Several species of aphids infest crabapple. The aphids do little damage in small numbers, but they are extremely prolific, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from crabapple leaves. The aphid is unable to fully digest all the sugar in the sap, and it excretes the sugary material in a fluid called honeydew. The honeydew often drops onto the leaves below. Plants or objects beneath the tree may also be coated with honeydew. A sooty mold fungus may develop on the honeydew, causing the coated leaves and plants to appear black and dirty.
Spray with an insecticide labeled for aphids, following label directions.
This plant disease is caused by a fungus (Venturia inaequalis). It is a serious problem on crabapples and apples in areas where spring weather is humid with temperatures ranging from 60° to 70°F. The fungus spends the winter in infected fallen leaves. In the spring, spore-producing structures in the dead leaves continuously discharge spores into the air. The spores are blown by the wind to new leaves and flower buds. If water is on the tissue surface, the fungus infects the tissue and a spot develops. More spores are produced from these spots and from twig infections from the previous year. The spores are splashed by the rain to infect new leaf and fruit surfaces. As temperatures increase during the summer, the fungus becomes less active.
Rake up and destroy infected leaves and fruit in the fall. When planting new trees, use resistant varieties. Apply a fungicide labeled for apple scab, following label directions.
This plant disease is caused by a fungus called Gymnosporangium juniperi-virginianae that affects both crabapples and certain species of juniper and red cedar. This disease cannot spread from crabapple to crabapple, or from juniper to juniper, but must alternate between the two. In the spring, spores from brown and orange galls on juniper or cedar are blown up to 3 miles to crabapple trees. During mild, wet weather, the spores germinate and infect the leaves and fruit, causing spotting and premature leaf and fruit drop. During the summer, spores are produced in small cups on the undersides of leaves. These spores are blown back to junipers and cedars, causing new infections.
Cedar-apple rust cannot be controlled on the current season''s foliage and fruit. The following spring, spray trees with a fungicide labeled for this disease, following label instructions. If possible, do not plant crabapples within several hundred yards of junipers or red cedar. Infestation can take place from junipers as far away as 3 miles.
This disease is caused by a bacterium (Erwinia amylovora) that is very destructive to many trees and shrubs. The bacteria spend the winter in the sunken areas (cankers) on the branches. In the spring, the bacteria ooze out of the cankers. Insects that are attracted to this ooze become smeared with it, and when the insects visit a flower for nectar, they infect it with the bacteria. The bacteria spread rapidly through the plant tissue in warm (65°F or higher), humid weather. Insects visiting these infected blossoms later carry bacteria-laden nectar to healthy blossoms. Rain, wind, and tools may also spread the bacteria. Tender or damaged leaves may be infected in midsummer.
Prune out infected branches 12 to 15 inches beyond any visible discoloration and destroy them. Disinfect pruning shears after each cut. Avoid overfertilization, which makes lush growth more susceptible to fire blight.
Several different types of scales infest crabapples. They lay their eggs on leaves or bark, and in spring to midsummer the young scales, called crawlers, settle on leaves and twigs. The small (1/10 inch), soft-bodied young feed by inserting their mouthparts and sucking sap from the plant. The legs usually atrophy, and a hard crusty or waxy shell develops over the body. Mature female scales lay their eggs underneath their shells. Some species of scales that infest crabapples are unable to digest fully all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew, which often drops onto the leaves below. A sooty mold fungus may develop on the honeydew, causing the crabapple leaves to appear black and dirty. An uncontrolled infestation of scales may kill the plant after two or three seasons.
An uncontrolled infestation of scales may kill the plant after 2 or 3 seasons. Spray with an insecticide labeled for these pests, following label directions.
Malacosoma species Tent caterpillars feed on many ornamental and fruit trees in the garden. The insects are found in nearly all parts of the United States. In summer, tent caterpillars lay masses of 150 to 300 eggs in bands around twigs. The eggs hatch in early spring when leaves are beginning to unfold; the young caterpillars immediately begin to construct the webs. On warm, sunny days, they devour the surrounding foliage and may strip trees in just a few days. The caterpillars feed for 4 to 6 weeks and then pupate. In mid- to late summer, brownish or reddish moths emerge and lay the overwintering eggs.
Spray with an insecticide labeled for these pests, following label directions. Cut out and destroy large webs. To prevent damage the following year, destroy the brown egg masses that encircle the twigs during the winter.