Some strawberry varieties are genetically resistant to the Fusarium wilt disease. For example, Fronteras, Portola, and San Andreas are Fusarium wilt-resistant varieties from the University of California. A description of disease resistance for currently available varieties is on the California Strawberry Commission web site. All Fusarium wilt-resistant varieties have the same resistance gene, called “FW1”. Until now, this gene was effective against every known strain of the Fusarium wilt pathogen, called Fusarium oxysporum f. sp. fragariae race 1 (Fof race 1), in California.
In fall of 2022, Dr. Peter Henry (USDA-ARS) consulted with a grower at a summer-planted, organic field in Oxnard, CA. This field had severe wilt disease with more than 50% mortality in some areas (Fig. 1). Macrophomina phaseolina was suspected to be the causal agent, because Portola was planted and it is resistant to Fof race 1. However, Dr. Henry observed foliar chlorosis symptoms that are associated with Fusarium wilt, suggesting that Fof could also be present (Fig. 2).
Two plant samples were collected at the field. One sample tested negative using molecular (RPA) methods for the four known soilborne pathogens at the USDA-ARS in Salinas. Concerned by the presence of Fusarium wilt symptoms, petioles were also plated on a Fusarium spp.-selective medium. These petioles yielded abundant growth of F. oxysporum. Pure cultures isolated from these plates were tested for pathogenicity on Fusarium wilt resistant (Fronteras and San Andreas) and susceptible (Monterey) varieties.
All three varieties were highly susceptible to the new isolates and the pathogen could be recovered from diseased tissues. These results confirmed that a new, “resistance-breaking” strain of Fof was present in this field. This strain is called Fof “race 2” to indicate it can cause disease on varieties with the FW1 gene. The new Fof race 2 could not be identified by commonly used qPCR and RPA protocols. Instead, the frequent recovery of F. oxysporum isolates from petioles and negative results for Fof race 1 qPCR assays were important clues leading to its identification (Fig. 3).
Once Fof race 2 was discovered, researchers from the USDA-ARS, California Strawberry Commission, Cal Poly Strawberry Center, University of California Cooperative Extension, and Driscoll’s rapidly organized a survey of summer-planted fields in Ventura County. Symptomatic plants were collected from 25 fields with evidence of wilt disease. Plant samples were subjected to petiole plating (to isolate potential Fof cultures) and molecular RPA assays (to determine if other known pathogens were present).
Based on these assays and subsequent genetic testing, Fof race 2 was confidently detected at one other location in Oxnard. Plants from multiple nurseries were equally affected, indicating Fof race 2 was already in the soil and did not arrive with transplants. Growers at affected sites are working to minimize the potential for Fof race 2 to spread. At present, Fof race 2 is likely to be limited in distribution. In fields where Fof race 2 is not present, varieties with resistance to Fof race 1 (e.g. Fronteras, Portola, and San Andreas) remain effective at preventing disease.
Isolates of Fof race 2 appear to be identical, based on initial genetic tests. Whole genome sequencing indicates this Fof race 2 strain is new to science – it has never been observed worldwide. The current hypothesis is this strain evolved recently near Oxnard.
Research on the prevalence, origin, and management of this new pathogen is ongoing and likely to progress rapidly over the next year. The objectives of this work include: 1) extensive, statewide sampling to identify other Fof race 2 infested fields, 2) developing a molecular diagnostic method that is specific to Fof race 2, and 3) developing best management practices for affected fields. The industry will be updated as new information becomes available.
Contact Peter Henry:
Peter M. Henry, Ph.D.
Research Plant Pathologist
1636 E Alisal St.
Salinas, CA 93905