Viburnum
Symptoms on viburnum foliage, like on other hosts, resemble those of sunscald or fungal infection. Lesions due to P. ramorum tend to form where water collects and usually involve the petiole. A leaf lesion can grow through the leaf, into the petiole, and into the branch, causing a canker. Cankers develop on the stem near the soil line and on branches. As the canker expands, leaves attached to the branch die and defoliate. When the branch is girdled by the canker, the remaining leaves wilt and turn brown, usually staying attached to the stem. Occasionally bleeding is seen on infected stems.
Viburnum species and cultivars that have been found positive for P. ramorum in Washington State are
Viburnum tinus “Spring Bouquet”
V. davidii
V. plicatum “Mariesii”
Other diseases that appear similar are foliar blight caused by grey mold (Botrytis cinerea). The spots first appear at the leaf margins, then spread to the rest of the leaf. Infected flower clusters or twigs are killed. A bacterial infection can cause small water-soaked lesions on leaves and stems, and can resemble a P. ramorum infection. Several fungi cause leaf spots including the fungi Cercospora spp., Phoma spp. and Phyllosticta spp. Anthracnose is caused by various fungi and appears as black, sunken lesions is fairly common on viburnum foliage. Fungal leaf spots on viburnum typically are angular to irregular-shaped, and the leaf tissue in the spots is sunken and dry, where P. ramorum lesions are often water-soaked and have diffuse margins. Spots may begin small, but enlarge or merge together, and may be reddish to grayish brown. Fungal leaf spots typically occur during warm, moist summer months, and initially will occur on older foliage.
Abiotic conditions such as frost damage will cause blackening of young shoots and dieback. Drought stress caused by lack of water, anaerobic conditions from flooding, and root damage caused by other Phytophthora diseases will cause wilting of the whole plant in a similar manner to a girdling stem canker.
Viburnum plants suffering from drought and other stresses are susceptible to Botryosphaeria dieback and canker, caused by Botryosphaeria spp. Healthy plants are much more resistant to infection by Botryosphaeria spp., as they will wall off the fungus and prevent its spread through the branch.
Under the bark the wood is stained brown, as with a Phytophthora infection. The cankers are small initially, but enlarge or coalesce into large areas that girdle the branch or trunk. Water movement is stopped beyond that point and results in a rapid wilting or browning of foliage. Branches with cankers may fail to leaf out in the spring.
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P. ramorum symptoms on viburnum foliage (Photo: Jennifer Parke, OR) |
Stem lesion on viburnum caused by P. ramorum. Notice staining under bark. (Photo: J.M. Davidson) |
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Stem and leaf infection by Botrytis cinerea. Notice grey mold on leaves. (Photo: Ralph S. Byther) |
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