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Poster: Biology & Disease Mgmt: Chemical Control


Pythium species causing damping-off of alfalfa in Minnesota: Identification, pathogenicity and fungicide sensitivity
D. Samac (1), L. Berg (2), L. Radmer (2) (1) USDA-ARS, U.S.A.; (2) University of Minnesota, U.S.A.

Damping-off and seed rot is an important disease of alfalfa, severely affecting stand establishment when conditions favor the disease. Globally, 15 Pythium species cause damping-off and seed rot of alfalfa, although surveys of species causing disease on alfalfa in Minnesota are lacking. Eight species were isolated by a seedling baiting technique from soil of five alfalfa fields in Minnesota with high levels of damping-off. Three species, P. sylvaticum, P. irregulare, and P. ultimum var. ultimum, were pathogenic on germinating alfalfa seedlings at 21 C. Strains of seven species isolated from infected soybean, P. irregulare, P. intermedium, P. sylvaticum, P. recalcitrans, P. conidiophorum, P. ultimum var. sporangiiferum, and P. ultimum var. ultimum, were pathogenic on alfalfa. Assays with Apron XL (mefanoxam) treated seed showed that fungicide sensitivity varied between and with species with approximately 56% of strains insensitive. In Apron XL amended medium, hyphal density was reduced at all concentrations but all strains had a similar growth rate as on non-amended medium. Insensitivity to Stamina seed treatments (pyraclostrobin) occurred in 94% of strains tested. The presence of broad host range species and fungicide resistance suggests that crop rotation and these widely used seed treatments are not effective tools for managing this disease. These results indicate that resistant cultivars are needed for managing damping-off in alfalfa production systems.