Ruth's golden aster (Pityopsis ruthii (Small) Small: Asteraceae) is an endangered, herbaceous perennial that occurs only at a few sites along the Hiwassee and Ocoee rivers in Polk County, Tennessee. This species is drought, heat, and submergence tolerant and has ornamental potential as a fall flowering landscape plant. In 2012, we vegetatively propagated various genotypes and established plantings in a landscape at Poplarville, Mississippi. In June and July of 2013, during periods of hot and humid weather, several well-established plants exhibited black or brown necrotic aerial blight symptoms including desiccation of stems and leaves. Blighted leaf samples were surface sterilized (10% commercial bleach, active ingredient 8.25% sodium hypochlorite, 1 min), rinsed in sterile water, air-dried, and plated on 2% water agar amended with 3.45 mg fenpropathrin/liter (Danitol 2.4 EC, Valent Chemical, Walnut Creek, CA) and 10 mg/liter rifampicin (Sigma-Aldrich, St. Louis, MO). Rhizoctonia sp. was identified based on hyphal morphology and cultures were maintained on potato dextrose agar. Colonies were fast growing, consisting of light tan to brown mycelia and tufts of crystalline aerial hyphae. Within 10 days, brown exudates were present in cultures and there was no pigmented reverse to the agar. Hyphae were a mean of 5.2 μm wide (4.6 to 6.1 μm; n = 10) and each compartment contained three or more nuclei. Hyphae were constricted at septa with right angle branching and no clamp connections, which is typical for Rhizoctonia solani (1). Light- to medium-brown, oblong to irregularly shaped sclerotia measuring 1.2 mm long (0.7 to 2.1 mm) × 0.9 mm wide (0.5 to 1.2 mm; n = 20) were formed in cultures after 3 weeks of growth. Total genomic DNA was extracted from two different colonies grown in potato dextrose broth for 7 days, amplified with PCR using ITS1 and ITS4 primers for amplification of the 18S rDNA subunit (2), the products purified, and sequenced. A consensus sequence of 657 bp was deposited in GenBank (Accession Nos. KF843729 and KF843730) and was 96% identical to two R. solani Kühn ITS sequences in GenBank (HF678125 and HF678122). R. solani was grown on twice autoclaved oats for 2 weeks at 21°C and incorporated into Pro-Mix BX, low fertility soilless medium (Premier Horticulture, Rivière-du-Loup, Quebec, Canada) at 4% (w/w) to inoculate seven P. ruthii plants grown in 10 cm-diameter pots; seven additional plants were grown in the same medium amended with 4% (w/w) sterile oats. Plants were grown in a greenhouse and covered with a plastic dome to maintain high humidity. After 2 weeks, six of the seven inoculated plants exhibited the same aerial blight symptoms as did the original infected plants from the field; none of the control plants developed disease symptoms. Colony morphology and hyphal characteristics as well as the sequence for the ITS region of rDNA from the re-isolated fungus were identical to the original isolate. To our knowledge, this is the first report of R. solani infecting Ruth's golden aster. We are not aware of the disease occurring in wild populations of the plant, but may impact plants grown in the landscape or greenhouse.
References: (1) B. Sneh et al. Identification of Rhizoctonia Species. The American Phytopathological Society, St Paul, MN, 1991. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, CA, 1990.
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