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First Report of Phoma strasseri as a Pathogen of Stachys officinalis in Bulgaria

For several years, a leaf spot disease has been observed on Betony, Stachys officinalis (synonym Betonica officinalis), in an experimental field in Kazanlak, Bulgaria. The round to somewhat angular spots (6 to 8 mm diameter) are dark brown with a pale center and have a chlorotic halo. A Phoma species isolated from the lesions formed regular to irregular, light brown colonies on potato dextrose agar (PDA). The isolate was studied as described by de Gruyter and Noordeloos (2). After 7 days, the growth rate was 43 mm on oatmeal agar and 33 mm on malt agar; the colonies were olivaceous gray-to-glauceous gray with a regular outline and with finely floccose, white-to-olivaceous gray aerial mycelium. Pycnidia, produced after 2 weeks, were ostiolate, globose to subglobose, 120 to 280 μm in diameter, citrine or honey, and later olivaceous to olivaceous black. The conidiogenous cells were globose to bottle shaped, 2 to 6 × 3 to 5 μm. The conidia were hyaline and unicellular, 5 to 7.5 × 2.5 to 4.2 μm, cylindrical to ellipsoidal with several small, scattered guttules. Chlamydospores were absent. According to these in vitro characters and after comparing the isolate with several Phoma isolates present in the culture collection of the Dutch Plant Protection Service, Wageningen, the Netherlands, the fungus has been identified as Phoma strasseri Moesz. The pathogenicity of the isolate was confirmed by artificial leaf inoculation of potted S. officinalis plants with a spore suspension (8 × 10⁶ spores per ml) kept in a moist chamber for 48 h at a mean average temperature of 16°C. Leaf spots observed 4 to 5 days after inoculation were similar to those observed in the field. P. strasseri was subsequently reisolated from the spots. P. strasseri (synonym Phoma mentae Strasser) has been recorded as the cause of rhizome and stem rot on mint, Mentha spp., in Europe, Japan, and North America (3). In addition, this fungus has been found in New Zealand (strain identified at the Dutch Plant Protection Service, unpublished data). To our knowledge, this is the first report of P. strasseri on S. officinalis in Bulgaria. P. strasseri may produce septate conidia and, therefore, can be classified in Phoma section Phyllostictoides Zherbele ex Boerema (1). P. strasseri clearly differs from other Phoma species described on Lamiaceae: Phoma leonuri Letendre (Phoma section Plenodomus (Preuss) Boerema et al., pycnidia scleroplectenchymatous, conidia aseptate, 3.5 to 5.5 × 1.5 to 2.5 μm), Phoma dorenboschii Noordel. & de Gruyter (Phoma Sacc. section Phoma, conidia aseptate, 3 to 5.5 × 2 to 2.5 μm, producing dendritic crystals in vitro), and Phoma valerianae Henn. (Phoma Sacc. section Phoma, conidia aseptate, 2.5 to 4 × 1.5 to 2 μm). Occasionally P. strasseri has been isolated from other Lamiaceae, namely Monarda didyma (Dutch Plant Protection Service, unpublished data). There is also a report from Valeriana sp. (3).

References: (1) G. H. Boerema. Mycotaxon 64:321, 1998. (2) J. de Gruyter and M. E. Noordeloos. Persoonia 15(1):71, 1992. (3) C. E. Horner. Plant Dis. Rep. 55:814, 1971.