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Plant NameTypeDescription 
AnthracnoseOakDiseaseOak anthracnose is a fungal disease of leaves and twigs of many species of oak. Typical symptoms include the presence of small to large brown lesions on the leaves, usually along the midrib or main veins. Half or more of a leaf may be killed. Young leaves may be severely deformed (curled, puckered, or twisted) by infections occurring during development. Twigs may develop cankers and die back. The fungus may be visible as small brown specks on the lower surface of infected leaves or as pustules on infected twigs. Infections of mature, more resistant leaves are often limited to small brown spots. Premature leaf drop may occur. The disease is favored by prolonged moisture at the time leaves are developing. Symptoms first appear on lower part of tree and progress upward. The fungus may infect and girdle twigs causing dieback.
Armillaria root rotOakDiseaseArmillaria root rot is a fungal disease often found in newly cleared soils or soils which have been flooded. Above-ground 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. White thread-like masses of the fungus may be found beneath the bark near the crown of infected trees, and/or as shoestring-like rhizomorphs, which are dark strands of the fungus growing on or just beneath the soil surface. Honey-colored mushrooms often grow near the base of infected trees in the fall. Infected trees may also exhibit a dark black line in the infected area encircling the base of the plant. Native oak trees usually tolerate infection easily, unless damaged by human activity such as construction or improper irrigation.
Leaf scorchOakDiseaseLeaf scorch has many possible causes. Plants that are under stress (such as from drought or heat) may not provide sufficient water to leaves, causing the edges to turn bronze, then later brown and dry. In severe cases, the scorch may spread to areas between veins or entire twigs may die back. Trees placed near heat-reflecting surfaces such as buildings or pavement often suffer from heat stress. Other possible causes of scorch include damage to roots or stem, soil compaction (which can cause reduced drainage resulting in root damage), or the presence of excessive salts from overuse of chemical fertilizers.
MistletoeOakDiseaseMistletoe is a parasitic plant which grows on the branches of a host tree. True mistletoe, found on oaks and a few other hardwood species, does not grow on conifers. It is spread by seeds ingested by birds and carried to new hosts. True mistletoe is a perennial plant. Leaves are olive-green and leathery. Seeds are borne in white berries. Host tree branches often develop large swellings at the point of mistletoe attachment. Branches are weakened at the attachment point and often break in storms. Mistletoe is also an aesthetic problem, since the large growths can be unsightly. All parts of mistletoe are considered poisonous.
Sudden oak deathOakDiseaseSudden oak death (SOD) is the common name for a disease of oaks (Quercus spp.) and tanoak (Lithocarpus densiflorus) caused by Phytophthora ramorum, a previously unknown and exotic plant pathogen. This pathogen has been found on imported plants in several nurseries in Washington since 2002. Red oaks and tanoak are highly susceptible to SOD, but white oaks are considered resistant. Other common landscape plants that are highly susceptible include rhododendron, kalmia, camellia, viburnum, and Pieris (andromeda). Infected oak and tanoak trees develop stem cankers in the inner bark. These cankers cause the bark to bleed reddish or black droplets of sap. The bleeding is usually not associated with bark cracks or insect holes, although insect holes may be present in infected trees. Bleeding cankers usually occur first in the lower portion of tree trunks, but have been found as high as 60 ft. above the ground. Inside the bark, the cankers appear as discolored blotches, often bordered by dark lines. The cankers expand and girdle the trunk, causing widespread crown death and killing the tree. While IT IS NOT POSSIBLE TO CONCLUSIVELY DIAGNOSE SOD BASED ON VISIBLE SYMPTOMS, the combination of whole crown death accompanied by bleeding of the bark is a good indicator of SOD infection in oak and tanoak. There are many other problems that can cause similar symptoms, including bacterial wetwood, insects, mechanical injury, fungal and bacterial leaf and stem blights, and even other Phytophthora species. The similarity of P. ramorum symptoms to those of other plant diseases and abiotic problems makes field diagnosis very difficult. P. ramorum infection is NOT LIKELY in the landscape UNLESS the plant (1) is a highly susceptible species AND (2) was purchased since 2002 AND (3) is showing symptoms associated with P. ramorum infection (or is located near another plant that meets all these criteria). Currently, the only way to confirm a SOD diagnosis is with laboratory tests.
California gallflyOakInsectThe gall of this cynipid wasp larvae is the largest and most conspicuous of the galls on oak, achieving a diameter of one to four inches. It is a twig gall that is often called an oak apple. It starts out green and in time becomes red on the sunny side. Later in the season, the gall turns brown or grayish just before the adult wasps chew their way out. The gall does no apparent damage to the tree and is easily removed without harming the twig. The gall is round, but is sometimes kidney shaped. During its development, the gall may host up to a dozen larvae.
