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Plant NameTypeDescription 
Fire blightHawthornDiseaseFire blight is a bacterial infection of shoots which enters the plant through blossoms, vigorously growing shoot tips, young leaves, and wounds. Blossom clusters appear blighted. Shoots suddenly wilt, turn black and die back, presenting a "scorched" appearance. Purplish cankers may develop on the shoots. During wet or warm weather, the cankers may ooze brown sticky droplets. Newly infected wood is reddish, while older infections are black. The bacteria are easily spread by rain and pollinating insects. Fire blight also affects pear, pyracantha, apple, and related species. It is not a proven problem in western Washington.
Leaf spotHawthornDiseaseLeaf spot is a fungal disease which can infect hawthorn, photinia, pear, pyracantha, crabapple, and related species. Initial infections are small reddish-brown to grayish spots on the upper or lower leaf surface. The spots may enlarge and coalesce on very susceptible species. On hawthorn, the spots are followed by yellowing and premature drop of the affected leaves. A "green island" effect can arise when the leaf turns yellow, as the areas around the fungal lesions may remain green. The disease is favored by wet weather and is easily spread from infected tissues by rain and splashing water. It overwinters mainly on fallen infected leaves. Succulent new growth is particularly susceptible to infection.
RustHawthornDiseaseRust is a fungal disease typically infecting leaves of hawthorn (occasionally also apple and crabapple). It may also be found on green stems and fruit. Initial symptoms are the presence of yellow to orange spots or pustules on the leaves. Severe infections may cause leaves to turn yellow and drop. Rust on stems and fruit causes these parts to be deformed. Mature sporulating rust infections are characterized by hairlike projections which bear the fungal spores. The alternate hosts for hawthorn rust are junipers, which develop swollen galls along the twigs. The juniper galls release orange spores in the spring.
ScabHawthornDiseaseScab of hawthorn is caused by the same fungus that causes scab on apple and crabapple. 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 a velvety 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 to black blotches. The disease is most favored by cool, wet conditions and overwinters in infected plant debris.
Apple-and-thorn skeletonizerHawthornInsectThe adult of the apple-and-thorn skeletonizer is a small dark-brown moth. It overwinters as an adult, laying eggs in the spring. The caterpillars are yellow-green in color with black spots and brown heads. They feed on the leaves of several plants including crabapple, apple, pear, cherry, and willow. Characteristic damage includes skeletonized leaves, or leaves that are rolled and tied with webbing. The feeding caterpillars produce large amounts of webbing and small dark frass pellets (excrement).
Hawthorn aphidHawthornInsectHawthorn aphids are yellow-green to pinkish, soft-bodied insects typically found on the leaves in the spring. Aphid feeding causes young leaves to curl tightly and turn purplish. Older leaves are crinkled and deformed. Feeding aphids produce large amounts of honeydew, a sweet, sticky material that may attract ants or become covered with a dark growth of sooty mold.
Pear slugHawthornInsectPear slugs, or pear sawflies, are insect larvae which resemble a small greenish or black slug. They are typically 1/4" to 1/2" long, tadpole-shaped. and produce large amounts of slime. The adult sawfly is a small, dark, wasp-like insect about 1/4" long. The larvae feed on the leaves of cherry, plum, and pear trees, as well as hawthorn. Leaves are typically skeletonized, with only the veins and lower leaf surface remaining. Heavily damaged leaves may drop from the tree. The pear slug typically has two generations per season and can be found on trees from mid- to late spring and again in late summer.
Scale insectsHawthornInsectSeveral species of scales may be found on hawthorn. In general, scales are approximately 1/16" to 1/8" in diameter, dull or shiny, and grayish to dark in color. They are typically found on the bark of small twigs and sometimes on the lower surfaces of leaves. They are flattened, circular to oval or oyster-shaped in outline, and may be hard-shelled or relatively soft. The immature scales (crawlers) are usually yellowish to brown and soft-bodied. They may be found on leaves or twigs and are typically present in early summer, depending on species. Scale-infested plants may show signs of wilt, leaf yellowing, and/or premature leaf drop. Heavily infested branches may be killed. Honeydew, a sweet, sticky material, may be present as a result of scale feeding and may become covered with a growth of black sooty mold.
Spider mitesHawthornInsectSpider mites are tiny, eight-legged and variably colored from yellow or green to red or brown. When viewed through a hand lens, they appear as tiny moving dots. Spider mites typically feed on the underside of leaves, causing yellowish to bronze stippling or speckling. Severely infested leaves may turn yellow and drop from the tree. Fine webbing may be present on leaves or in branch crotches. Hot, dry, dusty conditions are especially favorable for development of severe spider mite infestations.