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Plant NameTypeDescription 
Armillaria root rotDouglas FirDiseaseArmillaria root rot is a fungal disease transmitted between plants by root contact. Armillaria is often found in newly cleared soils or ones which have been flooded. Symptoms typically include sudden or gradual slowing of growth, yellowish or undersized needles, needle loss, or dieback of branches. 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. Young, stressed trees are most susceptible to infection. Armillaria-infected trees have damaged root systems and are more likely to fall in high winds.
Laminated root rotDouglas FirDiseaseLaminated root rot is a fungal disease of conifers throughout the Pacific Northwest. Douglas-fir, grand fir, and mountain hemlock are highly susceptible and are usually killed by the disease; many other conifer species are moderately susceptible, especially when growing near highly susceptible species. Native pines and cedars (including western red cedar) are considered tolerant; they may develop a butt rot but are almost never killed by the disease. Hardwoods such as maple and alder are immune. Laminated root rot attacks the root system of susceptible trees of any age, causing roots to decay and resulting in poor water and nutrient uptake and decreased growth. Affected Douglas-fir trees often develop rounded crowns due to stunting and short needles on the leader and branches. Symptoms in the crown appear as yellowing and thinning of the canopy, production of a distress crop of cones (Douglas-fir), and decline and death of the tree. These symptoms often do not appear until root damage is advanced. In the early stages of disease, freshly cut stumps (within about 3 feet of the soil surface) will show reddish-brown semicircular stains that fade when exposed to the air. On living trees, grayish-white to tawny or purplish mycelia of the fungus may be found sheathing the outside of infected roots. The wood of infected trees shows a characteristic layered decay in the roots and butt of the tree. Annual rings separate easily, with pits in the wood on both sides of the sheets or laminae. Reddish-brown, whiskery fungal strands (hyphae) may be present between the layers. The disease spreads by root contact and will persist in infected stumps and large roots remaining in the soil. Trees infected with laminated root rot are also predisposed to attack by bark beetles. Tree death results either from nutrient and water deficiency or from affected trees blowing down or breaking in the wind. Damaged trees may be wind-thrown due to loss of anchoring roots before above-ground symptoms are obvious, especially west of the Cascades, making this disease both difficult to diagnose and potentially hazardous.
Rhabdocline needle castDouglas FirDiseaseThe first symptoms of this fungal infection are many tiny yellow spots on the current season's needles appearing in late summer or fall. By spring, these spots have expanded and become characteristic dark reddish-brown blotches. The blotches on the underside of the needles later swell and split open to reveal orange-brown fungal structures. Diseased needles typically are cast (dropped) by late June. Severely diseased trees may retain only the current season's needles and be weakened. The disease is favored by high humidity. Rhabdocline needle cast may be confused with Swiss needle cast.
RustDouglas FirDiseaseRust is a fungal disease infecting the needles of Douglas fir. Wind-blown spores infect the new needles in the spring. Yellow to orange bumps, or pustules, are found on the underside of needles in the spring or early summer. The infected needles are usually a discolored yellowish-green. If the disease pressure is very high, sunken red-brown cankers may be found girdling the current season's branches. Rust overwinters on the fallen leaves of black cottonwood and other poplars which are alternate hosts to the disease.
Stem cankersDouglas FirDiseaseStem cankers on Douglas fir are sunken and oval, occurring on branches or new shoots. Each canker is typically centered around a dead twig and can be small or up to 30" long or more. Cankers result in death of the stem or branch above the point of infection. Young, stressed trees are most susceptible. Infection often occurs following drought, frost damage, or mechanical injury. Two asexual fungi cause the disease. See also Douglas fir: Upper stem canker.
Swiss needle castDouglas FirDiseaseThis fungal disease causes infected needles to become off-color and mottled yellow or brown. The needles may turn completely brown and are usually dropped prematurely, often in early spring. Infected needles may also remain green and attached to the branches. Older needles typically drop first. Severely diseased trees may have only the current season's needles remaining on the branches. On the underside of infected needles, along either side of the midrib, straight rows of tiny black dots appear. These dots (fungal structures) are most common on needles one year old and older. Swiss needle cast is favored by high humidity. This disease may be confused with Rhabdocline needle cast.
Upper stem cankerDouglas FirDiseaseUpper stem cankers are caused by fungi in the soil. Seedling Douglas fir trees are affected, with the tops of infected trees turning yellow. By fall, the tops turn reddish-brown. Below the discolored portion of the seedling, on the middle or upper stem, a sunken canker can be found. It starts in a bark crevice and expands around the stem, turning red-brown and eventually girdling the tree. See also Douglas fir: Stem cankers.
Yellow-green mottle syndromeDouglas FirDiseaseThe cause of yellow-green mottle syndrome is unknown, but may be genetic. Needles of all ages are affected by yellow-green mottle syndrome. Yellow-green blotches may be small or may cover the entire needle, but the midrib is never affected. Mildly affected needles have small or large blotches on only one side of the midrib. Severely mottled needles are entirely yellow with a dull green midrib. Affected needles usually fall off the trees, sometimes causing severe defoliation. Trees sometimes grow out of the syndrome in two or three years.
