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Title: Sensitivity of isolates of phytophthora capsici from the eastern United States to fluopicolide

item KEINATH, A - Clemson University
item Kousik, Chandrasekar - Shaker

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2011
Publication Date: 6/21/2011
Citation: Keinath, A.P., Kousik, C.S. 2011. Sensitivity of isolates of phytophthora capsici from the eastern United States to fluopicolide. Plant Disease. 95:1414-1419.

Interpretive Summary: Vegetable crops in the cucumber (watermelon, melon squash etc) and tomato (pepper eggplant etc.) family are very susceptible to the fungus known as Phytophthora capsici. This fungus causes diseases such as fruit rots and leaf blights that can result in severe economic losses in some years. A new chemical (fungicide) called “Presidio” was recently registered to control diseases caused by this fungus in the U.S.A. A collection of 69 strains of the fungus from Florida, Georgia, Michigan, North Carolina, and South Carolina were tested to determine their sensitivity to the fungicide “presidio”. Results showed that all tested strains were highly sensitive. The sensitivity values determined in these assays will help in monitoring the fungus over a period of time as the fungicide is now being used by American vegetable growers routinely. These established values will prove invaluable as baseline numbers should this damaging fungus change in the future.

Technical Abstract: Fluopicolide, a pyridinylmethyl-benzamide fungicide, was registered in the United States in 2008 to control diseases caused by Oomycete pathogens, such as Phytophthora capsici, on cucurbit and solanaceous vegetables. The main objective of this study was to determine baseline sensitivity to fluopicolide in isolates of P. capsici from the eastern United States. A total of 69 isolates that had not been exposed previously to fluopicolide were grown on fungicide-amended medium to determine sensitivity of mycelia, sporangia, and zoospores to the fungicide. All isolates of P. capsici tested (range of 54 to 69 isolates per assay) were sensitive to fluopicolide in all four assays. The median EC50 fluopicolide concentration was 0.22, 2.08, 0.048, and 0.10 mg/liter in the mycelia growth, zoospore germination, sporangia production, and zoospore production assays, respectively. For mycelia growth and zoospore germination, isolates from Michigan had a higher mean EC50 value than isolates from the four Southeastern states. This is the first report of variation in sensitivity to a fungicide by P. capsici isolates from different regions of the United States. In the sporangia production and zoospore production assays, isolates from different states did not differ in sensitivity. For future monitoring, fluopicolide concentrations of 0.3 to 1.0, 5.0 to 10.0, and 0.1 mg/liter are recommended as discriminatory doses for assays of mycelia growth, zoospore germination, and sporangia or zoospore production, respectively.