch wing span, sphinx moths emerge in late spring and lay their greenish-yellow eggs singly on the undersides of the leaves of tomatoes, peppers, and eggplants in the garden. After feeding for about a month, the caterpillars hatched from these eggs drop to the ground to pupate and emerge as adult moths, either later that same season or the following spring. There is only one generation per year in northern areas, but in warmer parts of the country there may be as many as four.
Control measures start with keeping an eye out for the newly hatched caterpillars; when they're small they blend in easily with the foliage. Look for their black droppings on the leaves. When they are small (under about 2 inches), the microbial insecticide Bt (Bacillus thuringiensis) will control them. This insecticide only targets caterpillars, so it is a very safe product to use. The caterpillars consume the Bt as they feed, then sicken and die. Once they are larger than 2 inches Bt is not as effective; control bigger caterpillars by handpicking. Cultivating the soil in late fall will help reduce the number of overwintering pupae by exposing them to hungry birds and cold weather.
If you happen to notice a caterpillar that looks like it's covered with grains of rice, let it be. The ″rice grains″ are the cocoons of a parasitic braconid wasp whose larvae feed on and eventually kill the hornworm. Leaving the parasitized caterpillar in the garden allows a new generation of wasps to emerge and go on to infest more hornworms.
Photo courtesy of Jack Dykinga, USDA Agricultural Research Service, Bugwood.org
Tomato Fruitworms
This troublesome caterpillar travels under a couple of aliases. Also known as the corn earworm and the cotton bollworm, it may infest not only tomatoes, but a variety of other crops, including corn, peppers, beans, okra, eggplant, soybeans, and cotton. The larval stage of a moth, fruitworms are variable in color; they can be greenish-yellow, brown, or black, may have stripes on their sides, and can grow up to 2 inches long. After hatching out from eggs laid on foliage, the young caterpillars feed briefly on leaves and then bore into the tomato fruits (they prefer to attack green fruits) to feed inside, leaving deep holes. After feeding for 2-4 weeks, the caterpillars drop to the ground to pupate. Several weeks later they emerge as adult moths to continue the cycle. There are multiple generations a year, so damage can continue from early summer through frost.
To control these pests, check leaves near green fruits once fruit set begins for the white, dome-shaped eggs and crush any you find. As soon as you notice eggs or any signs of caterpillars, begin spraying plants with the microbial insecticide Bt. Once the caterpillars are within the fruits, they are safe from any insecticide spray, so it's important to monitor plants in order to begin spraying when the young caterpillars are still feeding on the leaves. If you find infested fruits with deep holes in them, pick and destroy them to get rid of any caterpillars inside; don't add them to your compost pile.
In southern states tomato fruitworms overwinter in the soil as pupae (the resting stage between caterpillar and moth); in northern states they migrate in from southern areas as adult moths in early to midsummer. In areas where they overwinter, tilling the garden in the fall may expose pupae to cold and predation and reduce the numbers that survive the winter.
Photo courtesy of USDA Agricultural Research Service Archive, USDA Agricultural Research Service, Bugwood.org
Flea Beetles
If the leaves of your young tomato plants are covered with small holes and look like they've been peppered with buckshot, flea beetles are at work. These tiny (1/16 inch) black insects jump like fleas when disturbed (hence their name) and dine on the leaves of many garden plants, not just tomatoes. Their feeding is mainly a threat to seedlings and young plants because it can damage such a large proportion of the young plants' limited foliage. Older plants with more leaves weather flea beetle feeding with little harm. Plants can also be damaged by the beetles' larval stage, a soil-dwelling grub that feeds on roots. Again, young plants are most vulnerable, but generally the feeding of adult beetles on plant leaves is the biggest cause of concern.
Adult beetles overwinter in the soil and on garden debris, so a good cleanup at the end of the season helps to reduce numbers the following season. If you can, rotate the location of your tomatoes in your garden to a spot where flea beetles were not a big problem the previous season. Then cover young plants with row covers immediately after transplanting, leaving the covers in place until plants are well-established (crop rotation helps to ensure that flea beetles don't emerge from the soil under the cover).
Another strategy is to plant a trap crop about 10 feet away from your tomatoes a couple of weeks before your tomato planting date. Some crops that are likely to be tastier to flea beetles than tomatoes are arugula, mustard, daikion radishes, and napa cabbage. Once your tomatoes have grown large enough to withstand some flea beetle feeding, pull up and compost the trap crop.
