A good fruit tree should not make a good shade tree. However, when pruning is neglected, many apples and pears become better shade producers than fruit producers. Standard-sized trees often outgrow the reach of ladders or pruning hooks. Backyard and commercial growers have come to prefer dwarf or semi-dwarf trees which are not as tall and are easier to prune, spray, and harvest without the use of ladders.
A neglected but otherwise healthy tree will usually show a marked improvement in fruit quality as a result of pruning. Fruit buds begin developing in the growing season previous to the one in which they mature into fruit, and more are initiated than can be fully developed into fruit. Growing conditions during the season of bud initiation and the subsequent winter will affect the number of buds which flower, and certain cultivars are "alternate bearers" that seldom initiate many buds during a year with a heavy fruit crop. In any case, by late winter the buds for the coming summer's crop will be very evident. Buds only appear on two or three year-old twigs or spurs which are no thicker than a pencil.
The primary purpose of pruning is to increase sunlight penetration, remove less productive wood, and shape the crown into an efficient, stable form. If left unpruned, the quantity of fruit produced might be greater, but the quality much lower. Pruning increases fruit size, promotes uniform ripening, increases sugar content, and decreases disease and insect problems by allowing better spray coverage and faster drying following rainfall. It also allows easier access for timely harvesting.
White Apple Leafhopper.
The white apple leafhopper (Typhlocyba pomaria McAtee; Homoptera: Cicadellidae) is a native insect that is widely distributed throughout the apple growing regions of the United States and Canada. Several leafhopper species are found in abandoned apple orchards, but the white apple leafhopper is the species that predominates in commercial plantings. While apple is probably the only host on which this leafhopper overwinters, it may also infest peach, plum, cherry, and hawthorn during the growing season.
The white apple leafhopper is a leaf feeder and does not directly attack the fruit. Leafhopper nymphs and adults insert their piercing/sucking mouthparts into plant cells and remove the contents. As sap is sucked from the leaves, green tissue is destroyed causing foliage to become speckled or mottled with white spots. Heavy feeding may cause the entire tree to appear white or silver. Damaged foliage interferes with photosynthesis and reduces plant vigor resulting in smaller fruit size, poor fruit color, decreased bud formation, and fruit drop.
In addition to direct leaf injury, these leafhoppers excrete resin-like material, in deposits called tarspots. Tarspots on the fruit substantially reduce quality and value. Most of the significant fruit spotting is associated with the second generation of white apple leafhopper.
Damage by white apple leafhopper is usually more prevalent in well-maintained orchards with succulent leaves. It is more common on Rhode Island Greening, McIntosh, and Red Delicious than on other apple varieties. White apple leafhopper outbreaks are favored by moderate drought conditions.
Description and Life Cycle
White apple leafhopper adults are creamy white to yellowish-green in color and about 3 to 4 mm (1/8 inch) long. They hold their wings in a roof-like position when resting and appear as tiny wedges when seen from above. Adults are active and fly readily when disturbed.
The white apple leafhopper overwinters in the egg stage under the thin bark of twigs that are approximately 1.25 cm (1/2 inch) in diameter. The eggs are less than 1 mm (1/25 inch) long, cylindrical with tapering ends, and creamy white in color. The egg-laying sites appear as elongate, oval, blister-like swellings about 1.5 mm (1/16 inch) in length, which characteristically run perpendicular to the terminal growth. Overwintering eggs are found on one- to five-year-old wood, but are most often present on two-year-old wood.
Overwintering eggs begin to hatch just before apple blossoms open and usually complete hatching by petal fall. The emerging nymphs move to the undersides of older leaves and feed almost exclusively on the same leaf throughout development. The nymphs, which are wingless, move little as they feed, but will run actively when disturbed.
White apple leafhopper has five nymphal stages. First and second stage nymphs are about 1.0 to 1.5 mm (1/16 inch) long, pale white, with dull red eyes. The third stage nymphs have dull white eyes and developing wing pads. The fourth and fifth stage nymphs are similar in appearance to third stage nymphs, but reach a length of 2.8 mm (1/8 inch). Later stages also become more yellowish or yellow-green in color. The nymphal stages are separated by molting periods when the exoskeleton is shed. These transparent cast "skins" frequently remain hanging from the underside of the leaf. Nymphs of the first generation are most abundant in May and early-June. They are found on cluster leaves close to the trunk or large limbs; they are not found on actively growing terminal shoots.
