Which soilborne pathogens are most common in California strawberry fields? Part 1 – Watsonville/Salinas district

In the California strawberry industry, it’s widely accepted that there are four major diseases caused by soilborne pathogens: Macrophomina root rot (caused by Macrophomina phaseolina) (Fig. 1), Fusarium wilt (caused by Fusarium oxysporum f. sp. fragariae), Verticillium wilt (caused by Verticillium dahliae), and Phytophthora crown and root rot (caused by Phytophthora spp.).

**Figure 1.** Example of plant death experienced by many California strawberry growers due to soilborne pathogens. In this case, it’s Macrophomina root rot. Note how plants near the edge of the bed die first. (Photo by M. Steele)

What we did not know is which of these four pathogens are most common? A survey conducted in 2021 in the Watsonville/Salinas growing district sought to answer this question. The survey was extended to the Santa Maria district in 2022 and to Oxnard in 2023. Here I report on the results from the Salinas/Watsonville survey.

Strawberry plants showing symptoms of root disease were taken from 69 commercial fields, representing 34% of the acreage throughout the Watsonville/Salinas district. Molecular diagnostics (recombinase polymerase amplification or RPA) were then conducted on crown tissue for each of the four pathogens, and if results were negative for all four pathogens, crowns, roots, and petioles were plated on semi-selective media for additional pathogen diagnostics.

Results

In Watsonville/Salinas in 2021, 74% of plant samples from 69 fields were positive for at least one of the four major pathogens, and the four pathogens had similar prevalence (Fig. 2):
• Fusarium oxysporum f. sp. fragariae in 31% of fields
• Macrophomina phaseolina in 30%
• Phytophthora spp. in 24%
• Verticillium dahliae in 22%

**Figure 2.** Relative prevalence of the four major soilborne pathogens of California strawberry detected in Watsonville/Salinas fields in 2021.

Co-infection: 35 samples (47%) were positive for a single pathogen, 16 samples (22%) were positive for two pathogens, and 4 samples (5%) were positive for three pathogens. The number of instances of each pathogen combination is shown in Table 1.

Table 1. Instances of co-infection for each possible combination among the four pathogens.

	*Verticillium dahliae*	*Macrophomina phaseolina*	*Fusarium oxysporum* f. sp. fragariae	*Phytophthora* spp.
*Verticillium dahliae*	—	4	1	7
*Macrophomina phaseolina*	—	—	7	6
*Fusarium oxysporum* f. sp. fragariae	—	—	—	3

What does this mean? In Watsonville/Salinas, all four pathogens are prevalent. Furthermore, infection with two or three pathogens in the same field is common. This tells us that we should be paying roughly equal attention to each of these pathogens and the diseases they cause in Watsonville/Salinas. It’s also important to remember that these results are from a single year and that results will likely vary based on weather conditions in a given year.

The Santa Maria survey is currently being submitted for publication and the Oxnard survey is currently in progress. We will report on those surveys in future blog posts.

Footnote: This project was a collaboration among Mary Steele (MS student, Cal Poly Strawberry Center), Shashika Hewavitharana (Assistant Professor and Program Leader, Cal Poly Strawberry Center), Peter Henry (Research Plant Pathologist, USDA ARS, Salinas), Poly Goldman (Research Technician, USDA ARS, Salinas), and Gerald Holmes (Director, Cal Poly Strawberry Center).

Reference

Steele, M., Hewavitharana, S., Henry, P., Goldman, P. and Holmes, G. 2023. Survey of late-season soilborne pathogens infecting strawberry in Watsonville-Salinas, California. Plant Health Progress 24:104-109. https://doi.org/10.1094/PHP-06-22-0056-S