First Report of Brown Root Rot of Alfalfa Caused by Phoma sclerotioides in Maine, Ontario, and Pennsylvania.

Brown root rot (BRR), caused by the fungal pathogen Phoma sclerotioides G. Preuss ex Sacc. (synonym Plenodomus meliloti Dearn. & G.B. Sanford), is associated with yield loss of alfalfa (Medicago sativa L.) in regions with severe winters (1). In the spring of 2007, 9 to 69 alfalfa plants were collected from each of five production fields in Maine, 10 fields in Ontario, and nine fields in Pennsylvania. All alfalfa stands existed at least two winters. P. sclerotioides was isolated from alfalfa root and crown lesions from five fields in Maine (Penobscot, Somerset and Waldo counties), seven fields from southwestern (Woodstock and Niagara), south-central (Lindsay and Belleville), and southeastern Ontario (near Ottawa), and four fields in Pennsylvania (Columbia, Crawford, and Jefferson counties; 41.1 to 41.6°N). BRR incidence was 9 to 29% in Maine, 5 to 29% in Ontario, and 8 to 22% in Pennsylvania. In Ontario, some lesions girdled the crown; in three fields in Maine, large pycnidia characteristic of P. sclerotioides were present on alfalfa crowns and overwintered stems. On potato dextrose agar, conidia (5 to 8 × 2 to 3 µm, unicellular, hyaline, and ovoid) and pycnidia (0.33 to 1.15 mm in diameter with multiple beaks) of single-conidium isolates were characteristic of P. sclerotioides (2). Diagnostic PCR (3) of isolates resulted in a single amplicon of expected size (500 bp). The internal transcribed spacer (ITS) 1, 5.8S, and ITS2 of the rDNA were sequenced for 12 representative isolates, and sequences (GenBank Accession Nos. FJ179151 to FJ179162) were 95.5 to 100% identical to P. sclerotioides ATCC isolate 56515 over a 488-bp alignment. Eight months after seeding, potted 'Vernal' alfalfa was inoculated (4), kept at 4°C for 8 weeks, 0 to -2°C for 12 weeks, 4°C for 8 weeks, and 10 to 15°C for 7 weeks. Of 108 plants inoculated with the Maine isolates, 35 developed severe cortical lesions and 16 died. Of 18 plants inoculated with the Ontario isolates, 16 developed severe cortical lesions and eight died. Of 18 plants inoculated with a Pennsylvania isolate, 11 developed severe cortical lesions and five died. Lesions were typical of BRR: light to very dark brown, sometimes with a darker border, and often containing abundant pycnidia. Plant mortality was associated with lesions that girdled the root and crown. Of 18 plants in the control treatment, three developed severe cortical lesions and none died. BRR is common in Alberta, Saskatchewan, and Manitoba, but in eastern Canada it has been reported only in Nova Scotia. To our knowledge, this is the first report of BRR in Maine, Ontario, and Pennsylvania and the southernmost report of BRR in eastern North America. References: (1) B. Berkenkamp et al. Can. J. Plant Sci. 71:211, 1991. (2) G. H. Boerema et al. Persoonia 15:431, 1994. (3) R. C. Larsen et al. Plant Dis. 86:928, 2002. (4) M. J. Wunsch et al. Plant Dis. 91:1293, 2007.

First Report of Brown Root Rot of Alfalfa Caused by Phoma sclerotioides in Colorado and New Mexico.

