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Yellow Dryad, a New Host Plant of Colletotrichum acutatum in Switzerland

In spring 2008, yellow dryad (Dryas drummondii L.), an ornamental plant, was studied at the Research Center Conthey (Switzerland) for its possible use by the cosmetical industry. Plants grown from wild-type seeds were multiplied by transplanting cuttings in pots that were later transplanted in an experimental field plot. Before field planting, partial wilting occurred on several plants. The petioles of affected leaves appeared dry, tan, and covered with black acervuli containing black setae. Isolation from the acervuli on potato dextrose agar (PDA) containing chlortetracycline (25 mg/liter) resulted in the growth of white-to-gray mycelium containing salmon-colored conidial masses but no setae. The underside of the colony was carmine red. Conidia were primarily fusiform with an average size of 13 × 4 μm. On the basis of these morphological traits, the pathogen was identified as Colletotrichum acutatum J.H. Simmonds (2). PCR using the species-specific primer CaInt2 in conjunction with the conserved primer ITS4 (3) was conducted on genomic DNA from a single-spore isolate. An isolate of C. acutatum from strawberry was included as a positive control. This primer pair produced a 490-bp fragment that confirmed the identification based on morphology. A pathogenicity test was conducted at the end of August and beginning of September 2009. The single-spore isolate from yellow dryad and a single-spore isolate of C. acutatum from highbush blueberry (Vaccinium corymbosum located at Dürrenroth, Switzerland) were grown on PDA at 23 ± 2°C for 1 week. Conidial suspensions were prepared with 0.9% sterile NaCl solution and were adjusted to 3 to 5 × 10⁵ spores/ml. For each spore suspension, five 3- to 5-cm tall, healthy, yellow dryad plants in the rosette stage were sprayed until runoff. Spraying five plants with a sterile 0.9% NaCl solution served as control treatment. Immediately after inoculation, plants were covered individually with a polyethylene bag and incubated at 23 ± 2°C for 5 days. Polyethylene bags were then removed and the plants were placed outdoors under a plastic shelter (18 ± 4°C). After 1 week, plants inoculated with either strain of C. acutatum showed the same symptoms. The most prevalent symptoms on leaves were brown necroses surrounded by a dark brown margin; the necrotic lesions were covered with black acervuli without setae. Less frequent were small, brown lesions that turned gray and were covered with acervuli containing setae. Acervuli with setae also occurred frequently on the petioles of the inoculated plants. On the control plants, none of these symptoms were observed. Leaves with lesions were incubated in a humid chamber for 1 day, resulting in abundant salmon-colored sporulation from the acervuli. C. acutatum was reisolated from such spore masses on PDA. To our knowledge, this is the first report of C. acutatum on yellow dryad. Since C. acutatum is a widespread pathogen worldwide (1), it represents a potential threat to yellow dryad crops grown for ornamental and potentially cosmetical use.

References: (1) S. Freeman. Page 131 in: Colletotrichum: Host Specificity, Pathology, and Host-Pathogen Interaction. D. Prusky et al., eds. The American Phytopathological Society, St. Paul, MN, 2000. (2) P. S. Gunnell and W. D. Gubler. Mycologia 84:157, 1992. (3) S. K. Sreenivasaprasad et al. Plant Pathol. 45:650, 1996.