Fungicide Sensitivity of Isolates from the Washington Red Raspberry Industry.

is an important pathogen causing Phytophthora root rot of red raspberries worldwide. Management of this disease is partially achieved with fungicides, but efficacy has been low, and growers are concerned about fungicide resistance. To determine whether fungicide resistance is developing, species were isolated from 26 raspberry fields with root rot, identified, and evaluated for sensitivity to four fungicides: mefenoxam, phosphorous acid, oxathiapiprolin, and dimethomorph. The majority of the recovered 152 isolates were (143 isolates, 25 fields), with (8 isolates, 2 fields) and (1isolate, 1field) being found much less frequently. These results confirm as the dominant species affecting the Washington red raspberry industry. Almost all tested isolates were sensitive to all four fungicide chemistries, although three isolates were less sensitive to mefenoxam, with effective concentration for 50% growth inhibition (EC) values ranging from 3.53 to 100 μg active ingredient/ml. No resistance was detected against current fungicide label rates. However, other reasons were identified for why fungicides have been ineffective. Label rates vary widely by brand, and most fungicides are applied in the fall when is inactive. In addition, some phosphorous acid products are only labeled for foliar applications, which have been shown to be less effective than soil applications in other agricultural systems. Efficacy trials are needed to compare foliar and soil fungicide applications at different times of the year for their ability to control Phytophthora root rot in red raspberry production fields.