Integrated Management Strategies for Phytophthora sojae Combining Host Resistance and Seed Treatments.

Phytophthora sojae has re-emerged as a serious soybean pathogen in the past decade. This may be due in part to changes in resistance levels in current cultivars, adoption of P. sojae populations to deployed Rps genes, and highly favorable environments in the past decade. This multilocation study evaluated the effect of seed treatments on the incidence and severity of Phytophthora root and stem rot on soybeans with different combinations of Rps genes and levels of partial resistance. The efficacy of the seed treatments was highly variable across locations. Seed treatments (metalaxyl and mefenoxam) provided protection and increased yields across cultivars in locations where rain or irrigation occurred shortly after planting (Ohio, South Dakota, and Ontario). However, there were no significant differences in stand or yield consistently across cultivars in Iowa, Nebraska, Wisconsin, or Ohio, where heavy precipitation did not occur until later growth stages. The environment, levels of inoculum, and pathogen complex may have played a role in the different responses to the seed treatments and to the different combinations of Rps genes and levels of partial resistance to P. sojae in the cultivars. Fields that are poorly drained and have P. sojae populations with complex pathotypes may benefit the most from seed treatments. Individual fields where producers may see the greatest benefit to utilizing these integrated management strategies will need to be identified.

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 Bean pod mottle virus in Soybean in Canada.

In 2001, soybean fields were surveyed to determine the incidence of viruses because soybean aphids (Aphis glycines Matsamura), known to transmit Soybean mosaic virus (SMV) (2), were found in Ontario. In addition, bean leaf beetle (Cerotoma trifurcata Forster) was found during 2000 to be contaminated with Bean pod mottle virus (BPMV), although soybean plants, on which the beetles were feeding, tested negative (3). In the current survey, young soybean leaves were selected at random in July and August from 20 plants per site at growth stages R4 to R5 (1) from 415 sites representing the entire soybean-producing area in Ontario. Samples were maintained under cool conditions until received at the laboratory, where they were promptly processed. A combined sub-sample was obtained from the 20 plants per site. The 415 sub-samples were tested for SMV, BPMV, Tobacco ringspot virus (TRSV), and Tobacco streak virus (TSV) using polyclonal antibody kits for double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) (Agdia Inc., Elkart, IN). The ELISA plates were read with a plate reader (MRX, Dynex Technologies Inc., Chantilly, VA), and results were analyzed using ELISA software (Leading Edge Research, Merrickville, Ontario) and compared positive and negative controls (Agdia). TRSV was detected in one sample from Essex County and another sample from Middlesex County. SMV, BPMV, and TSV were not found in commercial soybean fields. However, SMV and BPMV were found in samples originating from two soybean breeding nurseries, one in Essex County and one in Kent County. Seedlings of soybean cv. Williams 82 were inoculated in the greenhouse with sap from leaf samples that tested positive for BPMV. Leaves of plants that developed mosaic symptoms were retested using ELISA and confirmed to be positive for BPMV. SMV and TRSV have been found previously in commercial soybean fields in Ontario (4). To our knowledge, this is the first report of BPMV on soybean plants in Canada. References: (1) W. R. Fehr et al. Merr. Crop. Sci. 11:929, 1971. (2) J. H. Hill et al. Plant Dis. 85:561, 2001. (3) A. U. Tenuta. Crop Pest. 5 (11):8, 2000. (4) J. C. Tu. Can. J. Plant Sci. 66:491, 1986.

First Report of Fusarium solani f. sp. glycines Causing Sudden Death Syndrome of Soybean in Canada.

Sudden death syndrome (SDS), caused by Fusarium solani (Mart.) Sacc. f. sp. glycines, is a disease of soybean (Glycine max (L.) Merr.) in several central and southern states of the United States. In Ontario, Canada, individual soybean plants with typical foliar symptoms of SDS (1) have been observed annually in Kent County since 1993 but the causal organism was not isolated or identified. In 1996, plants with symptoms of SDS were observed in six fields located in Essex, Kent, and Lambton counties. Interveinal chlorosis and necrosis occurred on top leaves of affected plants and a pale brown discoloration occurred in the vascular system in lower stems and upper tap roots. Slow-growing isolates of F. solani f. sp. glycines with typical blue sporodochia were isolated from symptomatic plants on acidified potato dextrose agar (1). Root inoculation of 15 2-week-old seedlings with colonized oat kernals with each of five single-spore isolates caused typical SDS symptoms on 5-week-old soybean plants of cvs. Conrad, A2540, S19-90, and Ripley in the greenhouse. The severity and incidence of symptoms varied with cultivar and isolate. Of 125 plants inoculated, 6% of Conrad, 10% of A2540, 14% of S19-90, and 17% of Ripley plants developed foliar symptoms. Symptoms did not develop on noninoculated controls. F. solani f. sp. glycines was reisolated from roots of symptomatic plants. Although Ripley is known to have resistance to SDS (2), foliar and root symptoms developed following inoculation with each Ontario isolate of F. solani f. sp. glycines. Yield losses in 1996 were difficult to assess because of the scattered distribution of diseased plants in most fields. Diseased plants had few and poorly filled pods. In two fields, soybean growth was severely restricted in large areas covering 2 ha each; however, soybean cyst nematode (SCN) was present in both fields. SCN was present at all locations. Although total yield losses are currently low, it is evident that F. solani f. sp. glycines causing SDS has become widely distributed in southwest Ontario and disease severity is increasing. References: (1) K. W. Roy et al. Phytopathology 79:191, 1989. (2) P. A. Stevens et al. Crop Sci. 33:929, 1993.