Tomato Pests, Diseases & Physiological Disorders

Insect Pests of Tomatoes

Tomato hornworm, showing the typical rear horn and side markings.
Image courtesy Whitney Cranshaw—University of Colorado.

• Hornworms

  Hornworms are huge green caterpillars that strip leaves fast, eating whole leaves at a time. Defoliation on that scale can badly cut yield, and older hornworms also chew on fruit. Their dark droppings (frass) are often the first sign you'll notice.

  Two hornworm species feed on tomatoes: the tobacco hornworm (Manduca sexta) and the tomato hornworm (Manduca quinquemaculata). They are the caterpillars of the Carolina sphinx (tobacco hawk moth) and the five-spotted hawk moth, respectively. In the U.S., tobacco hornworm is most common in the Southeast and tomato hornworm elsewhere.

  They look much alike. The tobacco hornworm has diagonal white stripes and a red horn, while the tomato hornworm has V-shaped white markings and a dark horn.

  Control them by hand-picking and destroying the caterpillars; by encouraging natural enemies, such as braconid wasps that parasitize the caterpillars, Trichogramma wasps that attack the eggs, and predators like lacewings and lady beetles; or by spraying an insecticide containing Bacillus thuringiensis or spinosad, both allowed in organic production.

• Whiteflies

  Whiteflies occur worldwide, and more than 1,500 species have been described, but only two or three are important on tomato, feeding on leaves and fruit and spreading viruses such as TYLCV. They lay eggs on leaf undersides. In greenhouse tomatoes, whitefly feeding is a common cause of irregular ripening.

  The adult sweetpotato whitefly and the silverleaf whitefly are pale, white to light yellow, and look like a tiny grain of rice from above. At rest, the adult sweetpotato whitefly (biotype A) holds its powdery white wings at a steep angle, tented over its slender body. The silverleaf whitefly (biotype B) holds its white wings closer to the body, which is a deeper yellow. Both lay white eggs that darken to brown before hatching.

  Natural enemies, good sanitation, rotation away from host crops, and sprays such as horticultural oil, insecticidal soap and azadirachtin together give only limited control. Silver or aluminum reflective mulch helps repel whiteflies. Prevention is vital, since an infestation can grow so fast that the only remaining option is to pull the crop and leave the space empty for a while.

• Thrips

  Thrips are found across most of the U.S. Their biggest threat to tomatoes is spreading tomato spotted wilt virus, described above. They are tiny, slender and pale to brown, with fringed wings.

  Reduce thrips by rotating crops and planting tomatoes away from spots where other host plants grew.

• Aphids
  

  Potato aphids on a tomato leaf.
  Image courtesy Joseph Berger, Bugwood.org.

  Several aphid species attack tomatoes, but green peach aphid and potato aphid are the most important. The damage depends on the species. Research has also linked earlier aphid infestation with whitefly infestation and TYLCV spread.

  The green peach aphid occurs worldwide, including throughout North America, and is most abundant early in the season. It is slender and dark green to yellow with darker green stripes and no waxy coating. Its feeding can cause wilting but usually only slightly delays maturity; however, it does spread plant viruses.

  The potato aphid is considerably larger and longer than the green peach aphid and comes in green or pink forms. It is more common later in the season. In large numbers it can do serious harm, causing distorted leaves, stunting and dead tissue (necrosis). Its sticky excretion, honeydew, encourages mold, and heavy infestations can cut yields sharply.

  Natural parasites and predators help control aphids, and, as with whiteflies, reflective mulch laid early in the season helps keep them away.


Typical tomato psyllid droppings ("psyllid sugar") on tomato leaves.
Image courtesy Whitney Cranshaw, Colorado State University, Bugwood.org.
• Tomato/potato psyllid (Bactericera [Paratrioza] cockerelli)

  The tomato/potato psyllid (TPP) belongs to a group sometimes called jumping plant lice. In the U.S. it occurs mainly in western states. About 1/8" (3 mm) long and shaped like a miniature cicada, it feeds mostly on nightshade crops, though many other plants can shelter or host it.

  Nymphs feeding on leaf undersides inject a toxin in their saliva that causes yellowing and stunted shoot tips, a condition called psyllid yellows.

  This can progress to severe wilting and dieback. Adults can also carry the bacterium Candidatus Liberibacter solanacearum, which adds to the damage.

