Rhododendron
Phytophthora ramorum symptoms on Rhododendron consist of leaf blight and shoot dieback and are indistinguishable from symptoms of other Phytophthora species. Leaf lesions will have dark discoloration that often follows the midrib or petiole, and is severe at places where water accumulates, such as the leaf tips.
If the whole plant wilts and dies, it is likely caused by another species of Phytophthora, or perhaps a fungus such as Botryosphaeria. Several species such as P. cactorum, P. cinnamomi, and P. citricola cause root rot and stem canker. P. ramorum has not been found to cause these symptoms on Rhododendron.
Symptoms that may resemble those of P. ramorum are sunscorch and infection by several species of fungi. These symptoms differ from those of P. ramorum by having defined margins, fruiting bodies (in the case of fungi), and often do not follow the midrib or petiole. They can occur anywhere on the leaf, not necessarily where water collects.
Shoot blights caused by Botryosphaeria and Phomopsis look similar to those caused by P. ramorum. The wood is stained brown under the bark and the bark is blackened. However, tiny pinhead like fruiting bodies can sometimes be seen, where these are not present with a Phytophthora infection.
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Sun scorch injury on Rhododendron leaves (Photo: Rich Regan, Oregon State University) |
Leaf blight caused by P. ramorum (Photo: Bruce Moltzen, Missouri Dept. of Conservation) |
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Root rot on Rhododendron caused by Phytophthora spp. (Photo: Jay Pscheidt, Oregon State University) |
Shoot blight caused by P. ramorum (Photo: Everett Hansen, Oregon State University) |
Rhododendron species and cultivars that have tested positive for P. ramorum in Washington are
Rhododendron spp. “Anna Kruschke” R . spp. “Besse Howells” R . spp. “Blue Boy” R . spp. “Cheer” R . spp. “Christmas Cheer” R . spp. “Double Date” |
R . spp. “Night Watch” R . spp. “Pink Walloper” R . spp. “Vulcan” R . catawbiense “Grandiflora” R. pentanthera |
A complete list of Rhododendron diseases can be found here: