Identifying Plant Pests and Diseases

Aphids

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.1

Cabbage Worms

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.

Spider Mites

This boxwood suffered a severe spider mite infestation, causing the leaves to dry out and die. Spraying with horticultural soap controls spider mites.2

Squash Bugs

Squash bug eggs are hatching into nymphs on the undersides of these leaves. Remove any affected leaves and scout for adults.3

Tomato Hornworms

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 it is to simply remove it from the plant and dispose of it.4

Whiteflies

Whiteflies feed off plants by sucking the plant’s juices. They can cause wilting, stunting, and death.5 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 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 control6 to existing whitefly infestations. Coat both the upper and lower surfaces of leaves. Repeat applications will likely need to be made.7

Wireworms

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 or sweet potato as a decoy trap. Skewer a piece of raw potato 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.8

Cucumber Beetles

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 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 such as Rotenone or Pyrethrum on your plants.9

Plant Viruses

Mottled yellow leaves usually indicate a mosaic virus. Remove and destroy any affected plants as soon as possible to limit the spread of the virus.10

Gray Mold on Peony Leaves

Botrytis,11 or gray mold, is a fungus that frequently affects peony plants. Remove and destroy any affected leaves. Give the plants plenty of room for airflow, and cut back and remove foliage in the fall to lessen overwintering spores.

Powdery Mildew

Powdery mildew has formed on the leaves of this rudbeckia. If you see it on any of your plants, use a fungicide to control the problem.12

Leaf Spot

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.