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Plant NameTypeDescription 
Bacterial twig blightWillowDiseaseBacterial twig blight of willow is caused by the same bacterium that causes bacterial blight of lilac and other woody ornamentals. The disease attacks twigs and branches, causing them to die back. Leaves on affected branches turn brown, wilt, and drop, sometimes resulting in severe defoliation. Young shoots are often girdled. Affected branches may show brown streaks in the wood when cut. The bacterium overwinters in the twig and branch cankers and reinfects new leaves in the spring.
Marssonina leaf & twig spotWillowDiseaseMarssonina leaf and twig spot is a fungal infection of twigs and leaves. Initially, small brown to black spots develop on leaves and green twigs. The spots may have white centers. On twigs, the spots may expand to form large girdling lesions which cause twig dieback. The fungus probably overwinters on infected and dead twigs and in fallen leaf debris. This disease rarely does severe damage, although it can be an aesthetic problem.
RustWillowDiseaseWillow rusts are fungal diseases that begin in early summer, but are most noticed in late summer to early fall. Leaves first display yellow spots, which turn dark brown or black by late summer or early fall. Severe infections cause leaves to drop, sometimes resulting in considerable defoliation. The fungus overwinters on dead leaves and infected twigs or on alternate hosts. Alternate hosts for willow rusts include fir, larch, and Ribes species such as currants and gooseberries, depending on the rust species. Rust infections on willows are not typically a serious problem, although growth of young trees may be somewhat slowed by defoliation.
Twig blight (Venturia)WillowDiseaseAlso known as scab, willow twig blight infects leaves and twigs. The fungus first infects new foliage in the spring, causing brown to dark spots along the midrib of the leaves. The young leaves become scorched in appearance, wither, and drop, resulting in minor to severe defoliation (particularly near branch tips). The infection then grows into the twigs, where symptoms include small brown to black cankers which result in girdling and dieback of twigs and branches. Infected leaves and twigs may display a velvety, olive-brown fungal growth, especially along the leaf midveins. Older leaves and twigs are less susceptible to infection. The disease is apparently spread by splashing water and is favored by wet weather.
CarpenterwormWillowInsectCarpenterworms are the larvae of a mottled gray and black moth. The caterpillars are white or pinkish-red with a dark head and 1" to 3" long when mature. They bore in the trunk and main branches of willow, poplar, elm, locust, and other deciduous trees. Discolored or bleeding limbs, branch dieback, and irregular and gnarled trunks are indications of carpenterworm infestation. Sawdust-like material (frass) and wood chips mixed with loose webbing on or around infested trees is typical. Adults emerge from infested trees in early summer. Trees with severe carpenterworm infestations are susceptible to wind breakage and may need to be removed.
Poplar-and-willow borerWillowInsectThe adult poplar-and-willow borer is a black and white weevil (a flightless beetle) about 3/8" long. The larvae are white and legless and mine beneath the bark and in trunk and branches. Feeding larvae expel large quantities of frass (sawdust-like excrement) through holes they keep open in the bark. The feeding larvae may girdle branches, resulting in leaf wilt and eventual branch death. When mature, larvae are approximately 1/4" long. Severe infestations cause swollen, lumpy bark, bark scars with exposed wood, and sometimes considerable production of shoots from old wood. Willows are preferred, but the borer will also attack poplar species.
Satin mothWillowInsectThe caterpillars of the satin moth feed on leaves of willows and poplars. They may also occasionally attack oak and aspen. The adult moth is satiny and pure white, with a wingspan of 1 1/2" to 2". The caterpillars are voracious feeders, sometimes causing severe defoliation of trees. Mature caterpillars are approximately 2" long. They are black, with red and white patches and tufts of hairs along the sides.
Spiny elm caterpillarWillowInsectThe spiny elm caterpillar is the larval stage of the mourningcloak butterfly. The adult is approximately 2 1/2" across, with purplish-brown to blackish wings bordered by a creamy yellow stripe and a row of blue spots. The larvae are purplish-black with white specks and have a row of orange to red spots along the back. They also have long forked spines on their bodies. The caterpillars often feed in large groups and eat all leaves on a branch before moving. Spiny elm caterpillars also feed on elm and poplar trees.