Plant Pests: Insects and Diseases

Aphids
Aphids. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.
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    Nip Problems in the Bud

    Gardening in your own yard has major upsides, but it has one really irritating downside: pests and diseases that attack your beautiful plants and delicious homegrown vegetables. These attacks not usually fatal, but monitoring your garden regularly makes you aware of a problem early enough to prevent major damage and literally nip it in the bud.


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    Aphids: Small But Persistent

    Aphids
    Aphids. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    Aphids have infested this scabiosa plant. Spraying with a jet of water and/or insecticidal soap gets rid of the problem. Repeated sprayings will probably be necessary.


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    Signs of Cabbage Worms

    Cabbage Worm Damamge
    Cabbage worm damage. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    Holes in cabbage leaves are a telltale sign of cabbage worm feeding. Hand remove adults and look for eggs on the undersides of the leaves.


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    Spider Mite Damage

    Spider Mite Damage
    Spider mite damage. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    This boxwood suffered a severe spider mite infestation, causing the leaves to dry out and die. Spraying with horticultural soap controls spider mites.


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    Squash Bug Eggs

    Squash Bugs and Eggs
    Squash bugs. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    Squash bug eggs are hatching into nymphs on the undersides of these leaves. Remove any affected leaves and scout for adults.


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    Tomato Hornworms

    Tomato Hornworm
    Tomato hornworm. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    The tomato hornworm is the larval stage of the five-spotted Hawk moth. Both stages of this insect are formidable to see, but the hornworm is not a welcome sight in the vegetable garden. Hornworms favor the leaves of tomato and pepper plants. Since the hornworms blend in so well with the foliage, you might not notice them until you start to see the damage. Because the hornworm is so large (about the size of a pinkie finger), the easiest way to get rid of him is to simply remove him from the plant...MORE and dispose of him. 


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    Whiteflies

    Silverleaf whitefly. Photo: USDA Agriculture Research Service (ARS) / Scott Bauer.

    Whiteflies feed off plants by sucking the plant's juices. They can cause wilting, stunting and death. As with their relatives the aphids, mealybugs and scale, a whitefly population can grow quickly.​ ​The best way to control whiteflies is to constantly monitor for them and use a combination of techniques to keep their population down. Check to be sure any new plant you buy is not infested; just moving the plant will cause them to flutter around. If you discover a small infestation in your...MORE garden, wash it off with a blast of water from the hose or by sinking the plant into a bucket of water. Sprays containing pyrethrum or neem offer some control to existing whitefly infestations. Coat both the upper and lower surfaces of leaves. Repeat applications will likely need to be made.


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    Wireworms

    Wireworm on a Sweet Potato
    Wireworm. Photo: David Bradbeer's / flickr.

    Wireworms are very common in most types of soil and can be found year-round. If wireworms feed on the roots of plants, the plants will grow slowly and will be weak. Since wireworms are so common, they are extremely hard to control. However, if you suspect wireworms are present, here are a few techniques you can try to limit their destructiveness:


    • Cultivate your soil in May and June, when they hatch, to expose them to hungry birds.
    • Use chunks of potato as a decoy trap. Skewer a piece of raw potato...MORE and bury it near the problem area and dig after about a week and check for the presence of wireworms. Dispose of the potato piece, wireworms and all.
    • Remove and destroy infected crops after harvesting to limit overwintering.

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    Cucumber Beetles

    Cucumber Beetles - Cause of the Cucumber Disease Bacterial Wilt
    Cucumber beetle. Photo Courtesy of the USDA Agricultural Research Service (ARS).

    Cucumber beetles eat roots, leaves, and flowers and transmit bacterial wilt disease along the way. The damage usually isn’t enough to kill the plants, but the loss of flowers means a loss of fruits. The spread of bacterial wilt can be deadly and quick. It starts with one leaf wilting and spreads. A telltale sign of bacterial wilt is the sticky, white sap-like substance that oozes from snapped stems. Bacterial wilt is a serious disease of cucumbers and muskmelons. It affects squash, too, but to a...MORE much lesser extent. 


    Control the damage done by cucumber beetles by buying wilt-resistant plants and keeping plants off the ground by using a trellis. If an infestation still occurs, use a pesticide like Rotenone or Pyrethrum on your plants. Cucumber plants are sensitive to many chemicals, so follow the label’s instructions carefully and use chemicals only as a last resort.


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    Plant Viruses

    Cucumber Mosaic Virus
    Cucumber mosaic virus. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    Mottled yellow leaves usually indicate mosaic virus. Remove and destroy any affected plants as soon as possible to limit the spread of the virus.


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    Gray Mold on Peony Leaves

    Peony Botritis
    Peony Botritis. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    Botrytis, or gray mold, is a fungus that frequently affects peony plants. Remove and destroy any affected leaves. Give the plants plenty of room for air flow and cut back and remove foliage in the fall to lessen overwintering spores.


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    Powdery Mildew

    Powdery Mildew
    Powdery mildew. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    Powdery mildew has formed on the leaves of this Rudbeckia. If you see it on any of your plants, a fungicide will help to control the problem. 


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    Leaf Spot

    Tomato Leaf Spot
    Tomato leaf spot. Photo: © Marie Iannotti (2008) licensed to About.com, Inc.

    The lower leaves of this tomato plant are infected with leaf spot. A fungicide can be used and the leaves can be removed from the lower part of the plant to prevent spores from splashing up onto the foliage.