Which soilborne pathogens are most common in California strawberry fields? Part 2 – Santa Maria district

In 2021, an effort began to determine which of the four major soilborne pathogens of strawberry in California – Macrophomina phaseolina, Fusarium oxysporum f. sp. fragariae, Verticillium dahliae, and Phytophthora spp. – are most common. Pathogen surveys of strawberry fields in the three major growing districts of California were proposed: Watsonville-Salinas, Santa Maria, and Oxnard. Watsonville-Salinas was surveyed in 2021 and results are outlined in Part 1 of this two-part blog article series – each of the four pathogens was detected in diseased plants from 22.0 to 31.1% of strawberry fields.

The next district to survey was Santa Maria in 2022, an effort carried out by M. Steele, M. Mendez, S. Hewavitharana, and G. Holmes. While the Watsonville-Salinas district is comprised only of fall-planted strawberry fields, Santa Maria has both fall- and summer-planted fields. The fields sampled in 2022 comprise 28% of 2021 fall-planted acreage and 14% of 2022 summer-planted acreage (Fig. 1). Methods used in the Watsonville-Salinas survey were followed: symptomatic strawberry plants were sampled from 68 fields, molecular diagnostics were conducted on crown tissue for each of the four pathogens, and when molecular diagnostics did not yield any positive results, crowns, roots, and petioles were plated on semi-selective media for further diagnosis.

**Figure 1.** Map of surveyed strawberry fields in the Santa Maria growing district (photo by Jason Sharrett, CA Strawberry Commission).

Results

In Santa Maria in 2022, 79% of plant samples from 68 fields were positive for at least one of the four major pathogens. Macrophomina phaseolina, the causal agent of Macrophomina root rot, was by far the most common being detected in 52% of the 100 total samples (Fig. 2).

**Figure 2.** Overall prevalence of the four major soilborne pathogens in Santa Maria, CA in 2022.

Co-infection: Of 100 total samples, 61 were positive for one of the four pathogens, 15 were positive for two pathogens, and three were positive for three pathogens. The number of instances of each pathogen combination is shown in Table 1.

Table 1. Instances of pairwise pathogen co-occurrence.

	*Verticillium dahliae*	*Macrophomina phaseolina*	*Fusarium oxysporum* f. sp. fragariae	*Phytophthora* spp.
*Verticillium dahliae*	—	5	2	2
*Macrophomina phaseolina*	—	—	2	9
*Fusarium oxysporum* f. sp. fragariae	—	—	—	0
Phytophthora spp.	—	—	—	—

An unexpected pathogen: the root-knot nematode (Meloidogyne spp.) was detected in six samples during soilborne pathogen diagnostic procedures, and four of those samples were later confirmed as Meloidogyne hapla. Although unlikely to cause plant mortality alone, the root-knot nematode can certainly reduce strawberry plant vigor especially in the presence of another soilborne pathogen. Characteristic symptoms of root-knot nematode are above-ground plant stunting and yellowing, and galling and deformation of roots (Fig. 3).

**Figure 3.** Strawberry plant root galling, characteristic of infection by root-knot nematode (*Meloidogyne* sp.) (photo by M. Steele)

Conclusions: Unlike in Watsonville-Salinas, in Santa Maria there was a dominant soilborne pathogen in strawberry fields, Macrophomina phaseolina. This pathogen requires particular attention in this district, especially during hot and dry summers and in eastside fields, where there is often higher disease pressure with Macrophomina root rot.

This survey was published in the International Journal of Fruit Science and can be accessed here.