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First Report of Boeremia exigua var. exigua Causing Stem and Leaf Spot on Common Speedwell in Switzerland

Michel, V. V., Daepp, M., Woudenberg, J. H. C., de Gruyter, J., de Wit, P. J. G. M.
Plant disease 2018 v.102 no.2 pp. 440
Phoma exigua, Veronica officinalis, actin, aerial parts, agar, bags, calmodulin, chlortetracycline, culture media, germination, greenhouses, internal transcribed spacers, leaf spot, leaves, pathogenicity, pathogens, peat, peptide elongation factors, phylogeny, polyethylene, pycnidia, ribosomal DNA, runoff, seedlings, sodium chloride, spores, stems, surveys, translation (genetics), tubulin, vigor, Switzerland
Common speedwell (Veronica officinalis) is grown in Switzerland for the production of herbal candies. The above-ground plant parts are harvested and dried for further use. Since 2013, a yield decline was observed in the Emmental region, the main speedwell-growing area of Switzerland. In August 2014, a survey to determine the cause was conducted. In all five surveyed fields, plants with strongly reduced growth vigor were observed, and from each field plant samples were taken for further investigation. On all plants, brown necrotic lesions, with a size varying from 2 to 3 mm up to 2 cm and containing black pycnidia, occurred on leaves and stems. After 1 week of incubation in a humid chamber, white spore masses were ejected from the pycnidia. The spores were translucent, biguttulate, strait to slightly curved, ellipsoid-shaped with a size of 4 to 7 × 1.5 to 3 µm, and tentatively identified as a Phoma species (Sutton 1980). The morphology and molecular phylogeny of a monospore isolate, obtained from spores ejected from a pycnidium on a diseased leaf and designated MIV-262 (CBS 141361), was cultured as described before (Boerema et al. 2004) and studied in more detail by molecular methods (Aveskamp et al. 2010). Based on its internal transcribed spacer regions 1 and 2 and intervening 5.8S nrDNA, actin, beta-tubulin, calmodulin, and translation elongation factor 1-alpha gene sequences (GenBank nos. CBS141361 CBS141361act KY550225, CBS141361 CBS141361tub2 KY550226, CBS141361 CBS141361cmda KY550227, CBS141361 CBS141361tef1 KY550228, and CBS141361 CBS141361ITS KY550229), it could be placed in a consensus phylogenetic tree of 66 Boeremia species reported by Berner et al. (2015) and was identified as Boeremia exigua var. exigua. To confirm Koch’s postulates, in 2014–2015 pathogenicity of B. exigua var. exigua on speedwell was confirmed in a pot trial at Agroscope. Seeds were placed on water agar plates, and after germination, healthy seedlings in the dicotyledon stage were transplanted into 0.3 l pots filled with a commercial peat substrate. The plants were transferred to the greenhouse at 15 to 18°C, with additional light 16 h/day. Isolate MIV-262 was grown on potato dextrose agar (PDA) supplemented with chlortetracycline and incubated for 3 weeks at 24°C in darkness. A suspension of 2 × 10⁶ spores/ml was prepared in sterile water containing 0.9% (w/v) NaCl. Four 3-month-old plants with a diameter of 12 to 15 cm were sprayed with the spore suspension until run-off. Four plants sprayed with the 0.9% (w/v) NaCl solution alone served as controls. After inoculation, individual plants were covered with a polyethylene bag and incubated for 1 week. Then, the bags were removed and plants left in the greenhouse until brown lesions developed on leaves and stems of inoculated plants. After 3 to 4 weeks, lesions reached up to 2 cm in diameter and contained black pycnidia. On control plants no such symptoms were observed. B. exigua var. exigua could be reisolated and cultured on PDA from spores ejected from the pycnidia. To our knowledge, this is the first report of B. exigua var. exigua as a pathogen of V. officinalis (Farr and Rossman 2017). On an unidentified species of Veronica, B. exigua var. gilvescens was reported (Berner et al. 2015), but it was not mentioned whether this isolate was pathogenic on that species.