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Plant NameTypeDescription 
Armillaria root rotRhododendronDiseaseArmillaria root rot is a fungal disease transmitted between plants by root contact. Armillaria is often found in newly cleared soils or soils which have been flooded. Symptoms typically include production of smaller-than-normal leaves, leaf yellowing, leaf drop, and 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. Armillaria is also known as oak root fungus.
Botrytis shoot blightRhododendronDiseaseBotrytis shoot blight is a fungal disease found on shoots in early spring. Symptoms of infection include brown, water-soaked spots on the shoots and masses of gray spores which may be formed on infected tissues. Infected shoots either bend over at the lesion or die back.
Gray blightRhododendronDiseaseGray blight is a fungal infection of the leaves. It usually becomes established after an injury to the leaf, such as sunscald, frost damage, or some mechanical injury. White spots or blotches with dark margins can be found on the leaf surface. On cut leaves, the infection may spread from the edge of the wound into the leaf tissue.
Leaf spotRhododendronDiseaseSeveral fungi cause leaf spots on rhododendron. They often infect through wounds. Infected leaves display dark brown spots which may cover up to half of the leaf surface. These spots may feel bumpy on the surface due to the presence of fungal structures.
Lime-induced chlorosisRhododendronDiseaseLime-induced chlorosis, or leaf yellowing, is caused by alkaline soil conditions. High alkalinity of soils can make iron or manganese unavailable to plants, causing leaf chlorosis. Pale to bright yellow leaves, particularly on the new growth, are common symptoms. Typically the chlorosis begins at the edges and progresses into the leaf, with only the veins remaining green in severe cases.
Marginal leaf necrosisRhododendronDiseaseLeaf scorch on rhododendrons is a response to stress. Symptoms include browning of tips or margins of leaves, with the damage sometimes spreading to the center of the leaf. Water stress (too much or too little), root or stem damage, high soil pH, and exposure to drying winds are some of the possible causes of leaf scorch.
Physiological leaf spotRhododendronDiseaseRhododendron leaves sometimes develop variously-sized and -shaped spots. These are usually dark purple in color. The exact cause is unknown, but cultural or environmental stress may result in physiological leaf spot. One particularly susceptible variety is the rhododendron Mrs. G.W. Leak. This problem does not usually cause any permanent harm to the plant.
Phytophthora blightRhododendronDiseasePhytophthora blight is a fungal disease whose symptoms are often confused with chemical or mechanical injury or leaf scorch. Irregular dead spots on the tips or margins of leaves, shiny black cankers on shoots, and leaf loss and tip dieback on affected branches are among the common symptoms. Several species of <i>Phytophthora</i> fungi can cause these symptoms. This disease is spread by wind and splashing water, but does not typically spread to neighboring, unrelated plant species. Do not confuse Phytophthora blight with Ramorum leaf and shoot blight, also caused by a <i>Phytophthora</i> species. See <a target="blank" href=><u>Rhododendron: Ramorum leaf and shoot blight</u></a> for additional information.
Phytophthora root rotRhododendronDiseaseThe symptoms produced by Phytophthora root rot are often confused with those caused by environmental or cultural problems. Above ground, infected plants may show stunting, yellowish-green leaves, branch dieback, wilt, and even plant death. Dead leaves persist on the branches instead of dropping. Underground, the roots rot, progressing from the smallest to the largest. The fungus may also cause rot in the main stem. Phytophthora persists in the soil and can infect new plants. Diseased plant tissues and debris are other sources of infection.
Powdery mildewRhododendronDiseasePowdery mildew on rhododendrons is a fungal infection of the leaves. On rhododendrons, the disease often fails to develop the distinctive powdery white growth normally associated with powdery mildews. Leaves may display yellowish-green or purplish-brown spots or blotches on the upper surface. Lower leaf surfaces may show brown or purple areas of various shapes and sizes. This disease may cause extensive leaf drop and even death of very susceptible plants. Powdery mildew is more severe on shaded plants and is favored by the high humidity found in crowded plantings and damp locations.
Ramorum leaf and shoot blight (Sudden oak death)RhododendronDiseaseRamorum leaf blight is caused by Phytophthora ramorum, the same organism that causes sudden oak death (SOD). In Washington, most cases have been found in nurseries on rhododendrons and camellias, which are highly susceptible to this disease. Some oak species, viburnum, kalmia, and Pieris are also considered highly susceptible, along with native plants such as salal and evergreen huckleberry. On rhododendron, any part of the leaf may be affected; however, symptoms typically appear where water collects. Damage consists of discolored lesions at the tip or near the petiole, often spreading along the midrib. Lesions may move into the shoot, causing shoot dieback. In extreme cases, cankers may develop on branches, but wilting and death of the entire plant is NOT A SYMPTOM of infection by P. ramorum. Cold damage, sunburn, chemical injury, or other fungal diseases may cause similar symptoms; however, lesions caused by abiotic factors such as sunburn tend to have very distinct margins and are often not associated with the leaf petiole or midrib. P. ramorum has a wide and varied host range, so rhododendron is seldom the only species affected. Phytophthora blight is another disease of rhododendrons which may cause symptoms that resemble those of Ramorum leaf and shoot blight, but it is caused by different species of Phytophthora. For more information, see also the Hortsense fact sheet on Rhododendron: Phytophthora blight. IT IS NOT POSSIBLE TO CONCLUSIVELY DIAGNOSE THIS PROBLEM BASED ON VISIBLE SYMPTOMS ONLY. In Washington, Ramorum leaf and shoot blight has so far been found only in association with infected nursery plants and water runoff from infected nursery sites. While this disease has the potential to become a serious and economically significant problem, it is not yet considered to be widespread in Washington in either cultivated or native landscapes. 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.