First generation adults begin to appear in early-June, with males emerging a few days before females. They mate early in the morning and lay eggs about two weeks later. Eggs that will hatch into second generation nymphs are laid in petioles, mid-ribs and large secondary veins on the undersides of leaves. Females lay eggs for about three weeks, each depositing up to 60 eggs. First generation adults gradually die off after five or six weeks and are not observed during mid- to late-July.
Second generation nymphs appear in early-August with adults appearing from mid- to late-August. Adults remain active throughout September but diminish rapidly in number by October. The overwintering eggs are laid under apple tree bark from mid-August to mid-October.
Contrast with Potato Leafhopper
The damage caused by white apple leafhopper feeding is different than that caused by the potato leafhopper, Empoasca fabae (Harris) on apple; feeding by potato leafhopper causes the tips of leaves to turn yellow and brown and to curl up. The bronzed, dried appearance of leaf tips is referred to as "hopper burn," and heavy infestations can result in stunted plant growth. While white apple leafhopper is found on cluster leaves and not on actively growing terminal shoots, potato leafhopper is more of a threat to young, non-bearing fruit trees and young, tender foliage.
Potato leafhopper is light green. White apple leafhopper can be distinguished from potato leafhopper by the tendency of white apple leafhopper to walk forward and backwards while potato leafhopper walks sideways as well as forwards and backwards. Potato leafhopper has a wide host range, including potatoes, beans, and alfalfa. Potato leafhopper develops throughout the year in the southern United States near the Gulf of Mexico, and migrates northward each growing season rather than overwintering in northern states. The appearance of potato leafhopper is therefore less predictable because its migration is dependent upon the jet stream and weather patterns. White apple leafhopper overwinters in northern areas.
Monitoring and Action Threshold
The need for control of white apple leafhopper should be determined at petal-fall, when wingless nymphs of the first generation can be found. The presence of white stippling on the upper surface of leaves indicates that nymphs are feeding beneath. Leafhoppers should be sampled from the underside of leaves, especially on suckers or older terminal growth. When monitoring the second generation, keep in mind that watersprouts often have heavier populations of leafhoppers than other areas of the tree. Count the number of leafhoppers on ten leaves from each of ten trees, and calculate the average number of leafhoppers per leaf. Treatment is suggested if there is an average of more than 0.5 leafhopper nymphs per leaf.
Natural enemies include parasitic wasps that attack leafhopper eggs; predators such as spiders, lacewings, and minute pirate bugs; and a fungus. In commercial orchards, natural enemies usually do not adequately control the white apple leafhopper.
The insecticides most effective in the control of leafhoppers are methomyl (Lannate*) and formetanate (Carzol*). Dimethoate (Cygon) and endosulfan (Thiodan*) are also effective. Carbaryl (Sevin) and oxamyl (Vydate*) are very effective but will cause fruit thinning if used within 30 days of bloom; if they are used, it is best during very early petal-fall rather than late petal-fall for minimal thinning. Be careful with emulsifiable concentrate (EC) formulations used near petal-fall as they tend to increase the potential for fruit russetting. Note that insecticides commonly used at petal-fall for control of plum curculio and codling moth, such as phosmet (Imidan) and azinphos-methyl* (Guthion) are not effective in controlling leafhoppers. Good control of the first generation should help suppress second generation leafhoppers so that additional control is not needed.
Young leafhoppers are easier to control than adults. The first brood is a better target than the second brood because the hatch is relatively synchronous, making a greater proportion of the more susceptible nymphs present at one time. Thorough spray coverage of upper and lower leaf surfaces is necessary and considered essential for effective control with contact insecticides.
* indicates a restricted-use material.
NOTE:Disclaimer - This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and the Ohio State University Extension assume no liability resulting from the use of these recommendations.