Colletotrichum acutatum, Agent of Anthracnose on the New Host Black Elderberry (Sambucus nigra) in Switzerland.

Black elderberry (Sambucus nigra L.) is grown in Switzerland for flower and fruit production. Flowers are used for candy and syrup production, whereas the fruits are directly consumed as berries. In autumn 2008, the diagnostic laboratory of Agroscope ACW received a sample of strongly shriveled elderberry fruits from the extension office of the canton of St. Gallen. The sample originated from an experimental plot at Flawil, where 80% of the berries exhibited these symptoms. In years with high rainfall, infections of 100% of the berries can be observed in the production areas of Switzerland. Symptoms of anthracnose are only visible on the fruits, but not on the other plant organs. Berries start to shrivel when turning from green to black, and sporulation can be observed on ripe fruits under humid weather conditions. The sample was incubated in a moist chamber at room temperature, where it formed abundant acervuli producing salmon-colored spores at the fruit surface. Isolation from the acervuli on potato dextrose agar (PDA) containing an antibiotic (chlortetracycline) resulted in the growth of white to grey mycelium with salmon-colored spore masses. The reverse side of the PDA was red to violet. One-celled conidiospores were primarily fusiform, with an average size of 16.5 × 4 μm. Based on these morphological traits, the pathogen was previously identified as Colletotrichum acutatum J. H. Simmonds (2). A PCR using the primers CaInt2 and ITS4 (1) was run on a pure culture of the isolate from elderberry and confirmed this identification. A pathogenicity test was conducted from May to August 2010. The isolate from black elderberry and an isolate of C. acutatum from highbush blueberry (Vaccinium corymbosum) were multiplied separately on PDA on the laboratory bench (23 ± 2°C) for one week. Conidiospore suspensions of each isolate were prepared with 0.9% sterile NaCl solution and were adjusted to 1.2 × 10⁶ spores/ml. Flower clusters of a single black elderberry tree at the Agroscope Research Center were inoculated at full flowering stage on May 26, 2010. Two sets of three healthy clusters were sprayed separately with the two spore suspensions until run-off. Spraying three healthy clusters with a sterile 0.9% NaCl solution served as control treatment. Immediately after inoculation, flower clusters were enclosed individually in transparent polyethylene bags for 2 days. To avoid excessive temperature inside the bags caused by solar radiation, the bagged flower clusters were placed below the leaves of the elderberry tree. During the 2 days, the average air temperature measured at the research center was 17 ± 2.5°C. Bags were removed and fruits of the treated clusters were harvested on July 27, 2010. Each cluster was incubated individually in a moist chamber on the laboratory bench (23 ± 2°C) for 10 days. Abundant formation of acervuli producing salmon-colored spores occurred on the fruits inoculated with either strain of C. acutatum. No such symptoms were produced on incubated fruits of the control treatment. From acervuli of the inoculated fruits, C. acutatum was reisolated on PDA. To our knowledge, this is the first report of C. acutatum on black elderberry. In Switzerland, a fungicide containing the active ingredient trifloxystrobine is registered to control C. acutatum on black elderberry. References: (1) S. K. Sreenivasaprasad et al. Plant Pathol. 45:650, 1996. (2) B. C. Sutton. The Coelomycetes. CAB International, Wallingford, UK, 1980.