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United States Department of Agriculture

Agricultural Research Service

Research Project: Improved Strategies for Management of Soilborne Diseases of Horticultural Crops

Location: Horticultural Crops Research

Title: Sensitivity of Pythium irregulare, P. sylvaticum, and P. ultimum from forest nurseries to mefenoxam and fosetyl-al, and control of Pythium damping-off

Authors
item Weiland, Gerald
item Santamaria, Luisa -
item GRUNWALD, NIKLAUS

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 22, 2014
Publication Date: July 1, 2014
Citation: Weiland, G.E., Santamaria, L., Grunwald, N.J. 2014. Sensitivity of Pythium irregulare, P. sylvaticum, and P. ultimum from forest nurseries to mefenoxam and fosetyl-al, and control of Pythium damping-off. Plant Disease. 98:937-942.

Interpretive Summary: Fungicides, such as mefenoxam and fosetyl-Al, are often used to control Pythium disease in forest nurseries of the Pacific Northwest. However, it is not known whether fungicide resistance has developed in forest nurseries, or whether other fungicides would also provide good disease control. We evaluated three Pythium species for fungicide resistance and found that P. irregulare was more resistant to mefenoxam than either P. sylvaticum and P. ultimum. We also found two highly mefenoxam-resistant isolates of P. ultimum. All three species were equally sensitive to fosetyl-Al and no resistant isolates were found. In addition, we conducted a greenhouse study to determine which fungicides and biological control treatments were the most effective at preventing Pythium disease on Douglas-fir tree seedlings as caused by P. irregulare, and two other Pythium species, P. dissotocum and P. 'vipa'. We found that phosphorous acid and fosetyl-Al were consistently the most effective fungicides at preventing Pythium disease, but mefenoxam also worked. Our results show that mefenoxam resistance does occur in P. ultimum in forest nurseries. Therefore, growers should alternate using fungicides with different modes of action to prevent the continued development and spread of mefenoxam-resistant Pythium isolates. Growers should also be aware that Pythium species differ in their sensitivity to certain fungicides and should consider which Pythium species they have in their nursery before applying fungicides.

Technical Abstract: Fungicides are often used to supplement soilborne disease control in Pacific Northwest forest nurseries. Mefenoxam and fosetyl-Al are the most commonly used fungicides to suppress Pythium damping-off of tree seedlings. However, it is not known whether fungicide resistant Pythium isolates are present or if other chemistries are as effective for disease control. Soil isolates of P. irregulare, P. sylvaticum, and P. ultimum from three forest nurseries were evaluated for sensitivity to mefenoxam and fosetyl-Al in an in-vitro assay. In addition, a greenhouse study was conducted to compare disease control efficacy of fungicide and biological treatments in Douglas-fir seedlings inoculated with P. dissotocum, P. irregulare, and P. ‘vipa’. Pythium irregulare was approximately three times more resistant to mefenoxam (0.20 µg a.i./ml) than either P. sylvaticum (0.06 µg a.i./ml) or P. ultimum (0.06 µg a.i./ml). However, two P. ultimum isolates were 3000 to 5000 times more resistant to mefenoxam than any other P. ultimum isolate. All three Pythium species were equally sensitive to phosphorous acid and no resistant isolates were found. Both fungicides were primarily fungistatic and did not kill most Pythium isolates. In the disease control trial, phosphorous acid and fosetyl-Al consistently provided good protection against P. dissotocum, P. irregulare, and P. ‘vipa’. Mefenoxam provided good protection against damping-off caused by P. dissotocum and P. irregulare, and intermediate protection against P. ‘vipa’. Results of this study indicate that mefenoxam resistance is present among P. ultimum isolates in the forest nursery industry and growers should rotate fungicides with different modes of action (e.g., phosphonates) to prevent the continued development and spread of resistant Pythium isolates. Pythium community structure at individual nurseries should also be considered because of the differences in fungicide sensitivity observed among different Pythium species.

Last Modified: 8/19/2014
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