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Plant NameTypeDescription 
Armillaria root rotPlum, Prune (Fresh)DiseaseArmillaria root rot is a fungal disease often found in newly cleared soils or soils which have been flooded. Aboveground symptoms typically include production of smaller-than-normal leaves, leaf yellowing, premature leaf drop, and branch dieback, often on only a portion of the tree. The fungus may be found as white, felt-like masses beneath the bark at the crown, as honey-colored mushrooms at the base of infected trees in the fall, and/or as shoestring-like rhizomorphs, which are dark strands of the fungus growing on or just beneath the soil surface. Infected trees may also exhibit a dark black line in the infected area encircling the base of the plant. Trees damaged by human activity such as construction or improper irrigation are more susceptible.
Bacterial cankerPlum, Prune (Fresh)DiseaseBacterial canker can infect twigs, branches, fruit, or the trunk. Elongate, dark, purplish cankers develop during early spring, often producing bacterial ooze in wet weather. The infected tissues often produce gum, although gumming is also caused by other factors. The cankers can girdle twigs and branches causing dieback. Leaves on girdled twigs typically yellow and fall by late summer. The bacteria typically infect via wounds caused by disease, insect damage, pruning, or frost injury. Infection can be spread by wind, rain, insects, pruning tools, or by planting or grafting with infected stock. The disease may spread throughout the entire tree (systemic infection) with or without visible symptoms.
Black knotPlum, Prune (Fresh)DiseaseBlack knot is a fungal disease that infects cherry and plum. Early infections of twigs are elongated olive-green, corky knots. They turn black and hard as they mature. Every year they expand lengthwise. It is possible for the fungus to stunt and kill limbs as it grows around the twig or branch.
Brown rotPlum, Prune (Fresh)DiseaseBrown rot is a blossom-infecting fungal disease. Infected flowers first appear water-soaked, then wilt and die. The light brown blossoms remain attached to the twigs, becoming covered with a grayish-brown fungal growth during wet weather. Blossom or fruit infections may spread to spurs. Infected twigs develop sunken, elongate cankers with gumming at the margins. Leaves on girdled shoots turn brown and remain attached. Infected fruit initially show a small brown spot which rapidly enlarges. The fruit remains fairly firm and often becomes covered with gray-brown fungus. The rot may also occur in stored fruit. Fruit may dry and harden into mummies, which serve as a source of infection in the spring, either on the tree or on the ground.
Crown gallPlum, Prune (Fresh)DiseaseCrown gall is caused by a soilborne bacterium. The bacteria infect through wounds on the crown and roots. Young galls are fleshy, white, enlarged masses on the roots or stems. Older galls are hardened and turn dark brown and woody or corky in appearance. They range in size from less than an inch to several inches across. The bacteria can be spread from infected to clean soil by water movement. Damage varies with location and size of galls. Small galls are essentially harmless. Large galls on the crown may weaken or girdle trees. The growths can also be an aesthetic concern.
Cytospora cankerPlum, Prune (Fresh)DiseaseThe fungi which cause Cytospora canker attack through wounds on twigs and branches. Initial cankers are small, but enlarge quickly and may streak up and down the stems without girdling. The cankers may also girdle twigs, resulting in dieback above the infection site and causing "flags" of dead material to appear in the canopy. The leaves on the dead twigs turn color and droop, but often remain attached. The canker itself appears as a dark, sunken area of dead bark and wood. Amber gumming is often present. Pinhead-sized black fruiting structures of the fungi often erupt through the bark and produce reddish tendrils or droplets of spores in wet weather. Spores are easily spread by wind, rain, and insects. The cankers are often perennial, enlarging through several seasons.
Plum pocketsPlum, Prune (Fresh)DiseasePlum pockets is a fungal disease affecting fruit, twigs, and leaves. Infected fruit initially develop small, white spots which rapidly enlarge. The spots later turn reddish to velvety gray. The fruit become distorted, discolored, and much larger than normal. Pits may be absent in the spongy, bladderlike, dark brown fruit. The leathery skin may later become covered with a whitish coating of fungal spores. The fruit later turn dry and hard (mummified). Leaves and twigs show symptoms similar to those of peach leaf curl. Leaves and shoots may be discolored (reddish to yellow) and curled or twisted. The fungus can overwinter on twigs, bud scales, and infected fruit.
