Diseases and Insects

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Your ability to grow terrific tree fruits depends in large part on your ability to control pests and diseases. You will face many of the same challenges as commercial growers, but it's unlikely that you will have the same powerful pest control tools that they have. For example, home fruit growers typically use hand-operated sprayers or those run by small electric or gasoline motors. Compared with commercial-sized sprayers, these machines have a smaller capacity and lower pressure and require more energy to do an effective spraying job.

This makes it especially important for you to follow cultural practices that keep trees healthy and minimize disease and pest buildup. If you plan to spray your trees, it's easier to get good coverage with home-scale equipment if you plant dwarf or semidwarf cultivars.

In addition to doing a good job of site preparation, choosing a location with good air drainage, and planting disease-resistant cultivars, there are several easy steps you can take to help prevent pest and disease problems:

• Maintain a complex ecosystem around your plantings that provides habitat for beneficial insects.

Without the pest control tools of commercial growers, you need to follow best cultural practices to keep trees healthy.

Prune out dead twigs and branches during the dormant season.

Dormant oil sprays can control mites, scabs, and pear psylla.

Choose scab-resistant apple cultivars.

  • Rake up and destroy leaves and diseased fruit in the fall, after harvest.
  • Familiarize yourself with disease and insect life cycles so that you can correctly time control measures.
  • If sprays are necessary, always use pesticides according to the label.

One particularly effective treatment that home fruit growers should consider is the use of dormant oil spray. If applied when up to 1/2 inch of green is showing on the buds, it can effectively control mites, scales, and pear psylla.

Home gardeners should be aware of the following major diseases and insects of tree fruits. For a more complete description of pests and control methods, see the Cornell Cooperative Extension publication Pest Management around the Home: Part I, Cultural Methods. For ordering information, see: "Related Cornell Cooperative Extension Publications," page 103.

Apple scab. This fungal disease is easily recognized by the olive-green to black spots it causes on fruit and foliage. Severely infected leaves are dwarfed, cupped, or curled and drop prematurely. Fruits infected during the early season may be severely deformed or may drop by early June. The scab organism survives the winter in dead apple leaves on the ground. Primary infections occur during rainy periods from the time green tissue appears in the spring until the end of June. Many secondary spores are produced within the primary lesions. These are washed from the lesions by rain and are spread to other susceptible tissues, where, under appropriate environmental conditions, they cause secondary infections.

Good scab control early in the season makes control in late summer easier. Home gardeners should seriously consider selecting cultivars that are resistant to scab. Mcintosh and Cortland are likely to become infected with apple scab in home orchards. Freedom, Liberty, Prima, Jonafree, GoldRush, and MacFree are very resistant.

Powdery mildew. This fungal disease overwinters in dormant buds. Leaves that emerge from infected buds are covered by a white, powdery fungal growth. Secondary spread of the disease to other plant tissues occurs from the time the buds open until the terminal buds form in late June. If powdery mildew is a serious disease in your area every year, do not plant the susceptible cultivars Idared, Monroe, Rome Beauty, Jonathan, Paulared, Gingergold, or Cortland.

Fire blight. This bacterial disease causes severe damage to pear and apple trees during warm, rainy spring weather. Branches blacken and droop rapidly as if scorched by fire. Fire blight bacteria overwinter between live bark tissue and the tissue killed the previous season. In the spring, the bacteria are spread by windblown rain, insects, or pruning tools. The disease usually affects blossoms but also can directly infect succulent shoots during late spring and summer.

To avoid fire blight, do not grow trees on poorly drained, highly acidic, or overfertilized soils. Cut off infected twigs and branches in late winter, making the cuts at least 6 inches below the dead area. If you are pruning during the summer, disinfect pruning tools with denatured alcohol or a 10 percent bleach solution between each cut. Be sure to clean and oil pruning tools after use. All pear cultivars except Seckel are very susceptible to fire blight. Susceptible apple cultivars are Idared, Jonagold, Jonathan, Lodi, Crispin (Mutsu), Greening, Paulared, Rome, Sir Prize, Spigold, Twenty Ounce, York, and Gala.

Brown rot. This fungus attacks peaches, plums, cherries, and nectarines. This disease is easy to diagnose by the unsightly brown rot that forms on the fruit, rendering it inedible. Spores are released during rainy periods in the spring and summer, infecting the blossoms and fruit. The fungus overwinters in infected twigs or fruits that remain on the tree or nearby on the ground. Remove and destroy diseased fruits to help reduce infection.

Black knot. This fungus infects plum and cherry trees, causing rough, black enlargements on the twigs. The knots are often two to four times the diameter of the twigs and up to 8 inches long. Prune black knot-infected twigs at least 8 inches below the knot in winter or early spring and destroy them. Do not allow this disease to build up, or severe pruning will be necessary.

Cytospora canker. This fungal disease is very devastating to peach trees and also infects apricot, plum, nectarine, and cherry trees. Cytospora canker can girdle scaffold branches, reducing yields or even killing trees. The disease first appears in April or early May as an oozing, light amber to dark brown gum near the point of infection. Beneath the gum, the inner bark begins to collapse, leaving a depressed area on the surface. By the second year, this area develops into an elongated or elliptically shaped canker. Although the bark dries out, it usually remains intact during the first year. In later years, the bark becomes broken, malformed, and covered with a black fungal overgrowth.

To avoid cytospora canker, plant cold-hardy cultivars, fertilize only in the early spring, and follow proper pruning procedures. Do not apply excessive nitrogen, and do a good job of controlling other stressors, especially brown rot and insects such as the peach tree borer and the lesser peach tree borer. Trees already stressed are most likely to suffer from cytospora infection.