Jumping oak gallOakInsectJumping oak galls cause small yellow spots on the upper surface of infested leaves. On the underside of the leaves are tiny seedlike galls. These appear in mid-summer or later. Inside each gall is the larvae of a tiny wasp. In the fall, the galls drop from the leaves to the ground, where the activity of the larva causes the gall to jump an inch or more off the ground. The wasps overwinter in the galls on the ground. In the spring, females lay eggs in opening oak buds. The larvae cause blister-like galls, deforming the leaves. Adult females emerging from these galls lay eggs on the leaves, producing larvae which cause the jumping oak galls. Although they may be unsightly, jumping oak galls do relatively little damage. Some yellowing and premature leaf drop may occur.
Leaf gallsOakInsectLeaf galls are caused by the feeding of several species of oak gall wasp larvae. Typical galls are enlarged or swollen plant tissues. These may assume several forms, depending on the species of wasp. Galls may be round, smooth, woolly, spiny, thorn-like, or dish-shaped. Colors vary from white to bright red to greenish, depending on wasp species and age of the gall. Many galls are formed on the leaves, but some may be found on twigs. Leaf deformities and twig dieback can result from infestations.
Oak leaf phylloxeraOakInsectThe oak leaf phylloxera is a pest of growing importance in the Pacific Northwest. It’s believed to have come from Europe in the early 1960’s. It is is a yellowish orange homopteran related to aphids and is about 1mm long (less than 1/16 of an inch) and like its aphid relatives it has several generations each year. It is a sucking pest that causes chlorotic mottling. At first the damage is seen as yellowish spots in spring and early summer. Gradually the damage becomes overwhelming and browning and defoliation occurs in late summer. Some trees may produce a second set of leaves after complete defoliation in August, but this severely weakens the trees and some will die. The oak leaf phylloxera attacks mainly Garry oak and English oak. However, other species such as pin oak have also been attacked. Canadian researchers expect that 10% of the trees that are susceptible will eventually die from continuous defoliation. Resident natural enemies will feed on this pest, but are currently not suppressing it to acceptable levels.
Oak ribbed casemakerOakInsectOak ribbed casemaker larvae mine and skeletonize leaf tissues. Young larvae feed inside leaves, chewing away tissues and leaving a linear mine. Older larvae feed on the underside of leaves, leaving only the upper surface and veins, which turn brown. Severely damaged leaves may be almost transparent. The mature larvae are about 1/4" long, and pale yellow to green. Immature larvae make flat, white, circular cocoons, while cocoons of mature larvae are white, with distinctive longitudinal ribs. The latter can be found on leaves, twigs, or other nearby objects. The adult moth emerges from the ribbed cocoons. It is mottled white, brown, and black with a wingspread of about 1/3". The oak ribbed casemaker is reported on California white (valley) oak (Quercus lobata) and coast live oak (Q. agrifolia).
Oak treehoppersOakInsectThe oak treehopper belongs to a family of plant lice distantly related to aphids. However, they are fairly hard-bodied and the adults bear a pointed hood on the back behind the head. They are quite attractive in that they are variably colored with red and cream stripes with some black spots below the hood in front. When numerous, these insects can kill twigs both by sucking the sap and by laying eggs in slits in the bark. They rarely cause serious damage to the trees.
Pit scaleOakInsectPit scales of oak may be brown, gold, or greenish in color. They feed on twigs and small branches, with young wood most commonly infested. Pit scale feeding causes the tissue of the twig to swell around the insect. The growth makes the scale appear to be in a pit on the bark. Numerous scales on a twig cause it to appear roughened. Adult scales are covered with a hard shell and are about 1/16" in diameter. Severe infestations cause twig dieback. Heavy infestations of pit scale combined with drought stress or anthracnose, a fungal disease can result in severe weakening or death of trees.
Stem gallsOakInsectStem galls are caused by the feeding of several species of oak gall wasp larvae. Typical galls are enlarged or swollen. Galls associated with oak stems are often smooth and round, but may be elongate, thorny, or rough. They can be single galls or clusters of galls. Colors vary from green to reddish to brown, depending on wasp species causing the gall and the age of the gall. Many galls are also formed on the leaves. Twigs can be weakened or killed by infestations.
Western oak looperOakInsectThe Western oak looper feeds mainly on the foliage of the Oregon white (or Garry) oak (Quercus garryana). Mature larvae of the Western oak looper are about 1" long and light brown with black spots when mature. They move in a characteristic "looping" or measuring-worm fashion. The adult moth is yellowish to dark brown. Severe infestations are rare, but can result in almost total defoliation of trees. No permanent damage is done by occasional outbreaks.