RussetingPlum, Prune (Fresh)DiseaseRusseting of plum can be caused by several factors. Rain during the period of full bloom, mechanical rubbing against other fruit or branches caused by wind, or other injury of the (russet scab) fruit surface can result in russet symptoms. High winds following full bloom and copper fungicide applications are other causes of russeting on French prunes. Affected fruit initially show shiny areas on the surface where normal wax is lacking. The shiny area develops into a brown, corky or scabby area just before fruit harvest. Affected fruit may be misshapen. Thrips may cause similar russeting symptoms on all stone fruits.
Shothole (Coryneum blight)Plum, Prune (Fresh)DiseaseCoryneum blight is a fungal disease. Lesions on leaves are initially small, purplish, round to oval areas which expand into brown spots with light centers. The lesions are typically up to 1/4" in diameter. The infected spots on leaves often die and drop out in warm weather, giving leaves the characteristic "shothole" appearance of the disease. The fungus probably overwinters on the bark and in infected buds. Spores are easily spread by water.
Silver leafPlum, Prune (Fresh)DiseaseSilver leaf is a fungal disease affecting cherry, apricot, plum, and other stone fruits. The fungus typically grows on dead wood, but it can infect living tissues through wounds and become systemic (throughout the plant). The leaves of affected branches turn silvery to ashy in color and the margins may curl slightly upwards. The branch may either die quickly or show symptoms for several seasons before dying. Affected branches have a dark staining in the heartwood. The fungus produces fruiting bodies only on dead wood. The shelf-like fruiting bodies push through the bark and are light brown to purple on the upper surface and pinkish to purple beneath. Trees are least susceptible to infection summer through fall in dry weather.
Virus diseasesPlum, Prune (Fresh)DiseaseVirus diseases in prunes and plums cause many variable symptoms depending on the virus strain and the cultivar affected. For instance, tomato ringspot virus can cause failure of the graft union, ultimately killing trees. Trees infected with a virus may also show various symptoms of decline, collapse, or breakage, which may be confused with root rots or other disease problems. Virus disease transmission is specific for each virus. Insects, nematodes, and grafting are possible modes of transmission.
AphidsPlum, Prune (Fresh)InsectSeveral aphid species can be found on plums, including the hop aphid and the leafcurl plum aphid. The hop aphid is yellowish to dark green. The leafcurl plum aphid is dark green to reddish in early spring and fall, and pale green in the summer. Aphids typically feed in colonies on the tips of new growth and the undersides of leaves. Infested leaves and shoots are curled and distorted. Feeding aphids produce large amounts of honeydew, a sweet, sticky material which may attract ants or become covered with a dark growth of sooty mold. Honeydew and sooty mold can reduce fruit quality.
Apple-and-thorn skeletonizerPlum, Prune (Fresh)InsectThe adult of the apple-and-thorn skeletonizer is a small dark-brown or reddish-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 apple, crabapple, pear, cherry, and hawthorn. Caterpillars are about 1/2" long at maturity. Characteristic damage includes skeletonized leaves, or leaves that are rolled into a cone and tied with webbing. Damaged leaves are brown and papery and drop prematurely.
Brown marmorated stink bugPlum, Prune (Fresh)InsectThe brown marmorated stink bug (BMSB) is an introduced pest species from Asia that is spreading quickly across the United States. Nymphs and adults feed on a wide variety of plant hosts. BMSB prefers to feed on fruit, seeds, and seed pods, but will also feed on stems and leaves of some hosts. Both adults and nymphs have piercing-sucking mouthparts and inject digestive enzymes into plant tissues to aid in feeding. Sunken areas and deformities (catfacing) on the surface of the fruit are typical symptoms. Damaged areas are discolored beneath the fruit’s skin and become hard and pithy or corky in texture. Stone fruits such as cherry, peach, plum, and apricot may show gumming at the injury site and from the corky areas beneath the surface. Other known tree fruit/nut hosts of BMSB include apple, pear, filbert, and stone fruits including apricot, cherry, and peach. One or two generations of BMSB per year are expected in the Pacific Northwest. Adults overwinter in sheltered locations (including houses, where they can become a significant nuisance pest). In the spring, light green to white eggs are laid in groups of about 20 to 30 on the underside of leaves. Young stink bugs, or nymphs, are black with a red-and-black striped abdomen. Nymphs often feed in groups when young. Older nymphs are dark with white bands on body, legs, and antennae. They may feed in groups or singly. Adults are a little over 1/2 inch long, with a shield-shaped body. Body color on adults is mottled gray and brown, while the legs and antennae have alternating dark and light bands. The abdomen also has dark and light bands which are visible at the edge of the wings. NOTE: BMSB adults closely resemble other stink bugs found in WA and OR. For more information on BMSB identification, see FS079E, Pest Watch: Brown Marmorated Stink Bug, available at https://pubs.wsu.edu/.
