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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: Phytophthora rubi isolates collected from raspberry (Rubus idaeus) in the Western United States show no differentiation in pathogenicity, fungicide resistance or genetic structure

Authors
item Stewart, Jane
item Kroese, Duncan
item Tabima, Javier -
item Larsen, Meredith
item Fieland, Valerie -
item Press, Caroline
item Zasada, Inga
item Grunwald, Niklaus

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 4, 2014
Publication Date: N/A

Interpretive Summary: Root rot caused by the water mold Phytophthora rubi is an economically important disease of red raspberry in the United States. Left unchecked, this disease will cause stunted plant growth and reduced yield, and in extreme cases can cause plant death. In order to effectively manage this disease more information is required regarding the diversity of P. rubi in western US raspberry production fields. To this end, multiple populations of P. rubi were collected from Oregon, Washington, and California and the ability of these populations to cause disease to raspberry as well as the sensitivity of these populations to a common fungicide were determined. In addition, the genetic diversity among these populations was determined using molecular techniques. It was found that P. rubi populations collected from the western US represent one large mixed population; in other words they did not differ in their ability to cause disease to raspberry, their sensitivity to common fungicide, or in their genetic structure. These results are significant because they will enhance raspberry plant breeding efforts because identified resistance will have broad applications across regions that produce raspberries. In addition, our results indicate that fungicide resistance may not currently be an issue within the western US raspberry industry. Every effort should be made to deploy a fungicide program in raspberry to avoid emergence of resistance.

Technical Abstract: Root rot of raspberry (Rubus idaeus), thought to be primarily caused by Phytophthora rubi, is an economically important disease in the western United States. The objectives of this study were to determine which Phytophthora species are involved in root rot, examine the efficacy of different isolation methods (cane, root, and root/soil baiting with young raspberry plants) relative to PCR-based detection, and to determine if pathogenicity, fungicide resistance, and/or genetic variation exists among P. rubi isolates collected from raspberry fields in the California, Oregon, and Washington. Of 275 samples, direct isolation from cane material resulted in a greater number of P. rubi isolates (39%), whereas root/soil baiting yielded the least (11%). The PCR method detected higher rates of P. rubi presence (68%) than any of the isolation methods. Sequencing of the internal transcribed spacer region of 293 of the total 745 collected Phytophthora isolates showed that all but one isolate (identified as P. bisheria) were P. rubi. Results of the pathogenicity and fungicide resistance to mefenoxam comparing 14 total isolates from WA, OR, and CA showed that isolates were similarly virulent against red raspberry and the EC-50 frequency distributions showed no significant difference. These results, combined with Amplified Fragment Length Polymorphism results show that P. rubi isolates from WA, OR and CA represent one large mixed population. This work provides novel insights into the detection and biology of P. rubi in western U.S. raspberry production systems.

Last Modified: 9/22/2014
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