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Plant Pathology


Apple Aphids

Generally three species of aphids - the green apple aphid, rosy apple aphid, and apple-grain aphid attack apple foliage in Kentucky. These are very similar in appearance. However, it is the rosy apple aphid which causes the most severe damage and is the most difficult of the three to control. While large numbers of any type of aphid can stunt new growth and cause sooty mold to develop on fruit and leaves, the rosy apple aphid injects a toxin with its saliva that causes the leaf to curl and the fruit to be distorted. Often these damaged leaves turn bright crimson in color. Relatively low numbers of rosy apple aphids can cause considerable damage.

Rosy Apple Aphid

Apple aphids are small pear to tear-drop shaped insects. Color varies from purple to rosy to light green. Generally a pair of projections (cornicles) will be present on the fifth or sixth segment. Mouthparts are piercing-sucking. The rosy apple aphid overwinters on apple trees as eggs laid on twigs, bud axils, or in bark crevices. The black eggs are 1/2 mm long and football shaped. Shortly after silver tip the eggs hatch. The nymphs color changes from dark green to purple as they grow. The overwintering eggs give rise to only female aphids which give birth to live young. The aphids continue to reproduce on apple until summer, then winged forms are produced which migrate to other hosts such as dock and narrow-leaved plantain to spend the summer. In the late fall, winged forms migrate back to apples and lay eggs in bark crevices and on twigs.

Wooly Apple Aphid

Unlike the other aphids, the woolly apple aphid (WAA) feeds on all parts of the tree including the roots. It differs from other apple aphids in appearance, life cycle and the type of damage inflicted. A colony of WAA's will appear as a cottony mass generally clustered in wounds of the trunk and branches of the tree. The aphids themselves will be purplish in color.

The life cycle of WAA is very complex. The winter may be passed in two forms, the egg or immature nymph. Nymphs will hibernate underground on roots of apple. Wherever apples and elms are close together, overwintering eggs will be deposited in cracks or protected places on the elm. During spring, eggs will hatch into wingless nymphs which feed on elm buds and leaves. In early June, a winged form is produced which migrates to apple and other hosts. These individuals will feed on wounds in the branches and trunk, and many work their way down to the roots, and trunk below ground surface. It is the feeding on the roots that produces the greatest damage. Rootstocks vary in susceptibility to WAA, use M111 or M106 if WAA is a serious problem. Whereas, Bud 9, M9, M26, and P series rootstocks are very sensitive to woolly apple aphid damage particularly prior to bulk root formation.

During the summer, repeated WAA generations of wingless individuals will be produced. In the fall, winged individuals are produced which fly back to elm and lay overwintering eggs, while some wingless forms will remain on the apple on both above and below ground parts throughout the winter.

SCOUT: Symptoms of aphid damage include curled and crinkled leaves, especially on new growth.

RECORD: The total number of aphid infestations per 100 leaf terminals, fruit clusters or pruning scars (WAA) by species.

ACTION THRESHOLD: Rosy Apple Aphid - 5 infested leaf terminals or fruit clusters /100 leaf terminals or fruit clusters; Apple-Grain & Green Apple Aphid - 50 infested leaf terminals or fruit clusters /100 leaf terminals or fruit clusters; Woolly Apple Aphid - 50 colonies/100 pruning scars.

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Apple IPM web site created by Kerry Kirk - 2001 - Send questions or comments to Patty Lucas