In September of 2008, declining grapevines were observed in two vineyards (Vitis interspecific hybrids ‘Canada Muscat’ and ‘Chardonel’) in Fennville, MI. Affected vines were stunted with shortened internodes and yellow leaves; others had dead cordons or were entirely dead. The grower reported that vines were losing vigor and collapsing during a period of 2 years. Renewal trunks would collapse during the second season of growth. Several symptomatic vines showed signs of root decay. On one vine, distinctive fruiting bodies (mazaedia) were apparent on the roots below the soil line and resembled those of Roesleria subterranea (Weinm.) Redhead (2,3,4). The mazaedia were 4 to 5 mm tall and 1 mm in diameter with white-to-tan stipes and powdery, gray-to-greenish hemispherical heads. Ascospores were hyaline to light grayish green, disk shaped, and 4 to 6 μm in diameter. This fungus, also known as R. hypogaea Thüm & Pass., has been previously reported to cause grape root rot, vine decline, and replant problems in North America and Europe (2,3,4). The fungus was cultured from ascospores on potato dextrose agar (PDA). Colonies grew slowly (approximately 2 mm per day at 22 to 24°C) and were green in the center. No spores were produced. DNA was extracted, and internal transcribed spacer (ITS) sequences obtained by PCR were compared with known sequences using BLASTn (1). Our isolate had 100% ITS sequence homology to an isolate from Germany, Roesleria subterranea strain IB (Accession No. EF060304.1). To test for pathogenicity, the fungus was grown in potato dextrose broth for 14 days at 22 to 24°C. An aqueous suspension (0.1 g of fungus per ml) was prepared by blending mycelia with sterile deionized water (SDW) in a food processor. Five two-node, rooted ‘Chardonnay’ cuttings (45 days old) were placed in the suspension. Five other cuttings were placed in SDW (control). After 3 h, plants were removed and repotted in fresh BACTO soil (Michigan Peat Company, Houston, TX) and kept in a growth chamber at 23°C with a 16/8-h light/dark cycle. After 25 days, inoculated plants were, on average, 63% smaller with 77% lower fresh-root weight and fewer fine roots than the control plants. The pathogen was recovered from surface-disinfested roots of all five inoculated plants but not from the control plants. Cultures appeared similar to the original isolate and their identity was confirmed by ITS sequencing. To our knowledge, this is the first report of R. subterranea on grapes in Michigan and the Midwest. This fungus needs to be recognized as a potential cause of vine decline and replant problems in Midwestern vineyards.
References: (1) S. F. Altschul et al. J. Mol. Biol. 215:403, 1990. (2) W. Gärtel. Page 40 in: Compendium of Grape Diseases. R. C. Pearson and A. C. Goheen, eds. The American Phytopathological Society, St. Paul, MN, 1988. (3) M. Kirchmair et al. Mycol. Res. 112:1210, 2009. (4) J. R. Liberato et al. Pest and Diseases Image Library. Online publication, 2009.
