Globally, 15 Pythium species have been found to cause damping-off and seed rot of alfalfa, although surveys of species causing disease on alfalfa in the midwestern United States are lacking. Pathogens were isolated by a seedling baiting technique from soil samples of five alfalfa fields in Minnesota with high levels of damping-off. Of the 149 organisms isolated, 93 (62%) were identified as Pythium spp. and 43 (29%) were identified as Fusarium species. Pythium sylvaticum, P. irregulare, and P. ultimum var. ultimum were aggressive pathogens on germinating alfalfa seedlings. Strains of seven Pythium spp. pathogenic on soybean and corn were also pathogenic on alfalfa. The majority of the Fusarium isolates were identified as F. solani and F. oxysporum with a low number of F. red-olens and F. incarnatum-equiseti. The F. oxysporum and F. incarnatum-equiseti strains were the most aggressive in causing seed and root rot. Pythium strains were sensitive to Apron XL (mefenoxam) and pyraclostrobin in vitro but efficacy varied when the fungicides were applied as a seed treatment. Seed treatments with Apron XL were more effective than treatments with Stamina against Pythium. The presence of aggressive, broad-host-range pathogens causing seed rot and damping-off suggests that new strategies are needed for managing this disease in alfalfa production systems.
Bibliographical noteFunding Information:
We thank L. Radmer, B. Bucciarelli, and F. Breuillin-Sessoms for their assistance. J. Kurle and D. Malvick, University of Minnesota, kindly provided soybean pathogens. Funding was provided by the Minnesota Corn Research and Promotion Council, the Midwest Forage Association, and USDA-ARS-CRIS project 5062-12210-002D.
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