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ARS Home » Pacific West Area » Corvallis, Oregon » Horticultural Crops Disease and Pest Management Research Unit » Research » Publications at this Location » Publication #411052
Research Project: Disease Management in Small Fruit and Nursery Crops Based on Knowledge of Pathogen Diversity, Biology, and Environmental Effects

Location: Horticultural Crops Disease and Pest Management Research Unit

Title: Fungicide sensitivity of Phytophthora rubi isolates from the Washington red raspberry industry

Author
item Weiland, Gerald - Jerry
item Scagel, Carolyn
item BENEDICT, CHRIS - Washington State University
item WASKO DEVETTER, LISA - Washington State University
item Beck, Bryan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2024
Publication Date: 3/11/2024
Citation: Weiland, G.E., Scagel, C.F., Benedict, C., Wasko Devetter, L., Beck, B.R. 2024. Fungicide sensitivity of Phytophthora rubi isolates from the Washington red raspberry industry. Plant Disease. https://doi.org/10.1094/PDIS-12-23-2641-RE.
DOI: https://doi.org/10.1094/PDIS-12-23-2641-RE

Interpretive Summary: Phytophthora rubi is a harmful pathogen that causes root rot of red raspberries, resulting in plant death and significant yield losses. Fungicides have not been effective and there is concern about fungicide resistance. We examined whether Phytophthora isolates collected from 25 Washington raspberry fields were resistant to four fungicides. Most isolates were sensitive to all four fungicides, indicating that they should be effective for root rot management. Instead, we determined that growers were likely applying the fungicides at the wrong rate, at the wrong time of year, or to the wrong part of the plant. Additional experiments are needed to optimize the best rates and methods for applying fungicides to achieve root rot control.

Technical Abstract: Phytophthora rubi 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 in Washington state, Phytophthora species were isolated from 25 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 Phytophthora isolates were P. rubi (143 isolates, 25 fields), with P. megasperma (8 isolates, 2 fields) and P. gonapodyides (1 isolate, 1 field) being found much less frequently. These results confirm P. rubi as the dominant species affecting the Washington red raspberry industry. Almost all isolates were sensitive to all four fungicide chemistries, although three isolates were less sensitive to mefenoxam with EC50 values of 3.53, 6.32, and 100 µg ai/ml. No resistance was detected to 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 P. rubi 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 systems. Experiments are needed to compare foliar and soil fungicide applications at different times during the year for their ability to control Phytophthora root rot in red raspberry production fields.