Ruth's golden aster (Pityopsis ruthii (Small) Small: Asteraceae) is an endangered, herbaceous perennial that occurs only at a few sites along small reaches of the Hiwassee and Ocoee rivers in Polk County, Tennessee. As part of a planned restoration program, Ruth's golden aster has been micropropagated in vitro and acclimatized to greenhouse conditions. In February 2011, several established plants in a greenhouse in Knoxville, TN exhibited signs and symptoms of powdery mildew including growth of white mycelium and conidiophores on the adaxial surface of leaves and slight curling upward of leaf margins. Mycelium was superficial and nipple-shaped appressoria were present. Mycelia, conidiophores, and conidia were removed from several leaves, mounted in water, and examined microscopically. Cylindrical to ovoid conidia (n = 100) lacking fibrosin bodies were borne in chains and had a mean length of 32.0 μm (19.2 to 38.7 μm) and width of 14.9 μm (6.3 to 21.2 μm). The description and dimension of the conidia agreed well with that provided for Golovinomyces cichoracearum (Erysiphe cichoracearum) reported on Coreopsis spp. (1,3) and Cirsium arvense (creeping thistle) (2). The teleomorph was not observed. Total genomic DNA was extracted from infected leaves, amplified with ITS1 and ITS4 primers for the 18S rRNA subunit (4), and visualized on a 2% ethidium bromide agarose gel. An amplicon of fungal origin, approximately 550 bp and smaller than the approximately 700-bp plant ITS amplicon, was excised, purified, and then sequenced. This sequence was deposited in GenBank (Accession No. JF779687) and was 99% identical to two G. cichoracearum accessions (Nos. AB77627 and AB77625). Infected leaves were rubbed on leaves of four healthy plants and healthy leaves were rubbed onto other healthy leaves of two additional plants as controls in the greenhouse. Signs of powdery mildew developed on those plants inoculated with infected leaves after 7 to 10 days and the morphology of the fungus was identical to our previous description. To our knowledge, this is the first report of G. cichoracearum (E. cichoracearum) infecting Ruth's golden aster. We are not aware of the disease occurring in wild populations of the plant, but it does impact the production of micropropagated plants in the greenhouse.
References: (1) D. A. Glawe et al. Online publication. doi:10.1094/PHP-2006-0405-01-BR. Plant Health Progress, 2006. (2) G. Newcombe and C. Nischwitz. Plant Dis. 88:312, 2004. (3) T. E. Seijo et al. Online publication. doi: 10.1094/PHP-2006-1214-01-BR. Plant Health Progress, 2006. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press Inc, New York, 1990.