AphidsDouglas FirInsectThe aphids that infest Douglas fir are quite large-- up to 1/5" long. They may range in color from gray to brownish to dark. The aphids establish large colonies on the twigs, but are rarely found feeding on needles. Large amounts of honeydew (a sticky material) are secreted, often attracting ants. The honeydew may become covered with a dark growth of sooty mold. Aphid feeding on Douglas fir may cause distorted stems or stunted growth.
ConewormsDouglas FirInsectConeworms attack Douglas fir by boring into shoot tips or stems, especially around wounds, and feeding on the soft bark tissues. The portion of the branch beyond the injured point may die back. Coneworms may also bore into green cones, feed on the soft bark of young growth, or feed inside the bark on the trunk. The coneworms are small and cream-colored or light brown with a darker head. The adult coneworm is a mottled gray moth. Coneworms also attack pines, hemlocks, true firs, and spruces.
Cooley spruce gall adelgidDouglas FirInsectThis aphid-like insect feeds on newly growing needles. Adelgids appear as woolly or cottony tufts on the needles, with heavily infested trees appearing "frosted" or flocked. Small purplish insects are found underneath the cottony tufts. Adelgid feeding can cause needles to become distorted or bent and yellowed. Infested needles sometimes drop prematurely. This pest is a serious concern in Christmas tree plantations, but is less important in the landscape. Cooley spruce gall adelgids also infest spruces, but cause distinctive galls on spruce that are not seen on Douglas fir. Chemical control must be aimed at crawlers during or shortly after bud-break.
Douglas fir needle midgeDouglas FirInsectThe tiny white larvae of the Douglas fir needle midge mine the inside of needles, which become yellowed and distorted. Infested needles often have a sharp bend at the injury site. Three different midges infest Douglas fir: one species feeds near the needle base, one feeds near the tip of the needle, and the third feeds near the middle. Damaged needles often drop from the tree, and heavy midge infestations can cause severe defoliation. The midges pupate in the ground, with the adults emerging around bud-break in the spring. The adult Douglas fir needle midge is a small fly. This can be a serious pest in Christmas tree plantations.
Douglas fir tussock mothDouglas FirInsectDouglas fir tussock moth larvae start at branch tips at the top of the tree and work down, feeding mainly on the new foliage and causing severe defoliation. They may be found under webbing on the branches. Severe tussock moth outbreaks are very sporadic and last usually around three years before subsiding. The larvae feed on the needles of Douglas fir, spruce, pine, larch, and true firs. They feed mainly on forest trees and are infrequent pests in the landscape. The caterpillars are distinguished by three long tufts of black hairs on their body (two in front, one in back) and lighter tufts along their back. The hairs from tussock moth caterpillars break off easily and may cause skin or respiratory irritation.
Douglas fir twig weevilDouglas FirInsectThe Douglas fir twig weevil is a small, wingless, grayish-black beetle with white markings and sometimes pinkish spots. The adult lays eggs in twigs in the summer. The larvae mine under the bark or inside twigs. Infested twigs and small branches often turn a reddish-brown and die back. Small, stressed trees are especially susceptible to attack by the weevils, particularly in dry years.
Sequoia pitch mothDouglas FirInsectThe larvae of the sequoia pitch moth feed by boring into branches or trunks. At the point where the larva enters the wood, small to large masses of white to pinkish pitch accumulate. The larva feeds locally underneath the pitch mass. Although healthy trees are occasionally attacked, the egg-laying moths are probably attracted to wounds such as those made by spring pruning. The moths may also be attracted to trees undergoing stresses associated with drought or saturated soil. This pest causes mainly aesthetic damage because of the pitch masses. Incidentally, they do not attack Sequoia.
Silverspotted tiger mothDouglas FirInsectThe caterpillars of the silverspotted tiger moth feed on the needles, often "tenting" branches with dirty-looking webs. Feeding occurs through fall and winter, with webs becoming more noticeable by spring. The caterpillars can reach 1 1/2" in length and are mostly reddish-brown with some blue-black or yellowish hairs. Adult moths are brownish or tan with distinct silver-white spots on the wings. Minor infestations cause little harm to trees. Douglas fir is the preferred host, but the caterpillars will infest other conifers. Caterpillar hairs can cause skin irritation if they are handled without gloves.
Spruce spider miteDouglas FirInsectThe presence of spruce spider mites is indicated by yellow or bronze stippling beginning near the needle bases. Infestations usually begin on older needles of the lower branches and spread upwards as the mite population increases. Damaged needles may turn brown or reddish-brown. Fine webbing may cover the needles and twigs. The actual spider mites are very small and vary in color from greenish to orange, dark green, or black, with orange legs. Spruce spider mites attack many species of conifers. They are easily spread by wind. These mites are often worst on dusty roadside trees.