Cedar-apple rust. This fungus can defoliate apple trees and blemish fruit. It requires two hosts to complete its life cycle—an apple tree and an eastern red cedar tree (Juniperus virginiana). It survives the winter in spherical galls

Remove and destroy diseased fruit to reduce brown rot.

on cedar trees. Spring rains promote the growth of hornlike structures that extrude from the galls. These structures release spores that travel by wind to apple trees and cause orange pustules on the upper surfaces of the leaves. One to two months after the appearance of these pustules, the fungal rust produces other structures on the undersides of the leaves or on the fruit. Fruit becomes infected during moist conditions when the temperature ranges between 46 degrees F and 75 degrees F. Spores produced on the leaves and fruit are released into the air during the dry conditions of summer and infect the leaves of cedar trees, completing the cycle.

Control strategies include applying fungicides, removing nearby red cedars, and using resistant cultivars. Very resistant apple cultivars include Delicious, Liberty, Nova Easygro, Novamac, and Tydeman. Susceptible cultivars are Golden Delicious, Jonathan, Lodi, Prima, Rome, Twenty Ounce, and York.

Apple maggot. This is the most destructive of all the insects that attack apples. The adult flies are slightly smaller than houseflies. They emerge from the soil between mid-June and mid-August and feed for about a week. Then the females lay eggs under the skins of apples. After hatching, the maggots bore through the fruit. In heavy infestations, many larvae can be found in a single fruit. Picking up and destroying fallen fruit once a week from early August through harvest reduces the potential for maggot infestations the following year. Apple maggot emergence and activity can be monitored by hanging red ball sticky traps.

Cherry fruit fly. This pest is closely related to the apple maggot and has a similar life cycle, but it attacks only sweet and tart cherries. Adults emerge for about a month beginning in early June. Use red ball sticky traps to monitor adult activity.

Plum curculio. This small, 1/4-inch-long weevil attacks all tree fruits. Adults overwinter in hedgerows or other sheltered areas and emerge in the spring when newly formed fruit is exposed—around petal fall in apples and shuck split in stone fruits. The most serious damage occurs when females deposit eggs in the fruit, causing small crescent-shaped scars. The larvae bore to the center of the fruit to feed. Infested fruits often drop to the ground in June. Picking up and destroying all drops in early June helps reduce developing larvae. If your planting is near woodland or other areas that provide good shelter for overwintering adults, infestations can destroy the entire crop. Chemical control measures are usually directed at the adults during the three weeks after bloom.

Apple tree borers. These pests are often found in home fruit plantings, especially in the trunks of young, unsprayed trees. Roundheaded apple tree borers are a particular problem. Adults feed on the fruit, bark, and foliage, but larvae cause the most significant damage. Females make a longitudinal slit in the bark of trunks and insert an egg. When the egg hatches, the larva bores into the sapwood and moves along the trunk, developing and enlarg ing tunnels. These tunnels weaken the tree structurally, cut off the flow of sap, and create a wound site that predisposes the tree to diseases and other insects. The downy woodpecker is the only known natural enemy of the roundheaded apple tree borer.

Pear psylla. This is the most common insect pest of pear trees. Adults resemble tiny cicadas. The nymphs secrete a sticky exudate called honeydew, which supports the growth of an unsightly black, sooty mold that soils leaves and fruit. If not controlled, psylla can cause early defoliation and crop loss.

Peach tree borers. These pests feed on the inner bark of peach trees and other stone fruits, girdling the conductive tissue. The two species, the peach tree borer and the lesser peach tree borer, are both clear-winged moths that lay eggs on the bark of the trees. The peach tree borer deposits its eggs at the base of the trunk and can kill the entire tree. The lesser peach tree borer attacks individual branches. Look for a gummy exudate mixed with a sawdust-like material excreted from small holes in the trunk. Gum secretions also can be caused by other injuries to the tree. To ensure positive identification of borers, cut away the bark and look for the larvae in their burrows. Avoid borers by painting tree trunks with a mixture of one part white latex paint and one part water.

San Jose scale. This insect pest infests apple, pear, plum, sweet cherry, apricot, peach, and nectarine trees. It sucks sap from all parts of the tree, including the fruit, causing the tree to decline in vigor. Badly infested areas on the bark appear ashy gray with encrusted scales. Fruit spotting can occur, particularly on apples. Females are about the size and shape of a pinhead and do not look like insects. To control scale insects, prune infested branches and suckers.

Codling moth. The larva of this moth is a worm found commonly in homegrown apples. As an adult, this iridescent gray moth deposits eggs on leaves and fruit. The eggs hatch about 6 to 20 days later, usually after the petals have fallen. Although other generations may occur during the season, it is most important to control this first generation, especially the adults before they lay eggs and the larvae that hatch from eggs deposited on fruit and foliage. If you spray trees for plum curculio and apple maggot, codling moths probably won't be a problem.

  1. These tiny insects—often called plant lice because of their small size and large populations—cause apple leaves to twist and roll. Two species are destructive—the rosy apple aphid and the green apple aphid. The rosy apple aphid damages fruit early and causes the fruit of certain apple culti-vars to become dwarfed and deformed. Both aphids secrete honeydew, and sooty spots may develop on fruit and foliage. Green apple aphids should not require chemical control in home orchards.
  2. While technically not insects, these tiny pests can bronze or discolor the leaves of many fruit trees. The European red mite and the two-spotted mite are common in the Northeast and thrive in hot, dry weather. Both reside on the undersides of leaves and cause damage by sucking plant juices. Natural enemies usually keep populations low.

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