Root-knot nematodes that overcome tomato resistance can reproduce on multiple resistant cultivars, according to two separate University of California research accounts. A 2026 UC Davis abstract found differences in reproduction among nematode isolates but no detectable resistance differences among five tested cultivars, raising questions about how much protection a cultivar change alone can provide.
The findings address a vulnerability in a long-used tool for California processing tomatoes. In a February 2024 report, UC Riverside researchers Antoon Ploeg and Scott Edwards explained that nematode resistance in these tomatoes rests on the Mi-1 gene. Conventional breeding introduced that resistance from a wild tomato relative, and resistant varieties have helped manage nematodes for decades.
The challenge also fits a broader concern about crop protection. UC Davis nematologist Shahid Siddique said in an abstract published in October 2026 that plant-parasitic nematodes damage many California crops and that control options are narrowing. The tomato experiments examine one specific part of that problem: nematodes capable of infecting plants bred to resist them.
Testing nematodes from fields with resistance failures
Ploeg and Edwards began with samples from processing tomato fields where resistant varieties had become infected. Over two years, UC Cooperative Extension farm advisors collected roots and soil from 16 fields across seven Central Valley counties, stretching from Colusa to Kern. This was a targeted investigation of resistance failures, not a survey of how commonly they occur.
At UC Riverside, researchers extracted nematodes from the samples and placed them with resistant Celebrity tomato plants. Eight weeks later, nematodes from all 16 locations had caused clear root symptoms. Identification using physical characteristics and PCR-based methods found Meloidogyne incognita in 15 samples and M. javanica in one sample from Kern County.
The researchers then selected five populations for greenhouse experiments: four M. incognita populations and the M. javanica population. They tested them on five resistant processing tomato varieties, with resistant Celebrity and susceptible Daniela tomatoes also included as controls. A standard M. incognita population provided another comparison. The greenhouse trial was conducted twice.
That comparison separated ordinary susceptibility from resistance-breaking behavior. The standard nematode population reproduced well on susceptible Daniela but not on the resistant tomatoes. By contrast, all five resistance-breaking populations multiplied on every resistant variety tested. Root galling was obvious in almost all combinations of a tomato variety and a resistance-breaking population.
Cultivar and nematode population both influenced the size of the increase. Averaged across the five resistance-breaking populations, nematode numbers more than tripled on each tested tomato variety. Individual comparisons also differed: population 11 reproduced more than population 10 on H5608, while their relative performance reversed on N6366. No tested resistant variety prevented multiplication.
A newer comparison puts isolates in focus
The newer research offers a separate comparison. In an abstract published in 2026, UC Davis researcher Shah Alam described testing five tomato cultivars against four resistance-breaking isolates, meaning separately collected nematode populations used in the experiment. His test included two isolates each of M. incognita and M. javanica, the same two species identified in the earlier investigation.
Alam reported no detected differences in quantitative resistance among his tested cultivars, but he did find differences in nematode reproduction among isolates. He suggested that an isolate's aggressiveness may matter more than the cultivar grown in a particular field. That finding concerns the cultivars and isolates he tested, rather than all processing tomatoes.
The two studies should not be treated as interchangeable. Ploeg and Edwards reported some differences among tomato varieties, while Alam's abstract reported none in the resistance measure he examined. They used different sets of nematodes in separate experiments. Both accounts nevertheless found differences among nematode populations, making population identity an important part of interpreting the results.
What the findings do not tell growers
Neither account establishes how widespread resistance-breaking nematodes are across California processing tomato acreage. The 16 fields in the earlier work were selected because resistant tomatoes showed infection. Ploeg and Edwards noted reports of apparent increases in resistance failures but explicitly said hard data were lacking to support a trend. Those samples cannot supply a statewide rate.
Another unresolved question extends beyond tomatoes. In their 2024 report, Ploeg and Edwards said it was unknown whether these nematodes could also overcome resistance in other crop varieties. That distinction matters when considering rotations: failure of tomato resistance does not by itself show that resistance in another crop will fail, or that the alternative will control these populations.
