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A Guide to Common Garden Pests and Plant Diseases

Use this handy guide to identify and avoid common garden pests and plant diseases.
By Alex Mitchell
March 2012 Web
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You don't need a sprawling backyard or spacious raised beds to grow delicious fruits, vegetables, and herbs of your own. In “The Edible Balcony,” longtime urban gardener Alex Mitchell shows how to transform whatever space you have—from a balcony or rooftop to a fire escape or window box—into a profusion of fresh, seasonal produce.
Photo Courtesy Rodale Books
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Plant diseases and garden pests may seem like a fact of life to the avid gardener, but with a little advice, you can learn to halt these headaches and even avoid them in the future. This convenient reference list from The Edible Balcony (Rodale Books, 2012) lists and explains some common garden nuisances. Excerpted from the chapter “Pests and Diseases.” 

Aphids

See Blackfly, Greenfly, and Woolly aphid.

Apple and Pear Scab

Rain and wind spread the spores of this fungal disease that causes dark green patches to appear on the leaves of apple and pear trees and then corky scabs on the surface of the fruit. Remove and throw away fallen leaves to reduce the spread of the spores.

Bacterial Canker

This disease affects stone fruit trees, such as peaches, plums, cherries, and apricots, causing a clear brown gum to ooze from the branches. Prune out affected areas down to at least 8 in. below the canker and throw prunings away rather than composting them. To deter, prune stone fruits only in the summer months.

Beetles

Tiny holes in arugula and brassicas and a cloud of tiny black beetles that fly up when disturbed usually means flea beetle. A mild infestation doesn’t do too much harm, but if it’s a real problem, avoid growing arugula between late May and mid-summer when flea beetles are most prolific.

Birds

The fruit of trees such as plums, cherries, and peaches is a magnet for many birds, while brassicas can be targeted by pigeons. If you’re really bothered, you may have to net the trees or plants at vulnerable times. With trees, you could also try hanging plastic bottles, DVDs, or glass candleholders in the branches to deter the birds.

Blackfly

These little black aphids can be a particular problem on beans, clustering around the growing tips and sucking the sap. Either blast off the flies with a jet of the hose or brush them off or spray them with insecticidal soft soap—an organic aphid deterrent available from all garden stores. (You’ll need a plastic bottle with a nozzle to spray it on with.)

Blight

This is an airborne fungal disease that tends to strike in damp, cool summers, affecting tomatoes and potatoes. It is unlikely to be a problem for potatoes on balconies, since container-grown spuds are usually First or Second Earlies, ready for harvest before the disease takes hold. Look for brown patches on leaves with white rings on the underside, and if you see them, dig up potatoes immediately. Tomato stems may also show brown patches and their fruits can turn black and rotten overnight. With tomatoes, cutting off affected leaves and fruit as soon as you spot blight may stop the disease from spreading through the plant. If blight has struck your tomatoes or potatoes, don’t compost the leaves since the spores can survive and affect future crops. Instead, throw them away.

Blossom End Rot

A dark, leathery patch on the bottom of tomatoes indicates this problem, which is caused by irregular watering. Cut off affected fruits and prevent the issue by watering little and often rather than via an occasional deluge.

Blossom Wilt/Brown Rot

If new shoots on your fruit tree suddenly wilt, particularly if it’s a few weeks after flowering, chances are your tree is suffering from this fungal disease. Apples, pears, plums, cherries, nectarines, peaches, and apricots can be affected. Avoid by removing any affected fruit—throwing it away rather than composting it—and pruning out any diseased spurs over winter.

Botrytis

The tell-tale sign is a grey, furry mold on leaves or fruits. Remove any infected material and take care when planting lettuces not to bury the bottom leaves in the compost.

Brown Scale Insect

This is a pest of citrus and bay trees. Shield-like bugs cluster in the leaf joints and under leaves, sucking the sap and weakening the plant. Remove the insects by hand or with an old toothbrush and soapy water.

Carrot Fly

This is generally not a problem for balcony or rooftop gardeners since these pests can’t fly higher than 1 ft.

Caterpillars

Large holes in leaves without any slug or snail trails generally suggests caterpillars have been visiting. Look under the leaves for the culprits as well as for clusters of caterpillar eggs and squash them.

Cats

Cats can be a real pain, scratching up the compost of newly sown containers and using them as toilets. Twiggy or prickly sticks laid across or poked upright into the compost will generally keep them off.

Codling Moth

Watch out for this pest on apple and pear trees—the larvae tunnel into the fruit, not only spoiling it but also encouraging secondary rots to form. Deter by putting up codling moth traps in late spring.