Brown root rot (BRR), caused by the fungal pathogen Phoma sclerotioides G. Preuss ex Sacc. (synonym Plenodomus meliloti Dearn. & G.B. Sanford), is associated with winterkill, slow emergence from winter dormancy, and yield loss of alfalfa (Medicago sativa L.) (1,2). BRR is a problem in regions with severe winters and is common in Alaska and Alberta, Saskatchewan and Manitoba, Canada. It was first observed in the continental United States in Wyoming during 1996 (2) and has subsequently been found in Idaho, Minnesota, Montana, New Hampshire, New York, Vermont, and Wisconsin. In the intermountain valleys of northern New Mexico and western Colorado, winters can be severe; alfalfa winterkill events occur periodically, but it is unknown if BRR is present. In May 2006, alfalfa plants were collected from production fields in Huerfano, Otero, and Rio Grande counties in Colorado and Rio Arriba and Taos counties in New Mexico and assessed for BRR. Two to three fields were sampled per county and 20 or 40 plants were collected per field. All fields existed for at least two winters. Fields sampled in Rio Grande County exhibited severe winterkill, with most plants completely girdled by crown lesions. Plants from other fields exhibited a range of root and crown rots. Isolation of P. sclerotioides was attempted from all plants with a previously described protocol (4). The pathogen was isolated from crown lesions of one alfalfa plant each from Rio Grande and Taos counties. Both lesions extended into the cortex. On potato dextrose agar and water agar with barley (4), single-conidium cultures of each isolate produced large pycnidia (0.35 to 0.80 mm in diameter) with multiple beaks, white cirri darkening to yellow with age, and unicellular, hyaline, ovoid conidia 5 to 7 µm long by 2 µm wide. Diagnostic PCR of the cultures using P. sclerotioides-specific primers (3) resulted in a single amplicon of expected size (500 bp). The internal transcribed spacer (ITS) 1, 5.8S, and ITS2 of the rDNA were amplified and sequenced using primers ITS1 and ITS4. The ITS sequences (GenBank Accession Nos. EU265669 and EU265670) were >98% identical to P. sclerotioides ATCC isolate 56515 over 503 bp of aligned sequence. Potted 'Vernal' alfalfa was inoculated 4 months after seeding, kept at 4°C for 5.5 weeks, 0 to -2°C for 12 weeks, and 4°C for 3 weeks. Of the 14 plants inoculated with the Colorado isolate, 11 developed cortical lesions and 8 winterkilled. Of the 23 plants inoculated with the New Mexico isolate, 22 developed cortical lesions and 16 winterkilled. Lesions were light to very dark brown, sometimes with a darker border and often containing abundant pycnidia. Winterkill was associated with lesions girdling the crown. P. sclerotioides was isolated from the lesions. To our knowledge, this is the southernmost report of BRR in North America and the first report of BRR in New Mexico and Colorado. The incidence and severity of BRR in the region surveyed appear to be considerably lower than in the more northern regions. References: (1) B. Berkenkamp et al. Can. J. Plant Sci. 71:211, 1991. (2) C. R. Hollingsworth et al. Can. J. Plant Pathol. 25:215, 2003. (3) R. C. Larsen et al. Plant Dis. 86:928, 2002. (4) M. J. Wunsch et al. Plant Dis. 91:1293, 2007.

Distribution, Impact, and Soil Environment of Phoma sclerotioides in Northeastern U.S. Alfalfa Fields.

We report brown root rot (BRR) of alfalfa, caused by the fungal pathogen Phoma sclerotioides, for the first time in the eastern United States. Alfalfa production fields in New York, Vermont, and New Hampshire were sampled in spring 2005, and soil characteristics were related to variability in BRR incidence and severity in two New York fields sampled extensively. BRR was detected in 8 of 10 fields sampled in New York, 6 of 7 fields sampled in Vermont, and 5 of 6 fields sampled in New Hampshire. Lesions on both roots and crowns were common in all three states, and most BRR lesions extended into the cortical tissues. Diagnostic polymerase chain reaction (PCR) of P. sclerotioides isolates produced a single amplicon of the expected size. In vivo conidia and pycnidia morphology of northeastern isolates was consistent with published descriptions of P. sclerotioides, and P. sclerotioides was reisolated from symptomatic lesions after pathogenicity testing. In two New York fields sampled extensively, BRR severity varied with soil strength, soil texture, soil saturation, and alfalfa stand density. The spatial pattern of BRR within fields suggests the pathogen was not recently introduced. The results suggest BRR is widespread in alfalfa production fields in New York, Vermont, and New Hampshire.