  Signs of TPP include:

  • adults, which fly well and jump off the plant when disturbed;
  • tiny yellow, oval eggs on short stalks on the undersides of leaves;
  • little white beads of waxy excreted sap (psyllid sugar) from adults and nymphs, which attract ants and mold;
  • stunted, yellow, purplish, cupped or curled leaves, as noted above;
  • severe wilting and plant death.

  If you think TPP is present, contact your local extension office for identification and advice before treating. Both biological and chemical controls exist, and extension can help you choose and may want to record the find.

• Potato flea beetle (Epitrix spp.)

  Potato flea beetles feed on nightshade-family plants and are widespread in North America. They are mainly a seedling problem, since young plants are most vulnerable. Adults often gather on lower leaves and chew many small holes.

  Where flea beetles are common, cover transplants with floating row cover on planting day and remove it once plants are established.

• Tomato fruitworm (Helicoverpa zea)

Formerly placed in the genus Heliothis, this caterpillar of the Helicoverpa zea moth also goes by corn earworm and cotton bollworm. It doesn't survive winter in the northern U.S. but migrates north from the South. Larvae bore into green or ripe fruit near the stem and feed inside, ruining it for sale. Newly hatched larvae are white with a black head and black bumps, each bearing two bristles. Older larvae range from yellow-green and green to red, brown or almost black, with fine spines.

Releasing the egg parasitoid Trichogramma pretiosum while fruitworm moths are laying eggs can help. A trap crop of corn in the silking stage can also draw egg-laying moths away from tomatoes.

Nematode Pests of Tomato

Southern root-knot nematode (N) on tomato.
Image courtesy Gerald Holmes—California Polytechnic State University.

Nematodes are a huge, varied group of roundworms found everywhere; more than half of them are parasites.

• Root-knot nematodes (Meloidogyne arenaria, M. incognita, M. javanica) • N

  The nematodes that attack tomatoes are named for the galls, or knots, they cause on roots. Galled roots take up water and nutrients poorly, so plants are stunted, weak and less productive.

  Modern hybrids very commonly carry resistance to the three main root-knot nematode species that attack tomatoes.

  Tomatoes resistant to nematodes

Not every tomato problem comes from a pest or disease. Environmental and nutritional stresses can also upset normal growth, causing disorders that harm plant health, fruit quality or yield.

Physiological problems in tomatoes include:

• Blindness

  A tomato normally grows one main stem, the leader, plus side shoots that can be pruned off or limited, usually to one extra leader. Sometimes the main stem stops growing, often when the plant has only a few true leaves; this is called "blindness." Only the main growing point is affected, so side shoots keep growing, and one or more can be kept to take over as the fruiting stem.

  The cause isn't fully understood, but genetics, day length and temperature have all been suggested.


Blossom end rot on tomato, outside and cut open. It tends to be worse in paste tomatoes than in other types.
Images courtesy of a longtime grower.
• Blossom end rot

  Early blossom end rot looks like a sunken, brown, water-soaked patch at the bottom of the fruit. As it grows it turns dark and leathery. Like tipburn in lettuce and brassicas, it's caused by a localized shortage of calcium in the tissue.

  These steps help reduce the causes of blossom end rot:

  • Test your soil and correct it, ideally the season before planting. Soil pH that's too high or too low, high soil salts, and occasionally low soil calcium can all contribute. Keep pH around 6.5–6.8 for good nutrient uptake, make sure nitrogen, phosphorus and especially calcium are adequate, and ensure good drainage to avoid salt buildup.
  • Uneven moisture is the more common culprit, so keep soil moisture steady all season and never let plants wilt. Without enough water, the plant can't move calcium and other nutrients to the fruit. Drip lines or soaker hoses help deliver steadier moisture and nutrient movement.
  • Don't injure roots when cultivating. Warm soil also helps tomatoes build a healthy root system.

  Affected fruit won't recover, so pick and discard it so the plant can put its energy into the remaining fruit. With good watering and feeding, later fruit should develop normally.


Abiotic stress manifesting as radial cracking, starting at the stem end and running toward the blossom end of the fruit.
Image courtesy of a longtime grower.

Abiotic stress manifesting as concentric cracking around the stem end.
Image courtesy Paul Bachi—University of Kentucky Research & Education Center.
• Cracking, splitting and catfacing

  Cracks and splits are named by the direction they run on the fruit.