RustRhododendronDiseaseYellowish spots on the upper leaf surface and dark brown or purple spot on the lower leaf surface can indicate the presence of a rust infection. Later, the underside develops blisters which break open to reveal characteristic yellow to orange or reddish spores. Rusts can also affect azaleas. The disease overwinters on the host plant.
Salt injuryRhododendronDiseaseSalt injury to rhododendrons is typically a result of culture or environment. Excessive salt (mineral) levels in the soil or irrigation water, excessive use of chemical fertilizers, or lack of water can be at fault. Edges of older leaves "burn" or become brown and dry and the plant may lose its healthy green color.
SunscaldRhododendronDiseaseSunscald is caused by exposure of the leaves to bright sunlight during hot weather. Sunscalded leaves on rhododendrons generally have a yellow to brown blotch in the center. Often this blotch has a yellow halo surrounding it. Typically the outside leaves on the southern and southwestern sides of the plant are most severely affected.
Tissue proliferationRhododendronDiseaseThere is no known cause of tissue proliferation, but it is currently believed to be a genetic abnormality. A swollen area or gall develops at the base of the trunk or (more rarely) on a shoot. Otherwise, the plants appear to be healthy. Numerous small sprouts may develop from the swollen area, which may deteriorate after several years. Plants may break at the site of the swelling. Tissue proliferation is apparently noninfectious, and does not seem to be spread by grafting. Tissue proliferation is also occasionally observed on azaleas.
AphidsRhododendronInsectAphids cause twisting, curling, or puckering of new growth by their feeding. Whitish cast skins and green live aphids may be visible on the undersides of leaves. Honeydew, a shiny, sticky substance excreted by aphids, may be present on the leaves, as may sooty mold, a black fungus which grows on the honeydew.
Azalea bark scaleRhododendronInsectThe azalea bark scale is a soft scale resembling mealybugs. Red females cover themselves and their eggs with a white, waxy protective coating. Cottony masses of the scales are often located on twigs and in the forks of the branches. Sticky honeydew is excreted by the pests, and may have black sooty mold growing on it.
CaterpillarsRhododendronInsectSeveral caterpillar pests occur on rhododendrons, most notably cutworms, loopers, and the obliquebanded leafroller. Cutworms and loopers (various species) feed on leaf tissue, removing large, irregularly-shaped pieces. They are night-feeders and may not be seen on the leaves in the daytime. Obliquebanded leafrollers are usually found on the new growth. Their presence can be determined by the presence of rolled leaves held in place with webbing. The feeding caterpillar can be found inside the rolled leaf.
Lecanium scaleRhododendronInsectLecanium scale adults are found on the twigs in late spring. At this stage they are usually brown or mottled brown and white bumps on the stems. During the summer and fall, scale nymphs can be found on the foliage and are flat and light brown. The partially developed nymphs overwinter in crevices in the bark.
Rhododendron lace bugRhododendronInsectThe rhododendron lace bug causes stippling on the upper surface of the leaves, and deposits crusty or tar-like excrement on the lower surface. Adult insects are whitish-tan and approximately 1/8" long with lacy-looking wings. Nymphs grow to about 1/8" and are spiny. Infestations are more severe on plants in the sun. Damage is usually apparent by early to mid-July. While almost never fatal, repeated infestations of rhododendron lace bugs may result in yellowed, sickly plants. This insect is not found on azaleas. Rhododendron lace bug has one generation per year. It overwinters as eggs laid on the underside of leaves, usually along leaf veins. See also Azalea: Azalea lace bug.
Rhododendron whiteflyRhododendronInsectWhiteflies are small white insects that hold their wings rooflike over their abdomens rather than flat like true flies. Infested leaves, usually the tender young leaves at the branch tips, are mottled yellow on the upper surface, and the margins often curl. Rhododendron whiteflies often leave white powdery residues on the leaf surface. Rhododendrons with smooth surfaces underneath the leaf are most affected. Whiteflies can produce large amounts of sticky honeydew, which can develop a heavy coating of black sooty mold. The rhododendron whitefly is mainly an aesthetic pest, although heavy infestations can damage susceptible plants.
Root weevilsRhododendronInsectSeveral species of weevils feed on rhododendrons. Characteristic damage includes notching of leaf edges by adults and minor to severe root and bark damage by larvae. Occasionally the larvae girdle the trunk at or just below the soil surface, killing the plant. Adults are wingless and begin feeding around June to August, depending on the species.