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Plant NameTypeDescription 
Fire blightMountain AshDiseaseFire 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.
Nectria cankerMountain AshDiseaseNectria canker is a fungal disease often found on twigs and branches that have been weakened by drought, frost damage, insect damage, or other diseases. Cankers are initially distinguished by discolored bark with coral or reddish fungal fruiting bodies. The cankers are sunken and often associated with wounds. Older cankers develop concentric, target-like rings of wood. Affected branches may have stunted or wilted leaves, or fail to produce leaves in the spring. Branches may be girdled.
AphidsMountain AshInsectAphids are small, soft-bodied insects that typically feed near the tip of growing shoots. Their feeding may deform leaves (curling, distortion) and stunt terminal growth. Aphids range in size up to 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. It may also attract honeydew-feeding ants.
Ash borerMountain AshInsectThe ash borer, also known as the lilac borer, is a member of the clearwing moth family. The adults of many clearwing moths mimic yellowjackets or paper wasps. The larvae of this moth cause damage to ash, lilac, and privet. The larvae tunnel in the bark of stems (trunk) and branches causing a gradual weakening of the plant. Other than emergence holes, one can determine the pests' presence by the pupal skeletons attached to the bark at the emergence sites. There is usually only one generation per year. However, a two-year cycle can occur in the northern part of its range. Adult numbers usually peak in May or June and females lay their eggs singly in bark crevices where they soon hatch and burrow their way into the phloem tissue and continue feeding. Emerging full-grown larvae produce a round hole about 1/16 inch in diameter. Urban, open grown trees are highly susceptible to infestation.
Leaf blister miteMountain AshInsectThe leaf blister mite is a very tiny white, sausage-shaped eriophyid mite. They are seldom visible to the unaided eye. Eriophyid mite feeding on the underside of mountain ash leaves causes development of blister-like spots on the upper leaf surface. These raised spots are generally light green, round and somewhat wrinkled in appearance. Older leaf blisters may turn brown. Severe infestations may result in premature leaf drop.
Mountain ash sawflyMountain AshInsectMountain ash sawfly is a relatively new pest to western Washington. First reported in 2009, the larval stage of this insect feeds on leaves of both European and American mountain ash (Sorbus aucuparia and S. americana). Adult sawflies are a stout, thick-bodied wasp. The females lay eggs in slits cut along leaf margins. Caterpillar-like larvae emerge in early spring and feed in groups or rows along the leaf margins, often with their rear ends curled. These voracious feeders skeletonize leaves, leaving only the stem, midrib, and occasionally some larger veins. Young larvae are greenish with black spots and black heads. As they mature over a period of 3-4 weeks, the larvae reach about 5/8 to 3/4 inch long and turn yellow-orange with black spots. They drop from the plant to the ground, where they pupate in soil or leaf litter. A second generation of larvae begins to feed in August. It is believed that a third generation may occur in some years. Damage is first seen on lower branches and may seem to appear overnight. Early detection is important to limit defoliation. While mountain ash sawfly is considered to be mainly an aesthetic problem, repeated severe defoliation may injure trees.