Plant Disease Management Handbook

Cause Ascochyta rhei and Ramularia rhei, fungi that overwinter on old rhubarb leaves and stalks. These fungi also can spread through infected rootstock. Leaf spots are common on field-grown rhubarb west of the Cascade Range due to frequent rains in March and April when leaves and stalks are actively growing. Both pathogens produce spores that move by wind and splashing water.

Image related to Rhubarb (Rheum rhabarbarum)-Leaf Spots

This is leaf spot caused by Ramularia sp.

Photo by D. Ormrod, 1983.

Ramularia stalk infections.

Photo by D. Ormrod, 1983.

Cause Botrytis cinerea, a fungus that overwinters on decayed rhubarb plant parts. Spores also are abundant in the air and on decayed parts of many other plant species. The disease can be a problem in the field but is more destructive in forced crops.

Image related to Rhubarb (Rheum rhabarbarum)-Gray Mold

Gray mold develops on older leaves and stems as reddish brown, water-soaked areas that produce additional spores.

Lisa Jones, 2015.

See:

Azalea (Rhododendron spp.) - Leaf and Flower Gall

Brooms can be found on otherwise healthy bushes. The bush shown here has healthy branches on the left and a broom on the right. Leaves on the witches broom are smaller, chlorotic and may have yellowed to cream colored thickenings.

Jay W. Pscheidt, 2019.

Image related to Rhododendron-Witches' Broom

Rhododendron macrophyllum with several woody shoots arising from one part of the plant. Brooms can become very large and numerous.

OSU Plant Clinic Image, 2013.

Note the many short branches of the broom relative to the longer branches above.

Photo by Jay W. Pscheidt, 1998.

The cause of Witches' brooms in azalea is unknown.

Jay W. Pscheidt, 2020.

Cause Unknown. A genetic change that occurs during tissue-culture propagation that predisposes plants to express symptoms when grown under accelerated growth conditions. The disease was a major problem for growers in the 1980's but with changes in production of tissue culture plants it has not been a problem for many years. Tissue proliferation has occurred on seedlings and on plants propagated by tissue culture and vegetative cutting.

Note the mass of small shoots near the base of the plant.

Photo by Jay W. Pscheidt, 1992.

Crown gall is solid, looks somewhat uniform, and has malformed vascular tissue.

Melodie Putnam, 1989.

See:

Rhododendron - Leaf Scorch

Symptoms only started to show up once summer temeratures got over 90F.

Jay W. Pscheidt, 2018.

Note the chlorotic areas near the leaf midribs. Some necrosis is also apparent.

Photo courtesy Jim Green.

Note that only the rhododendron is sunburned, while the other plants are able to tolerate the full afternoon sun.

Photo by Jay W. Pscheidt, 1998.

Cause Chrysomyxa piperiana is probably the main species on native R. macrophyllum in Oregon and has the alternate host, Sitka spruce, which is reported in Oregon. The rust of cultivated rhododendrons is C. reticulata, a native North American rust that spread to nurseries from Ledum.

Orange to yellow-orange spores of the fungus develop in pustules on the underside of the leaf.

Photo by Ralph S. Byther

Note the pustules on the underside of the leaf.

OSU Extension Plant Pathology Slide Collection, 1972.

See:

Tanoak (Notholithocarpus densiflorus) - Sudden Oak Death

Rhododendron - Phytophthora Blight and Dieback

Image related to Rhododendron-Ramorum Leaf Blight and Shoot Dieback

The rhododendron cultivar Unique, grown in a nursery showing necrotic leaf spots along the margins of the leaves.

Photo by Jennifer Parke, 2003.

It is uncommon to see necrosis occur out to the leaf tip. Usually the necrosis affects the midrib, as pictured here, and then goes down the petiole.

Photo by Everett Hansen

Leaves collapse once the stem has been girdled.

Photo by Everett Hansen

The bark has been stripped back to expose the discolored vascular system below.

Photo by Everett Hansen

This shows the typical tip dieback to Rhododendron spp. from this disease. Leaves will collapse soon.

Photo by Everett Hansen

Cause The fungus Erysiphe azaleae (formerly Microsphaera azaleae) is found throughout the Pacific Northwest on garden azalea and rhododendron species and hybrids. Molecular techniques have identified a similar looking fungus as E. digitata which may have been introduced into the PNW in the 1990's.

Note the subtle, fuzzy, gray leaf spots on the lower part of leaf and the faint, yellow areas on top of the leaf.

Photo by Jay W. Pscheidt, 1994.

Spots due to powdery mildew can be seen on the upper leaf surface. Some cultivars show these kinds of symptoms.

Photo by Jay W. Pscheidt, 1994

This shows the range of symptoms to be expected on Rhododendron spp. from powdery mildew.

OSU Extension Plant Pathology Slide Collection, 1990.

A few evergreen cultivars of rhododendron, such as Vulcan''s Flame, have the typical white powdery colonies of powdery mildew.

Photo by Jay W. Pscheidt, 1994

Cleistothecia of Microsphaera azaleae showing the typical multiple but short appendages.

Photo by Jay W. Pscheidt, 1994

Cause Phytophthora root rot is caused by a number of Phytophthora species, including P. cactorum, P. cambivora, P. cinnamomi, P. citricola, P. cryptogea, P. plurivora, and P. pini. These pathogens are fungus-like microorganisms and are also known as water-molds.

A dead rhododendron on the left, a wilted plant on the right, and the plant in the center appears to be OK but most likely it also is infected with Phytophthora.

Photo by Jay W. Pscheidt, 1991.

Healthy rhododendron root system (left) and three rhododendron root systems with Phytophthora root rot (right).

Jerry Weiland, 2016.

Image related to Rhododendron-Phytophthora Root Rot

Rhododendrons at the bottom of the hill where water collects are dying from root rot.

Jay W. Pscheidt, 1991.

Phytophthora root rot in container grown Rhododendron 'Purple Gem'.

Jerry Weiland, 2017.

Phytophthora root and crown rot of these new seedlings.

OSU Plant Clinic Collection, 2011.

Container rhododendrons showing symptoms of Phytophthora root rot when located in low lying areas of the canyard where water collects.

Jerry Weiland, 2013.

Top picture shows wilting symptoms while bottom picture of the same plant (Rhododendron 'Taurus') shows the transition zone with discoloration of the cambium below and healthy above.

Neil Bell, 2012.

See:

Rhododendron - Phytophthora Root Rot

Rhododendron - Ramorum Leaf Blight and Shoot Dieback

Image related to Rhododendron-Phytophthora blight and dieback

Note the numerous necrotic leaves.

Photo by Jay W. Pscheidt, 1991.

Rhododendron 'Spring Dawn' with a lot of defoliation from Phytophthora blight.

Jay W. Pscheidt, 2000.