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Plant NameTypeDescription 
Powdery mildewCrabappleDiseasePowdery mildew of crabapple is typically seen on the foliage, but may also occur on twigs, blossoms, and fruit. Characteristic patches of white, powdery fungus develop on the leaves and terminal shoots. Infected young leaves often become deformed and curled. Infected shoot tips may be deformed or die back. Small black fungal fruiting bodies may develop in the white areas. Symptoms of fruit infection include white powdery growth and russeting (development of brown or rust-colored, corky patches on the fruit). The fungus may overwinter in infected buds.
ScabCrabappleDiseaseScab of crabapples is caused by the same fungus which causes scab on apple and hawthorn. The disease infects during wet weather in the spring and initially causes small, pale, watersoaked spots on the leaves. The spots enlarge and darken, first to olive-green then to black. Leaves may become distorted and often drop, sometimes resulting in severe defoliation of susceptible trees. Scab can also affect fruit, with pinhead spots enlarging to velvety olive then black blotches. The disease is most favored by cool, wet conditions and overwinters in infected plant debris.
Stem rotCrabappleDiseaseStem rot of crabapple is caused by a soil-borne fungus. This disease is most prevalent in the winter on nursery stock. The fungus infects through wounds on the stem where the bark is broken. Cankers develop on the stems of young plants and near the base of older plants. Active cankers are orange to purple-brown with a darker margin. Older cankers are brown, slightly sunken, and may develop cracks. The tissues beneath the canker may be discolored. Twigs and branches above the canker may show yellowing leaves or die back. Cankers can grow to 8" in length and may expand to completely girdle the stem.
AphidsCrabappleInsectAphids on crabapples may be green or reddish in color. They typically feed near the tip of growing shoots, sometimes deforming leaves and stunting terminal growth. Fruit may also be deformed. These soft-bodied insects are approximately 1/8" long and often produce honeydew, a sweet sticky material. The honeydew may develop a growth of black sooty mold, which is an aesthetic problem but seldom harms the plant.
Apple-and-thorn skeletonizerCrabappleInsectThe adult of the apple-and-thorn skeletonizer is a small dark-brown moth. It overwinters as a pupa or an adult, with the females laying eggs in the spring. The caterpillars are yellow-green in color, have black spots and brown heads, and feed on the leaves of several plants including crabapple, apple, pear, cherry, and hawthorn. Characteristic damage includes skeletonized leaves, or leaves that are rolled into a cone and tied with webbing. Damaged leaves turn brown and drop prematurely.
Leafrollers and leaftiersCrabappleInsectSeveral species of caterpillars will roll and tie leaves of fruit trees. Characteristic signs of feeding include leaves that are webbed, rolled, and tied, often with frass in the webbing. Damaged leaves often turn brown and may drop prematurely. The larvae often group together to make nest-like structures of chewed leaves and webbing. Leafrolling and leaftying caterpillars are usually about 1-2" long at maturity.
San Jose scaleCrabappleInsectSan Jose scale is an armored (hard) scale found on many deciduous trees and shrubs including (but not limited to) apple, cherry, elm, maple, poplar, and willow. The scale insects are about 1/16" in diameter. The female is gray with a yellow spot in the center. The crawlers are yellow and are easily spread by wind, birds, or people. San Jose scale may be found on twigs, branches, leaves, and fruit. Heavily infested trees may wilt and appear water-stressed. Severe infestations may cause twigs and branches to die back. The scale overwinters as black immature scales.
Spider mitesCrabappleInsectSpider mites on crabapple vary in color. They may be yellow, green, red or brown. They have eight legs and are very small, appearing as moving dots when viewed through a hand lens. Spider mites often feed on the underside of leaves, causing yellowish to bronze stippling or speckling. Fine webbing may be present on leaves or in branch crotches. Hot, dry, dusty conditions are especially favorable for development of severe infestations.