For severe infestations, insecticides containing spinosad or pyrethrin can be used according to label instructions. Neem oil sprays may act as a repellent.
Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org
Stink Bugs
These pests are real stinkers for tomato growers, especially in southern areas (although there are also some beneficial predatory stinkbugs that feed on pests). A number of different species of stink bugs feed on tomatoes. Some are green, some are brown, but all the adult bugs have a distinctive shield shape. The immature stage or nymphs are similar in shape to the adult insects, but smaller and often differently colored. For example, the nymphs of the green stink bug start out black, then change to green with orange and black markings. Stinkbugs overwinter as adults in sheltered spots, emerging in spring to lay clusters of barrel-shaped eggs on the undersides of leaves. There are multiple generations a year.
Both adults and nymphs damage tomato buds, blossoms, and fruits as they feed with their piercing-sucking mouthparts. Developing tomatoes are dimpled and deformed, with light yellow blotches or whitish, cloudy looking spots. Beneath these discolored areas inside the fruits is spongy or pithy white tissue.
To control these pests, look for eggs on leaf undersides and destroy them. Handpicking adults and nymphs is also an option if numbers are relatively small, but be sure to wear disposable gloves, as bugs can emit an unpleasant odor when handled or crushed. Go out in the early morning when bugs are sluggish and knock them off plants into a container of soapy water. For larger infestations, plants can be treated with an insecticide labelled for this use. Lower-toxicity insecticides such as insecticidal soap and horticultural oil are more effective against nymphs than adult bugs. Contact your local Extension Service for advice on specific products labelled for use in your state.
Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org
Aphids
These small, soft-bodied, pear-shaped pests may be pale yellow, green, pink, or pink and green mottled, depending on the species (potato aphids and green peach aphids are common on tomatoes). But they all feed on tomatoes by sucking out the plants' juices, causing curling, distorted leaves, especially on the new growth. In small numbers they do little damage. But when the population of aphids is high, their feeding can weaken and stunt plants; they can also transmit virus diseases as they feed.
To keep aphids in check, encourage the presence of beneficial predatory insects in your garden by including flowering plants in among your vegetables and by minimizing the use of insecticides, especially broad-spectrum products, in your garden. Knock aphids off of plants with a strong stream of water from a hose; once they're off your plants it's unlikely that many will be able to make their way back on. For serious infestations, spray with a low-toxicity insecticide such as insecticidal soap, horticultural oil, or neem oil according to label instructions.
TOMATO DISEASES
Photo courtesy of William M. Brown Jr., Bugwood.org
Late Blight
This devastating disease, caused by the fungus-like pathogen Phytopthera infestans, has caused big problems for tomato growers in many parts of the country in recent years. It also infects potatoes. Unlike many other tomato diseases, late blight can kill plants rapidly, going from a few water-soaked spots on the leaves and stems to the entire plant collapsed in a heap, sometimes within a matter of days. The wind-borne spores allow the disease to spread readily from plant to plant, as well as over long distances.
Cool nights, warm days, and wet weather set the stage for the development of this disease. The first sign is the appearance of dark, water-soaked, irregularly shaped spots, about the size of a nickel or a quarter, on the leaves. These spots become covered with a fuzzy white mold on the undersides of the leaves. They enlarge quickly, turn black and kill the entire leaf. The infection then spreads to the leafstalks and main stem, eventually causing the entire plant to collapse and die. Tomato fruits and potato tubers can also be affected.
If you see spots on just a few leaves of tomatoes or potatoes, you can pick them off, put them in plastic bags and toss them in the garbage. But if lots of leaves or the stems are infected, it's best to destroy the entire plant to prevent the spread of spores to uninfected plants, both yours and those of other gardeners and farmers miles away. Don't add infected plants to your compost pile; seal them tightly in a plastic bag and throw it away. Fungicide sprays will not cure infected plants, but may protect uninfected tissue. Check plants regularly when weather conditions are favorable for the disease. If you see signs that the disease is just beginning, it may be helpful to begin spraying with a fungicide labelled for this use to prevent the spread of the disease.
The late blight pathogen needs living plant material to overwinter. It can overwinter on potatoes left in the ground or in the center of a compost pile, so clean up the garden well at the end of the season and don't compost infected tubers. Late blight can be introduced early in the season on infected transplants shipped for sale from warmer parts of the country. Check transplants carefully before buying for any signs of disease, or grow your own. Consider selecting late blight-resistant varieties; look for the letters ″LB″ after the variety name. Use drip irrigation or soaker hoses to avoid wetting foliage and space plants so that there is good air circulation around them. If problems have been severe in the past, consider preventative fungicide sprays repeated according to the label instructions.
