Tomatoes are the most commonly grown plant in the home vegetable garden. Tomatoes are relatively easy to grow, and there is a wide variety of different types. If you’re growing tomatoes, you’ll more than likely encounter a few pests and diseases along the way. So, let’s take a moment and talk about some common issues we encounter when growing tomatoes.

Insects

Aphids are small, soft-bodied, pear-shaped insects with long legs and antennae. They can vary from green, yellow, black, gray, or red. They are most commonly found in clusters on succulent young shoots and leaves.

Whiteflies are also small insects. The bodies and wings of adult whiteflies are covered with a white powdery wax, thus their name. They can be found on the undersides of leaves, and, when disturbed, will fly off. The nymphs (immature stage) will attach themselves to the undersides of leaves and don’t move.

Both aphids and whiteflies have straw-like mouthparts (insects with this type of mouth are commonly called sucking insects), and when they feed leaves may become curled, puckered, and begin to turn yellow. While they rarely kill plants, if populations get high, it can result in stunted growth (reduced vigor) of the plant.

Aphids and whiteflies can often be managed by spraying them with a strong stream of water to knock them off of plants. They are commonly attached by natural enemies such as lacewing larva, lady beetles, and parasitoid wasps. Be on the lookout for these natural enemies and take steps to encourage them in your garden; they may take care of the problem for you! If you have a large infestation of whiteflies, you can try placing yellow sticky cards near the plants to trap the adults. Finally, if populations get too high, pesticides can be used to manage them.

Hornworms are large (3+ inches when fully grown) green caterpillars that can cause quite a bit of damage to tomato plants. There are two species you may encounter on tomatoes: tobacco hornworm and tomato hornworms. Tobacco hornworms have seven diagonal white lines on each side, & a curved red horn at the hind end. Tomato hornworm has eight V-shaped marks on each side, & the horn is bluish-black. These caterpillars will eventually become hawk moths.

Hornworms will overwinter as a pupa underground. One way to prevent hornworm problems if you’ve had them in the past is to destroy these pupae, either by tilling or hand picking them out of the garden. Caterpillars can also be handpicked off of plants and disposed of. You may also find caterpillars that have white projections coming out of them. These are the pupa of parasitoid wasps that have been eating the insides of the caterpillar. If you find these leave them in the garden so the wasps can go and attack other caterpillar pests for you.

Tomato fruitworms are caterpillars that may occasionally show up in your tomatoes (they are the same insects we call corn earworm).  The larvae are green to reddish to brownish in color with light and dark stripes on their bodies. They will burrow into and consume fruit. You will usually find a hole at the base of the fruit stem. When the tomato is cut open, you will discover tunnels that will often have frass (insect poop) and decay inside.

Since they usually damage less than 5% of a crop, management of them may not be needed. If you do have issues with them, a pesticide can be applied to tomatoes at 4- to 10-day intervals (read the label to see how often) starting when the fruit is small.

Diseases

Early blight primarily infects the foliage of plants, but it can also affect the stem and fruit. It appears as irregularly shaped brown spots that have concentric rings (resembles a bulls-eye or target), commonly on older leaves. The spots are often surrounded by yellow tissue. The spots can grow to be ¼ to ½ inch in diameter and will often grow together (coalesce), forming large brown areas. Eventually, leaves will drop off of plants.

Septoria leaf spot also primarily infects leaves, but will also infect the stems. It forms small (1/16 to 1/8 of an inch) circular spots on leaves. These spots have a tan or light-colored center with dark purple or brown margins. Like early blight, the individual leaf spots will often coalesce, forming large areas of diseased tissue. Heavily infected leaves will turn yellow and fall off of the plant. Unlike early blight, it does not form concentric rings. But, if you look closely (may need to use a magnifying glass), you can often see small black pimple-like fruiting bodies in the center of the leaf spots.

Anthracnose is primarily a fruit disease. Circular sunken lesions develop on fruit. Over time these spots will enlarge and darken. Often pink to orange masses spores form in concentric rings on the surface (when it’s humid). Anthracnose may occasionally be found on leaves and stems where it will cause irregularly shaped brown spots with dark brown edges.