EarwigsPlum, Prune (Fresh)InsectEarwigs are reddish-brown insects about 3/4" or less in length. Both males and females have pincers at the rear end. Earwigs are largely beneficial, feeding on many pests such as aphids, mites, and nematodes, as well as on algae, fungi, and decaying plant material. However, earwigs can also damage plants. They sometimes feed on flowers, shoot tips, leaves, or fruit. Damaged shoot tips may fail to develop properly, sometimes stunting growth. Damaged leaves exhibit small to large holes. Fruit damage consists of shallow, irregular areas chewed into the surface.
Fruittree leafrollerPlum, Prune (Fresh)InsectThe larvae of the fruittree leafroller feed on leaves, flower parts, and sometimes young fruit. They are pale to dark green with a shiny black head and are about 3/4" long at maturity. Typical symptoms of leafroller feeding are leaves that are rolled and tied in place with webbing, but surface damage on the fruit is more common on prune, pear, and apricot. The caterpillars are active when disturbed, wiggling vigorously or dropping to the ground on a thread. Larvae are mature by the end of May, and adults may be found from early June through mid-August. The adult moth is 1/2" to 3/4" long and mottled tan and brown.
LeafhoppersPlum, Prune (Fresh)InsectLeafhoppers are typically found on the underside of leaves. Immature leafhoppers (nymphs) are usually less than 1/10" long and white to greenish or yellowish in color. Adults are white and about 1/8" long. Leafhoppers resemble aphids but are larger and more active. They feed by sucking plant juices, often causing damaged leaves to develop a white to yellow speckling or mottling. Severely damaged leaves may turn brown and shoots may curl and die back. Feeding leafhoppers produce honeydew, a sweet, sticky material which may attract ants or become covered with a dark growth of sooty mold. Leafhoppers rarely cause serious damage to plants, although very heavy infestations may result in premature leaf drop and small fruit.
Pacific flatheaded borerPlum, Prune (Fresh)InsectThe Pacific flatheaded borer typically attacks weakened, injured, or stressed trees. The larvae feed between the bark and the sapwood. They can weaken and girdle trees. Injured bark may show dark depressions on the surface or cracks through which the sawdust-like frass can be seen. The larvae are white or cream-colored with broad, flattened heads. They can reach up to 1" in length. The adult borer is a dark, coppery-brown beetle. The broad, flattish adults are about 1/4" to 1/2" long. Adult females lay eggs in bark crevices on the trunk below the lowest branches. The larvae burrow into the bark after hatching. The borer prefers to feed in and lay eggs on wood that is exposed to sunlight. This insect feeds on many species of trees and shrubs.
Peach twig borerPlum, Prune (Fresh)InsectThe peach twig borer attacks both shoots and fruit. In the spring, overwintering larvae bore down the center of shoots, causing the tip to wilt or "flag". Each caterpillar can damage several shoots. Later generations of larvae feed on both shoots and fruit. They often burrow into the stem end of young fruit. Pits of infested fruit are often split, with the larvae inside. They may also feed on the fruit surface. The caterpillars are reddish-brown with black heads and yellowish rings on the body. Mature caterpillars are up to 1/2" long. The adult moth is dark gray and about 1/3" long. Apricot, nectarine, and peach may also be attacked. The peach twig borer can be an important pest of peaches, particularly in eastern Washington.
Peachtree borerPlum, Prune (Fresh)InsectThe peachtree borer adult is a dark, bluish, clear-winged moth which somewhat resembles a wasp or hornet. The female, which has dark forewings and a red-orange band on the abdomen, lays eggs on the tree. Emerging larvae move to ground level or just below ground, where they burrow beneath the bark and feed in the crown region. Signs of larval feeding include the presence near ground level of jelly-like gum mixed with dirt and pellets of excrement. Heavy infestations can weaken older trees, and foliage may be yellowed as if nitrogen-deficient. Young trees may be girdled and killed.
