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Sorbaria sorbifolia (L.) A. Braun ‘Sem’ is a woody shrub of the Rosaceae valued for its ornamental pinkish-red spring foliage and white blossoms. Plants with symptoms typical of infection by the bacterial phytopathogen Rhodococcus fascians were observed in a nursery in Minnesota in 2008. These observations followed confirmed infections by R. fascians in the same nursery on a variety of herbaceous ornamentals. Plants with shoot proliferation (Fig. 1) and leafy galls (Fig. 2) were submitted to the Oregon State University Plant Clinic for diagnosis in April and May 2008. Symptomatic tissue was removed and washed briefly in tap water, then single tissue pieces of 3 to 5 mm2 were incubated individually in 3 ml sterile 0.85% NaCl containing 2% (w/v) cycloheximide for 60 min at room temperature. The suspension was streaked onto D2 medium (1) and incubated at 27°C for 3 to 10 days. Small, domed, moist-looking chrome-yellow bacterial colonies were selected for further culturing; single colonies were streaked onto fresh D2, and individual colonies were streaked again. This process was completed a minimum of three times or until uniform cultures were obtained. Three single-colony isolates were identified by substrate utilization (Biolog, Hayward, CA) as R. fascians. Two of these single-colony isolates were streaked to Miller Medium (MM), consisting of the following ingredients (g/liter): mannitol (10), casein (4), peptone (4), NH Cl (1), yeast extract (3), MgSO (0.3), NaCl (0.2), KH PO (0.5). The pH of the medium was adjusted to 6.5, and 15 g of agar was added to the medium before autoclaving for 20 min at 103.4 kPa. After two days growth on MM, bacteria were loaded onto sterile toothpicks and applied to leaf axils of 3-cm-tall tissue-cultured S. sorbifolia ‘Sem’ plants. Three R. fascians strains isolated from other hosts and an avirulent R. fascians strain were similarly inoculated to plants. For each strain three ‘Sem’ plants were inoculated, and the experiment was performed twice. All plants inoculated with virulent isolates of R. fascians began to form leafy galls three weeks postinoculation whereas no symptoms were observed on plants inoculated with the avirulent strain or on non-inoculated controls. Bacteria were isolated from inoculated plants as above and identified as R. fascians using substrate utilization. These tests confirm S. sorbifolia as a newly identified host for R. fascians, further expanding the host range among ornamental plants (3). This bacterium is known to infect primarily herbaceous plants, and S. sorbifolia is one of only a very few woody plants known to be naturally infected. This finding indicates woody plants may be at risk of disease in those nurseries that grow a range of woody and non-woody materials close together in the same location. R. Fig. 1. S. sorbifolia ‘Sem’ with an abnormal number of shoots at the base of the stem due to natural infection by R. fascians.

Sorbaria sorbifolia is a New Host for Rhodococcus fascians