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Poster: Biology & Disease Mgmt: Chemical Control


Prevalence of metalaxyl resistance in Pythium populations in dryland agriculture in the US Pacific Northwest
W. CHEN (1), S. Guy (2), R. McGee (3), T. Paulitz (1), L. Porter (4), K. Schroeder (5), G. Vandemark (1) (1) USDA ARS, Washington State University, U.S.A.; (2) Wasington State University, U.S.A.; (3) USDA ARS, U.S.A.; (4) USDA ARS, Washington State Univer

Metalaxyl (mefenoxam) has been the primary fungicide used in managing Pythium damping off of chickpea and other pulse crops. Metalaxyl resistance among Pythiun spp. has been reported from the irrigated Columbia basin in the state of Washington where metalaxyl has been applied multiple times annually to manage potato diseases, but not from dryland agriculture where metalaxyl has been used only once per year in seed treatments. However, in the spring of 2014 and 2015, severe seed rot and damping-off of metalaxyl-treated chickpea seeds were found in the dryland agriculture region in southeastern Washington and northern Idaho, and were associated with Pythium isolates with high levels of resistance to metalaxyl. High densities of metalaxyl-resistant Pythium populations were found in areas of severe chickpea seed rot and damping-off, and were observed in seven commercial chickpea fields in southeastern Washington and northern Idaho. P. ultimum var. ultimum dominated the populations that were highly resistant to metalaxyl. ED50 values for metalaxyl-resistant isolates were significantly higher than those of metalaxyl-sensitive isolates. Under controlled conditions, metalaxyl treatments failed to protect chickpea seeds from seed rot and damping-off following inoculation with metalaxyl-resistant Pythium isolates. An effective alternative measure is urgently needed to successfully manage metalaxyl-resistant Pythium in chickpea production.