An Integrated Approach for Controlling Verticillium Wilt of Strawberry

Strawberry (Fragaria x ananassa, Duch.) is an important crop in California, with more than 35,000 acres planted in 2018 resulting in a farm gate value of $3.1 billion. In 2020, California strawberry production accounted for more than 85% of national strawberry production and faces serious threats to production due to various soil-borne diseases. One such disease, Verticillium wilt, is caused by the fungal pathogen Verticillium dahliae and is commonly found in temperate zones around the world where strawberries are grown. Due to the phase-out of efficacious fumigants like methyl bromide, alternative disease management methods have become necessary to alleviate threats to production. Alternative fumigation practices such as crop termination have recently been investigated, and the integration of crop termination with bed fumigation and host resistance can play an integral role in control of Verticillium wilt. A field trial was established at California Polytechnic State University, San Luis Obispo to examine the efficacy of integrative management solutions for control of Verticillium wilt of strawberry in a naturally infested field. The efficacy of sequential fumigation applications of crop termination and bed fumigation was examined. Further, the integration of a resistant cultivar was also implemented in hopes of further decreasing plant mortality and increasing yield. Different fumigant products such as metam potassium, metam sodium, and chloropicrin were used to assess their performance in different fumigation applications. Metam potassium and metam sodium were used for crop termination. When used for crop termination, both products delivered significant reduction in soil inoculum density and adequate crop injury. Metam potassium, metam sodium, and chloropicrin were used for bed fumigation. All products reduced soil inoculum density. Lower plant mortality and higher yield resulted from sequential applications of crop termination and bed fumigation, with average plant mortality for non-treated control plots and sequentially fumigated plots being 67.2% and 24.1%, respectively. There were no significant increases in yield for plots bed fumigated and sequentially crop terminated and bed fumigated, but significant increases in yield for all plots treated versus the non-treated plots were found. The integration of a moderately resistant cultivar Valiant after the fumigation series showed lower mortality and higher yield versus a susceptible cultivar Seascape. A two-year study was also conducted in order to evaluate host resistance to Verticillium wilt in 74 cultivars and elite breeding lines from five strawberry breeding programs. Genotypes were established in a field naturally infested with V. dahliae on the campus farm at California Polytechnic State University, San Luis Obispo. All five breeding programs had a wide range of susceptibility to Verticillium wilt, ranging from 1.5% to 100% mortality for both years of the trial. Twenty-three cultivars and elite breeding lines were common to both years of the trial; of these, five cultivars showed vastly different results between the two years. For example, ‘Monterey’ showed 78.8% mortality in 2021 and 11.5% mortality in 2022. This demonstrates the importance of evaluating host resistance over multiple years under different environmental conditions and field locations