The nematode species most commonly associated with California strawberries is the northern root-knot nematode (N-RKN), Meloidogyne hapla (Fig. 1). At the Strawberry Center’s Disease Diagnostic Service, we observed an increase in submitted samples with root-knot nematodes present. Four of these nematode samples were submitted to the CDFA diagnostic lab for confirmation of the species. They performed a DNA sequence analysis which confirmed the species as Meloidogyne hapla. Now that we have a significant number of findings, it’s important to update the industry and look for any consistent patterns among sites where N-RKN was found. 

Number of samples, locations and soil types 
Since 2022, we received plant samples from 17 fields showing the presence of N-RKN (5 fields in 2022, 6 fields in 2023, 5 fields in 2024 and 1 field so far in 2025). Some of these samples were submitted to the diagnostic service and the others came from our Santa Maria disease survey (15 fields) and Oxnard district disease survey (2 fields). In the Santa Maria district, 5 fields are located east of the 101 freeway and the rest were from fields west of the 101. Most of the samples were from fields with sandy or sandy loam soil. For the Salinas-Watsonville district, we asked Steve Koike (Director at TriCal Diagnostics) for his observations over the last 3 to 4 decades as a diagnostician. His recollection is that there were approximately 6 samples positive for root-knot nematodes. (S. Koike, personal communication). 

Fumigant history
Since fumigation is an important means of controlling nematodes, it’s important to know if the fields where N-RKN was found were recently fumigated.  Out of the 17 N-RKN positive fields, 5 were organic (not fumigated), 4 were fumigated with Pic-Clor 60 and 4 were fumigated with chloropicrin.  We were not able to get the fumigation history for some of the fields. Bottom line: there was no consistent pattern with regard to fumigation and the presence of N-RKN.

Co-infection with fungal pathogens
A total of 23 plant samples were collected from the 17 N-RKN fields and 7 were also positive for Macrophomina phaseolina, 5 were positive for Fusarium oxysporum f. sp. fragariae (race 1), and 1 was positive for Phytophthora sp.. Additionally, 2 of the samples were positive for both M. phaseolina and F. oxysporum f. sp. fragariae (race 1). Four samples were positive for minor pathogens and one sample was positive exclusively for N-RKN. So, all but one of the 23 plants samples were co-infected with fungal or Oomycete root pathogens.

Symptoms and signs
If you suspect root-knot nematodes in your fields, look for symptoms of root galling and stubby or swollen roots (Fig. 2 and 3). Using a 14X or 20X hand lens you should be able to find some gravid female nematodes protruding from the root surface (Fig. 2). If you suspect N-RKN in your field/s, please submit a sample to the Cal Poly Strawberry Center’s Disease Diagnostic Service to confirm the diagnosis.

Interaction between N-RKN and soilborne pathogens
N-RKN is known to interact synergistically with other pathogens, often making disease more severe than with either pathogen alone. This interaction has been documented with pathogens like Verticillium dahliae (in tomato, eggplant and pepper), Fusarium spp. (in cotton, tomato and watermelon), Pythium spp. (in cucurbits, lettuce and tomato), and Rhizoctonia solani (in soybean, sugar beet and potato). All these fungal pathogens are common in California strawberries. We are investigating the possibility of an interaction between M. hapla and Fusarium oxysporum f. sp. fragariae race1 (causing Fusarium wilt) as well as M. hapla and Verticillium dahliae (causing Verticillium wilt). The goal of this study is to assess the damage to strawberries caused by infection with N-RKN alone and when co-infected with F. oxysporum f. sp. fragariae race 1 or V. dahliae.

Why the increase in N-RKN findings?
Finding N-RKN on California strawberry roots is not common but seems to be on the rise based on our recent findings and previous anecdotal observations. We don’t think our findings are cause for alarm, nor do we see a clear economic impact, especially since other fungal pathogens were present in nearly all samples. This could signal that N-RKN is on the rise in the absence of the fumigant methyl bromide or simply be the result of looking for it more intensively. Our recent disease surveys are an instance of looking more intensively.

We will continue to be vigilant for N-RKN and investigate its potential for economic damage, especially in association with other fungal root pathogens.