Photo courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org
Early Blight
This fungus disease causes brown spots that are generally round, about 1/4 to 1/2 inch in diameter, with characteristic dark concentric rings on them, sort of like a target. Sometimes the spots will have a yellow margin. Symptoms usually begin on the lower leaves first and work their way up the plant. Badly infected leaves turn yellow and drop. Infections can also occur on the stems, where they cause spots with concentric rings, and on the fruits, which develop dark, leathery, sunken spots, again with the characteristic ring pattern.
Warm, humid or wet weather favors the development of this disease. While plants can be infected at any stage, they are most susceptible as the fruits are beginning to mature. Loss of leaves weakens plants and increases the chances of sunscald on exposed fruits.
Alternaria solani, the fungus that causes early blight, can survive as spores in the soil and on plant debris for at least a year, so a good garden cleanup at the end of the season and a 2-3 year rotation of tomatoes and related vegetables (peppers, eggplant, and potatoes, which are also susceptible) in the garden will help to reduce problems. A thick organic or plastic mulch will help to keep spores from splashing up on to the lower leaves from the soil. Avoid wetting the tops of plants when watering. Consider planting resistant varieties; look for the letters ″AB″ after the variety name.
If early blight is regularly a significant problem in your garden, consider using an appropriate fungicide preventatively or at the first sign of disease to protect uninfected tissue, making repeat applications according to the label instructions. Fungicides won't cure existing infections.
Photo courtesy Thomas Zitter, Cornell University
Septoria Leaf Spot
This common fungus disease causes many small (1/8 inch diameter) brown or grayish spots with dark margins on tomato leaves, beginning on the lowest ones first, usually after the plant has begun to set fruit. Spots have a yellow halo and often you can see black specks in the centers of the spots. Severely infected leaves turn yellow and drop, weakening the plant and leaving fruits exposed to sunscald. Warm, wet weather favors the development of this disease.
To reduce problems with Septoria, try to keep the tops of plants dry, use mulch to prevent spores splashing up from the soil, and avoid working among plants when they are wet. As with early blight, the fungus can survive in the soil and on plant debris for up to three years, so rotate crops and clean up the garden well at the end of the season. If Septoria is a big problem in your garden, consider preventative fungicide applications.
Wilts
Fusarium wilt and verticillium wilt are soil-borne fungus diseases that harm plants by plugging up their water-conducting tissues. Fusarium is more of a problem in warmer weather; cool, moist soils favor verticillium wilt. But the symptoms of infection are similar for both -- lower leaves wilting, turning yellow, and dropping, followed by wilting of the top portion of the plant and possibly its eventual death. In Fusarium infections wilting is often seen on one side of the plant initially and leaf yellowing moves upwards in the plant as the disease progresses. In Verticillium wilt, the tips of the lower leaves often show yellowing with a distinctive V-shape.
There are no chemical controls available to home gardeners for either of these diseases. Pulling up and destroying infected plants, cleaning up the garden well at the end of the season, and rotating crop locations of tomatoes in the garden are helpful in minimizing problems with wilt diseases. The best course of action is to choose varieties that have been bred to be resistant to wilts. The letter ″F″ (also ″F2″ and ″F3″) after a variety name indicates resistance to Fusarium; the letter ″V″ indicates resistance to Verticillium.
Photo courtesy of David Liebman
Anthracnose
This is a fungus disease that causes rot in ripe and over-ripe fruits. While infection often occurs while tomatoes are still green, it doesn't become evident until the fruits begin to ripen. First you'll see round, sunken spots on the fruits, about 1/2 inch in diameter; then the spots darken and develop concentric rings, and small, black specks appear in them. The spots enlarge and merge, making infected fruits inedible.
Warm, wet weather favors the development of the disease. Try to avoid overhead watering; stake and mulch plants to keep developing fruits away from the soil; space plants to allow for good air circulation; don't work among plants when they are wet; and pick tomatoes as soon as they ripen and use them promptly.
The fungus that causes anthracnose overwinters in infected plant debris, so be sure to do a thorough garden cleanup at the end of the season. Because the fungus can persist in the soil, a 2-3 year crop rotation of tomatoes and related plants is helpful. If anthracnose is a severe problem in your garden, consider applying preventative sprays of a fungicide labelled for this use according to label instructions when the first fruits begin to develop.
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