Pear slugPlum, Prune (Fresh)InsectThe pear sawfly is also known as the pear slug because of its resemblance to a small, dark slug. These insects are the larval stage of a glossy, black sawfly about 1/5" long. The caterpillar-like larvae are covered with a dark green to black slime which gives them the slug-like appearance. The larvae are yellow immediately after molting until the slime is produced. Larvae are also yellow-orange immediately before pupating. Pear slugs feed on upper leaf surfaces, skeletonizing leaves. Severe infestations can cause defoliation, weaken trees, and affect fruit development. The pear slug prefers cherry (where it is called the cherry slug) and pear, but will also feed on plum.
Scale insectsPlum, Prune (Fresh)InsectScale insects found on prune and plum include the San Jose scale and the lecanium scale. San Jose scales are about 1/16" across and are found on twigs, branches, leaves, and fruit. The female is gray with a yellow spot in the center. The crawlers (immature) are yellow. Infested fruit develops sunken spots surrounded by reddish areas. Lecanium scales are shiny brown and found on twigs. The turtle-shaped adults are 1/8"-1/4" across with light markings, while the crawlers are more flat. Both scales produce large amounts of honeydew, a sweet, sticky material which may attract ants or become covered with dark sooty mold. Heavily infested branches may be wilted, yellowish, or show other signs of stress. New growth may be stunted or lacking. Twigs, branches or entire trees may be killed by repeated infestations.
Shothole borerPlum, Prune (Fresh)InsectShothole borers are small (1/10" or less), brown to black, stubby-nosed beetles. Adults feed at the base of leaves or twigs. Later, they bore into the bark and lay eggs along a narrow gallery paralleling the grain of the wood. The white, legless grubs (bark beetles) feed by boring between the bark and sapwood, making narrow tunnels filled with sawdust-like frass (excrement). Feeding larvae can weaken or girdle trees. Larval galleries are typically at right angles to the first gallery, with the grubs pupating at the end of the galleries. The emerging adults leave tiny round "shotholes" in the bark, giving the beetle its name. Shothole borers are especially attracted to injured, stressed, or dying trees, but can also attack healthy trees. Fruit, ornamental, and forest trees and shrubs can be attacked.
Spider mitesPlum, Prune (Fresh)InsectSeveral species of spider mites can be found on plums. They range in color from pale yellowish or green to red or brown. Spider mites feed on the leaves (typically on the underside), causing a whitish, yellow, or brown stippling. More severe damage to leaves results in a bronzed appearance. Heavy infestations may cause leaf drop. Spider mite feeding is usually accompanied by webbing on leaves and between leaves and twigs, although the European red mite produces smaller amounts of webbing than other species. Spider mite infestations are worse in hot, dry, dusty conditions.
Spotted wing Drosophila (SWD)Plum, Prune (Fresh)InsectSpotted wing Drosophila (SWD) resembles other Drosophila species (fruit flies or vinegar flies) in appearance, but unlike other members of the family which attack only overripe, damaged or decaying fruit, SWD attacks healthy fruit as it ripens on the plant. Adult SWD flies are about 1/8 inch long, with red eyes and a yellow-brown body. Darker bands may be visible on the abdomen. Male flies have a distinctive dark spot on the leading edge of the wing near the tip. SWD is the only fruit fly species in our area with this spotted wing, making identification of males relatively simple. Females lack the spotted wing, but have a large, sawlike egg-laying organ called an ovipositor at the tip of their abdomen. It is used to deposit eggs in fruit (oviposition). The eggs are laid beneath the surface of ripening fruit as it begins to soften and show color, continuing through to harvest. Scars left by oviposition may appear as indented, soft spots on the fruit surface. Small white- or cream-colored larvae hatch within a few days and feed in the fruit, causing the fruit to soften and collapse around the feeding site. Further damage may be caused by secondary pathogens (fungi and bacteria) which attack the damaged fruit. At maturity, the larvae may be up to 1/8 inch long. They may pupate inside or outside the fruit. The length of the life cycle depends on temperature, with adults most active at cool temperatures (around 68 degrees F). Most soft-skinned fruits are vulnerable to attack by SWD, including peach, plum, cherry, grapes (table and wine), strawberry, blueberry, and cane fruits. It has also been found in Asian pear, fig, and hardy kiwi. See SWD under Common Insects for an additional image of the larval stage.