Common Scab

Scabby, scurfy patches on the skin of potatoes won’t affect their flavor. Simply scrub off the marks before cooking.

Green Fruitworms

The moths are brown and 2 in. across. The greenish larvae are about 1 in. long and they hatch from eggs laid on the flower buds. The larvae eat the flowers and small fruits before dropping to the ground to pupate. Use an insecticide as soon as buds sprout in the spring.

Greenfly

These are aphids that suck the sap from the growing tips of many vegetable and fruit plants, whose excretions encourage fungal diseases. Spray any bugs you see with insecticidal soft-soap solution.

Leaf Miner

Chard, spinach, and beet leaves can be bothered by this pest, tiny larvae which make tunnels inside the leaves, causing dead, brown patches. Either remove individual leaves or squish the larvae inside the leaf with your fingers.

Peach Leaf Curl

This is a fungal disease affecting peach, nectarine, and apricot trees which causes the leaves to turn red and pucker before falling off. The spores are spread by rain splash, so contain the disease by covering fan-trained trees with a sheet of plastic from autumn to mid-spring (allowing air to get into the sides). Do the same with standard potted trees or bring them inside over the winter until mid-spring. It’s also worth investigating growing disease-resistant varieties.

Powdery Mildew

Peas, apples, grapes, melons, zucchini, squashes, pumpkins, and cucumbers are susceptible to this fungal disease, which makes it look as though the leaves are sprinkled with talcum powder. Improve air flow around the plants by removing leaves or even entire plants. Feed regularly and don’t let the compost dry out.

Raspberry Beetle

These beetles lay their eggs inside blackberries and raspberries, making them brown, hard, and inedible. Little white grubs may be found inside berries, so soak the fruits in salty water to bring the creatures out, then rinse thoroughly before eating.

Red Spider Mite

This is usually a greenhouse pest, but occasionally it can be a problem in dry, hot summers for citrus, cucumber, and pepper plants. Microscopic insects colonize the leaves, sucking the sap and weakening the plant. The signs to look out for are pale yellow dots on the undersides of leaves and white web fibres underneath. Mist the leaves to increase humidity. Indoors, the biological control Phytoseiulus persimilis can be effective; it is available by mail-order through garden stores or online.

Sciarid Fly

Brown flies, about 1/8 in. long, that are found on the surface of moist potting compost, these can be a problem with seedlings, particularly those sown inside in pots in the spring. Their larvae feed on roots, making the plants collapse and die. They thrive in damp conditions, so prevent by watering from below and keep watering to a minimum. A layer of grit or sand on the surface of the compost can also help.

Silverleaf

A fungal disease affecting apples and stone fruits such as peaches, plums, apricots, and cherries that causes the leaves to become silvery and the tree’s growth to be weakened. Avoid by pruning stone fruits only in the summer months. If mildly affected, a tree can be coaxed back to health via plenty of feeding; if severely affected, trees should be removed.

Slugs and Snails

Slugs and snails can reach the balcony, carried up on pots or in old compost, and, once established, can be real pests even several stories up. So, discard any snails in household garbage and keep an eye out for hiding places under pots. Organic slug pellets based on ferric phosphate are harmless to other wildlife and children and are very effective in dealing with slugs.

Split Tomatoes

Irregular watering causes the tomatoes to burst, so try to water little and often rather than via an occasional deluge.

Vine Weevils

These grayish-black beetles cut slits in the leaves of plants, but it’s the grubs that are the real menace for container gardeners, since these eat the roots of plants, killing them. If you find the adult beetles, squash them. If you find small, white, U-shaped grubs in the compost and the plant is not too badly affected, shake the compost off the roots, wash them thoroughly, and then replant in fresh compost. Throw away the infested compost as it will be full of eggs that you don’t want spread around your garden.

Whitefly

Clouds of tiny, white moths that fly up when disturbed suggest a whitefly infestation. Spray an infected plant with insecticidal soft soap or a few drops of dish soap diluted with water in a spray bottle.

Woolly Aphid

Apples can be susceptible to these pests that secrete a white, cotton-wool-like fluff on the stems in spring and summer. Wash them off with soapy water.

Woolly Vine Scale

Grapes and currants are particularly susceptible to these flat, brown insects which lay their eggs in cotton-wool-like threads on the bark. Remove with an old toothbrush and soapy water.

This excerpt has been reprinted with permission from The Edible Balcony by Alex Mitchell, published by Rodale Books, 2012. 


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