When it comes to managing diseases in tomatoes, there are several things you can do.

• Try to select cultivars that have resistance to disease, particularly if you’ve had issues in the past.  
• Practice crop rotation. Ideally, you wouldn’t grow any solanaceous (tomato family; includes peppers, eggplants, and potatoes) plants in the same area (this is often easier said than done in backyard gardens). Early blight can survive in the soil, on seeds or infected plant debris for a year. While Septoria can survive for up to three years on infected plant debris, as well as weedy hosts, it does not survive in the soil on its own.
• If you are purchasing transplants, inspect plants for any leaf spots (and insects) before purchasing.
• Make sure to space your tomatoes properly. Adequate spacing will allow airflow between plants, and it will allow them to dry-out faster (plant pathogens need moisture to survive).
• Using mulch on the ground creates a barrier between the soil, where disease spores may be, and the plants. This will help prevent any of the pathogens from splashing onto the leaves of your plants and infecting them.
• Avoid watering late in the day and try not to get the foliage wet. The longer the leaves remain wet, the higher the chance disease will develop. Additionally, when you water, try not to get the foliage wet (drip irrigation is good for this).  
• Make sure leaves are dry when handling plants. If leaves are wet, you may end up spreading the disease.
• Remove and destroy any diseased plant material. When removing it, don’t just throw it on the ground, get it out of the garden!
• Fungicides can also be used (along with these other strategies) to manage diseases in your tomatoes.

Other Problems

Contrary to popular belief, blossom end rot is not caused by a disease or insects. Blossom end rot is actually a physiological disorder. The cause of blossom end rot is poorly understood, it has commonly been believed that it is caused by low levels of calcium in developing fruit, or due to stress on the plant (read more about the causes of blossom end rot). One of the best ways to avoid blossom end rot is to provide adequate moisture from fruit formation to maturity. Avoid frequent shallow, instead, water deeply. Using mulch can help conserve soil moisture, even out the moisture in the soil, and help reduce how frequently you need to water. Unfortunately, once a fruit is suffering from blossom end rot, there is nothing you can do, so it is best to remove the fruit and discard them.

Sunscald on tomatoes are areas of the fruit that have been damaged and turn white,typically on the top of the fruit (although it can happen anywhere). It commonly shows up during hot, dry weather, especially on fruit that has recently been exposed to sun, such as when the leaves are killed by disease.

Fruit with yellow shoulder will have hard, yellow areas near the stem end that don’t ripen. These fruits will also often have internal white tissue. Plants that have yellow shoulder are often growing in soils with low potassium levels, low organic matter, and a high pH. If you have experienced yellow shoulder in the past, a soil test may be in order. High temperatures (>90ᵒF) are another possible cause.

Cracking is caused by rapid fruit growth, which causes the skin to crack because it can’t expand fast enough. This is commonly seen when plants receive moisture (rain or irrigation) after a dry spell. Tomatoes exposed to the hot sun often tend to crack more as well. Additionally, some tomato varieties are more prone to cracking than others. To try and prevent cracking, make sure plants receive an even water supply and maintain healthy foliage, so fruit is shaded (which will keep them cooler).

If you encounter any of these or other tomato issues, feel free to contact your local extension office for help.

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Photo Credits

Aphids - Joseph Berger, Bugwood.org; Whiteflies - Whitney Cranshaw, Colorado State University, Bugwood.org; Tobacco hornworm - Eddie McGriff, Eddie McGriff, University of Georgia, Bugwood.org; Tomato fruitworm - Bruce Watt, University of Maine, Bugwood.org; Early blight - Clemson University - USDA Cooperative Extension Slide Series , Bugwood.org; Septoria leaf spot - Bruce Watt, University of Maine, Bugwood.org; Anthracnose - Clemson University - USDA Cooperative Extension Slide Series , Bugwood.org; Blossom end rot - Scott Nelson; Sunscald - Clemson University - USDA Cooperative Extension Slide Series , Bugwood.org; Yellow shoulder - Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org; Cracking - M.E. Bartolo, Bugwood.org