  • Cracking comes in two main forms. Radial cracks run outward from the stem like spokes, while concentric cracks circle the fruit around the stem end.
  • Catfacing, unlike the other cracks, shows up at the blossom end, as scarring and cracks or, in bad cases, holes in the fruit.

  All share similar causes: a flood of water after dry weather, abrupt temperature swings, or extreme temperatures. When water rushes in, the inside of the fruit expands faster than the skin, and the skin cracks or splits.

  Besides uneven watering, cold weather during flowering can also cause catfacing.

  These blemishes give fungi and bacteria a way in and raise the risk of rot, especially on green fruit. Cracked, split or catfaced fruit is still fine to eat, just less attractive.

  Keep moisture steady through flowering and fruit development to prevent cracking and catfacing.

  Some varieties resist cracking better than others, and catfacing seems more common in large-fruited and heirloom varieties. If keeping moisture even is difficult, choose crack-resistant varieties.

• Misshapen fruit

  Now and then some fruits look quite different from the rest of the crop and from the variety description. Poor pollination is often to blame, and several conditions can interfere with it:

  • Temperature or humidity outside the best range. Pollination works best at about 70–82°F (21–28°C) with humidity around 70%.
  • Too little water.
  • Nutrient shortages or excesses.
  • Pollen not being released and moved. Tomato flowers pollinate themselves, but the pollen has to be shaken loose, by wind or by bees vibrating the flowers, so it falls from the anthers onto the stigma. In still air, such as in a greenhouse or tunnel, less pollen moves. Vibrating flower clusters with an electric pollinator wand or gently shaking the plants helps.

Tomato fruit with sunscald damage in the field.
Image courtesy Howard F. Schwartz, Colorado State University, Bugwood.org.
• Sunscald

  Sunscald is essentially sunburn on fruit, usually because leaves are sparse or have been lost. Affected fruit develops pale, blistered patches that can turn papery. Besides spoiling the fruit, sunscald makes it more open to disease and insects.

  Sunscald can't be reversed once it happens, but shade cloth over the greenhouse or row reduces light, and avoiding heavy leaf pruning keeps developing fruit covered.

• Yellow or green shoulders

  With this disorder, part of the fruit fails to ripen. The shoulders stay green at first, then turn bright yellow and remain firm as the rest of the fruit ripens.

  The cause isn't certain but probably involves several factors, including potassium deficiency, varietal susceptibility, viruses, and high temperatures (above about 90°F / 32°C). Heat may matter because lycopene, the red pigment in tomatoes, forms poorly at high temperatures.

  Some varieties are more prone to it. Manage it with good airflow and ventilation to control temperature, prune only as needed so ripening fruit keeps some leaf cover, and feed adequately to avoid deficiencies.

• Zippering and other ripening disorders
  
  Zippering happens when the flower's anthers stick to the skin of the young fruit as it sets.

  Several other physiological problems can cause uneven ripening or discoloration, such as puffiness, graywall or blotchy ripening, white internal tissue and zippering.

  They usually stem from some mix of temperatures that are too low or too high, nutrition problems, and other environmental factors.

  Zippering is cosmetic and is linked to high humidity at fruit set, which can make the anthers (the pollen-bearing tips of the stamens) cling to the skin of the newly forming fruit.

Tomatoes inspire real devotion, from the weekend gardener with a few patio pots to the professional running acres of greenhouse production. Whatever kind of grower you are, we hope this overview encourages you to keep learning and to use the help available in your area when you need it. Our aim is simply to help you grow better tomatoes. Even experts find that tomatoes can surprise them, usually pleasantly and occasionally painfully. Prevention is always the best approach, but if you get stuck, contact your local extension agent or plant diagnostic clinic, give us a call, or look through the resources below; everyone keeps learning something new about this "vegetable" each year. (Botanically, of course, the tomato is a fruit.) Until then, may your pommes d'amour (love apples) delight you and everyone you share them with.

• Tomato Diseases: A Color Handbook American Phytopathological Society

• Tomato Health Management American Phytopathological Society

• APS image database

• How to manage pests: tomato University of California Statewide Integrated Pest Management Program

• Managing blossom end rot University of New Hampshire Cooperative Extension Service

• Garden Insects of North America. Whitney Cranshaw & David Shetlar

• Physical, Biological & Botanical Controls • Comparison Chart of Hensley’s-Preferred Options PDF

• Top tomatoes for high tunnels and hoophouses: diseases and resistances, plant habit, variety picks article