Late summer disease symptoms in western Washington red raspberry fields associated with co-occurrence of Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans, but not Raspberry bushy dwarf virus

Authors: Weiland, Gerald - Jerry; BENEDICT, CHRIS - Washington State University; Zasada, Inga; Scagel, Carolyn; Beck, Bryan; Davis, Elizabeth; Graham, Kimberly; Peetz, Amy; Martin, Robert; DUNG, JEREMIAH - Oregon State University; REYES GAIGE, ANDRES - Oregon State University; THIESSEN, LINDSEY - North Carolina State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2017
Publication Date: 11/1/2017
Citation: Weiland, G.E., Benedict, C., Zasada, I.A., Scagel, C.F., Beck, B.R., Davis, E.A., Graham, K., Peetz, A.B., Martin, R.R., Dung, J.K., Reyes Gaige, A., Thiessen, L.S. 2017. Late summer disease symptoms in western Washington red raspberry fields associated with co-occurrence of Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans, but not Raspberry bushy dwarf virus. Plant Disease. 102(5):938-947. https://doi.org/10.1094/PDIS-08-17-1293-RE.
DOI: https://doi.org/10.1094/PDIS-08-17-1293-RE

Interpretive Summary: Plant and soil samples were evaluated from 24 red raspberry fields in northern Washington state to determine why raspberry plants were exhibiting symptoms of yellowing, stunting, wilting, and death in late summer. Our results showed that Phytophthora rubi, a root rot pathogen, was the primary pathogen most frequently associated with disease symptoms. However, Pratylenchus penetrans (a plant-parasitic nematode) was also more common on the roots of diseased plants, and the soil next to diseased plants contained higher numbers of Verticillium dahliae, a fungal wilt pathogen. Raspberry Bushy Dwarf Virus, on the other hand, was not associated with late summer disease symptoms. Together, all three soilborne pathogens (Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans) were twice as likely to occur on diseased plants than on healthy plants, indicating that a disease complex is likely responsible for causing the late summer disease symptoms. These results are important because they show all three soilborne pathogens are common in the Washington red raspberry industry and suggest that current disease control methods are not adequately controlling these pathogens. Disease control methods are being evaluated to improve control of Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans in red raspberry field soils.

Technical Abstract: 60% of the $109 million processed by the U.S. red raspberry industry is in northern Washington. In 2012, late summer disease symptoms were observed in many raspberry fields. These symptoms were initially attributed to Verticillium dahliae, but other soilborne pathogens (Phytophthora rubi, Pratylenchus penetrans) or Raspberry Bushy Dwarf Virus (RBDV) might also be involved. A survey was conducted to: 1) establish the incidence and soil population levels of V. dahliae in red raspberry production fields; 2) compare diagnostic methods for detecting V. dahliae from soil; and, 3) assess which pathogens are associated with late summer disease symptoms. Plant and soil samples were collected from 51 disease sites and 20 healthy sites in 24 fields. Samples were analyzed for the presence and quantity of each pathogen using traditional plating and extraction methods, quantitative qPCR, and ELISA. Results showed that V. dahliae was common in 88% of the fields and that qPCR was more sensitive at detecting the pathogen from soil than plating, particularly when populations were less than 1 ppg. Soil populations of V. dahliae were greater in disease sites, but the pathogen was detected with similar frequency from healthy sites and it was only isolated six times from the canes of diseased plants. Phytophthora rubi, P. penetrans, and RBDV were also common in production fields. Both P. rubi and P. penetrans (root populations only), but not RBDV, were more frequently found at disease sites than healthy sites and the amount of P. rubi detected was greater than from healthy sites. Regardless of detection method, V. dahliae, P. rubi, and P. penetrans, either with or without RBDV, were more likely to co-occur at disease sites than healthy sites, suggesting that a disease complex is present in raspberry production fields. Results indicate that P. rubi is the primary pathogen most strongly associated with late summer symptoms of disease, but that P. penetrans and high populations of V. dahliae may also be of concern. Disease control methods should therefore focus on all three soilborne pathogens.