Tomatoes, America’s number one vegetable, suffer from myriad pests and diseases. Virtually dozens of pathogens can cause spots on the leaves, and a whole host of insects can make a meal out of your plants and spoil your harvest. So many potential problems exist that you may be tempted to give up and buy them at the supermarket. Luckily, you will probably encounter only one or two of these problems in your own garden. We’ll review some of the more common ones, specifically leaf problems, in this issue.
Always remember how important it is to identify a problem before you spray. Take a diseased sample to your local garden center or Cooperative Extension office, or use your own reference materials to pinpoint the cause and find the right control. Sometimes there is no control available. When this is the case, our recommendation is to “destroy” the infected plants. You’ll have to choose one of three things to rid your garden of the problem: (1) Put the plant in a plastic bag and discard it in the trash; (2) bury the plants deeply in an area far from your garden; or (3) burn the plant. In the case of diseased plant material, any of these steps will help minimize spore production.
Symptoms: Leaves are sticky, yellow, and distorted.Cause: Aphids.
Aphids are tiny insects that come in many colors and suck plant juices from the leaves. You’ll often find them in vast colonies on succulent, new growth. Aphids excrete a sugary substance called “honeydew,” on which ants feed and on which black mold may develop.
Control: Spray with insecticidal soap, pyrethrins, malathion, or diazinon.
Symptoms: Leaves have tiny, shotgun-like holes in them, about 1/8 inch in diameter.Cause: Flea beetles.
Flea beetles are tiny black insects that hop like fleas when disturbed. They chew little holes throughout the leaf surface.
Control: Although flea beetles are rarely a problem to well-established plants, you can spray them with pyrethrins, methoxychlor, diazinon, or carbaryl.
Symptoms: Leaves have big, ragged holes in them, and some leaves even appear to be missing.Cause: Tomato hornworms or Colorado potato beetles.
A tomato hornworm is a large, green worm with white diagonal markings on its sides, and with a red or black curved “horn” at its rear. Only a few may be present but they are absolutely voracious eaters. You might come across a hornworm that has what looks like grains of rice on its back; these rice grains are actually the cocoons of the parasitic braconid wasp. When the wasps pupate, they will feed on (and eventually kill) the hornworm.
The Colorado potato beetle is a yellow, black-striped insect. Its larva is large and red, with two rows of black dots on its sides. Both the adults and larvae can severely damage plants by feeding on leaves and stems.
Control: For hornworms, here’s one of the few times when hand-picking the pest is suitable for the weekend gardener. You can also spray with the biological insecticide Bt (Bacillus thuringiensis). And if you find one that is parasitized with the wasp, don’t spray it. The wasps are beneficial insects that can infest other hornworms after they mature. Bt will also kill the Colorado potato beetle larvae. For the adult, though, you can spray with pyrethrins, diazinon, methoxychlor, or carbaryl.
Symptoms: Leaves are mottled and yellowing, and a puff of white insects arises if the plant is tapped.Cause: Whiteflies.
Whiteflies are tiny white insects that feed on plant sap. Like aphids, they excrete “honeydew” on which black mold may grow.
Control: Spray with insecticidal soap, pyrethrins, malathion, or diazinon. You can also set out yellow sticky traps whose color will attract them. The different stages of the whitefly’s life cycle each have their own tolerance to insecticides, so you’ll have to spray regularly and you may have to use more than one control method.
Symptoms: Leaves are yellow; plant is stunted and wilts in hot weather.Cause: Nematodes.
Nematodes are microscopic worms that live in the soil and feed on plant roots. If you pull the plant out of the ground, you’ll notice that the roots are swollen and distorted.
Control: Destroy infested plants; plant resistant varieties next season.
Symptoms: Leaves have light and dark mottling on them.Cause: Tobacco mosaic virus.
Tobacco mosaic virus is very infectious and can be spread by simply brushing against plants. The virus is seed-borne, but can also be passed to the plant from tobacco in cigarettes and other products (the virus actually survives tobacco processing).
Control: Destroy infected plants. Don’t handle tobacco before working around tomato your plants, and grow resistant varieties next season.
Symptoms: Older, lower leaves are yellow; shoots or the whole plant wilts.Cause: Fusarium, Verticillium, or Walnut wilt.
These three diseases are difficult to tell apart. Fusarium and Verticillium wilt are caused by soil-borne fungi, and walnut wilt comes from planting your tomatoes too close to a walnut tree. The tree’s roots produce a chemical called juglone which is toxic to tomato plants.
Control: After your harvest, destroy the plants. Next season, plant varieties resistant to Fusarium and verticillium, and practice crop rotation by growing your tomatoes in a different spot in your garden each time. If you suspect Walnut wilt, next season plant your tomatoes at least 50 feet away from your walnut tree, or else grow them in containers.
Symptoms: Older leaves have dark brown, concentric rings on them.Cause: Early blight (also known as Alternaria)
Early blight occurs in humid weather. The fungus overwinters in residue from diseased plants, and can also be present on the seed itself.
Control: Destroy or discard infected plants when harvesting is completed. In the meantime, spray them with copper, chlorothalonil, mancozeb, or maneb fungicides (look for the active ingredient on the label). These sprays won’t cure the disease, but they can protect new foliage from the fungus if they are used regularly all season.
Symptoms: Leaves have irregular, greasy-looking or water-soaked gray areas that expand rapidly in wet weather.Cause: Late blight.
Late blight is the fungus that caused the great potato famine in Ireland from 1845-1850. It occurs most often in humid weather, during which a gray mildew grows on the undersides of the affected leaves of tomato and potato plants.
Control: Spray with copper, chlorothalonil, mancozeb, or maneb fungicides (look for the active ingredient on the label) to protect new foliage. . You’ll have to continue your spray program all season long. Destroy the infected plants as soon as harvest is completed. If your plants are severely affected, dig them up and destroy them immediately.
Symptoms: Plant suddenly and rapidly wilts, even though the leaves are green and it has been watered regularly.Cause: Bacterial wilt.
This disease is caused by a soil-borne bacterium. It persists in the soil in the southern states, and can be found in greenhouses in the winter in northern areas.
Control: Dig up and destroy infected plants. Plant your tomatoes in a different spot in your garden for a few consecutive seasons.