Survey of late-season soilborne pathogens infecting strawberry in Watsonville-Salinas, California

Authors: STEELE, MARY - California Polytechnic State University; HEWAVITHARANA, SHASHIKA - California Polytechnic State University; Henry, Peter; Goldman, Polly; HOLMES, GERALD - California Polytechnic State University

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2022
Publication Date: 3/29/2023
Citation: Steele, M.E., Hewavitharana, S.S., Henry, P.M., Goldman, P.H., Holmes, G.J. 2023. Survey of late-season soilborne pathogens infecting strawberry in Watsonville-Salinas, California. Plant Health Progress. 24(1):104-109. https://doi.org/10.1094/PHP-06-22-0056-S.
DOI: https://doi.org/10.1094/PHP-06-22-0056-S

Interpretive Summary: Strawberry plants grown commercially are threatened by several fungal pathogens, some of which survive and spread in the soil. The diseases caused by soilborne pathogens that are currently focused on the most in the California strawberry industry are Verticillium wilt, Macrophomina root rot, Fusarium wilt, and Phytophthora crown rot. A disease survey was conducted between August and October 2021 in the Watsonville-Salinas strawberry district of California to see which of the four were most prevalent. Strawberry plants showing symptoms of disease were collected from 69 fields (representing 31% of strawberry fields in the district), then molecular and plating techniques were conducted on each set of plants to screen for the four soilborne pathogens. The four pathogens were recovered from 22 to 30% of samples, so all represented a significant problem for growers. Almost 30% of samples were not positive for any of the four soilborne pathogens. The cause of symptoms in cases like these is the subject of ongoing research at the Cal Poly Strawberry Center. More strawberry disease surveys in the Watsonville-Salinas district and elsewhere are needed to add to these findings.

Technical Abstract: To determine the soilborne pathogens involved in strawberry plant death, 74 plant samples were collected from 69 ranches in the Watsonville-Salinas growing district of California between 11 August and 15 October 2021. Field samples consisted of eight plants exhibiting moderate to severe plant collapse. Crown tissue from each plant was excised and pooled for recombinase polymerase amplification (RPA) to detect four major soilborne pathogens: Macrophomina phaseolina, Fusarium oxysporum f. sp. fragariae, Verticillium dahliae, and Phytophthora spp. Root, petiole, and crown tissue from plant samples in which no pathogens were detected by RPA was plated on semi-selective media to verify the absence of the four pathogens and screen for other pathogenic fungi. At least one of the four pathogens was detected in 54 of the 74 samples (72.9%). Fusarium oxysporum f. sp. fragariae was detected in 22 samples (29.7%), M. phaseolina in 21 samples (28.4%), Phytophthora sp. in 18 samples (24.3%), and V. dahliae in 16 samples (22.0%), demonstrating that all four of the major soilborne pathogens are prevalent in this growing district. No strong associations were found between the pathogens and field characteristics (i.e., cultivar, plant spacing, disease incidence, soil type, plastic mulch color, and fumigation history).