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By Jaspreet Sidhu, University of California Agricultural and Natural Resources
Some non-fumigant nematicides are showing potential to manage root-knot nematodes in carrots, according to research conducted at the University of California. This could be good news as the industry searches for alternative methods to control root-knot nematodes, which pose a major threat in carrot production regions including California.
California is the leading producer of carrots in the United States and produces about 80% of the total U.S. production. Carrots are grown year-round in diverse growing areas in California. The southern San Joaquin Valley has the most concentrated area under carrot production, with Kern County producing almost 60% of the total carrots in California. The nation’s two largest carrot growers and shippers are also located in Kern County. However, the pride of being the “Carrot Capital” in the nation also comes with several challenges for carrot cultivation, and one such challenge is the damage caused by root-knot nematodes.
Root-knot nematodes, Meloidogyne spp., are the most important plant-parasitic nematodes affecting carrot production in California, especially in light texture soils. The nematodes are widespread in central and southern California. Damage results from feeding of second-stage juveniles inside carrot roots, and the roots respond to nematode invasion by the formation of root galls. Root-knot nematodes can cause substantial damage by stubbing, forking and galling of the roots, thereby reducing marketable yields. Deformed roots due to galls are unable to sustain the water and nutrient needs of the plants, leading to wilting and poor growth of plants. Forked roots also pick up excess soil that increases the tare transported to processing facilities and increases processing costs for additional cleaning efforts.
Nematode Management
Currently, there are no resistant cultivars available for the California carrot industry, and management has mainly relied on the use of pre-plant soil fumigants. Management with these products is expensive and involves safety and environmental risks.
New fumigant regulations by the Department of Pesticide Regulations (DPR) have been put in place to restrict the emissions of volatile organic compounds (VOC) from the use of soil fumigants. These regulations include limits on the amount of soil fumigants a grower is allowed to use in a year, caps on the amounts allowed within a township, and newly expanded buffer zones, meaning large parts of a field may be left untreated. These new regulations by DPR may mean that there will be some fields not treated for nematodes because of caps placed on the amount a grower is allowed to use or caps on the amount of fumigants allowed in a township.
Rotation with non-host crops can be a viable option, but its utility is often limited due to the wide host range and reproduction potential of the root-knot nematodes.
Therefore, alternative control options that have high efficacy, are economically viable and are environmentally safe need to be evaluated under field situations. In addition, to prevent the possible development of nematode resistance to these new products, it would be imperative to have alternative chemistries available to avoid the development of nematicide resistance. An overreliance on any one nematicide product would likely lead to the loss of that product.
Field Trials
Hence, the purpose of research at the University of California was to continue evaluation of recently registered non-fumigant, novel nematicides and screening of potential nematicides for managing root-knot nematodes in carrots.
A promising nematicide, Nimitz (a.i. fluensulfone, ADAMA Agricultural Solutions Ltd.), was registered for use on carrots in California in 2021. Another nematicide, Salibro (a.i fluazaindolizine, Corteva), was recently registered for use on carrots in California. Velum One (a.i. fluopyram, Bayer Crop Science), a broad-spectrum fungicide and nematicide, is registered on several major crops in California but not on carrots. It has promising efficacy against root-knot nematodes in carrots, but its efficacy is sometimes debatable due to its limited motility in the soil. However, field studies have suggested that soil surfactants might improve fluopyram’s efficacy. Nimitz, Salibro and Velum have only “caution” on the label as a signal word. Salibro has high solubility and is highly selective against root-knot nematodes, but continued evaluation is needed. Another developmental nematicide (DP) also showed excellent performance against root-knot nematodes in our 2022 and 2023 trials.
Two randomized complete block field trials were conducted in the summers of 2022 and 2023 at the Kern County Extension Research Station in Shafter, California, where a root-knot nematode site is maintained to ascertain a uniform and moderate to high root-knot nematode population pressure for these studies. In 2022, the trial included four replications and six treatments. Each plot was comprised of two 30-inch-wide beds that were 20 feet in length with a 5-foot buffer between plots along the bed. Carrot seeds cv. Uppercut was seeded (three lines per bed) on June 23, 2022. In 2023, the trial had six replications and eight treatments. Each plot was 30 inches wide and 20 feet in length with a 5-foot buffer between plots along the bed. Carrot seeds cv. Uppercut was seeded (three lines per bed) on June 15, 2023. Rates, timings and methods of application for each treatment are listed in Table 1.
Treatments were applied as a pre-plant or post-plant application (as recommended by the manufacturer) as a soil drench using watering cans, incorporated into the top soil and sprinkler irrigated to move the products down into the soil. The trials were managed following grower standard agronomic practices.
At harvest, carrot roots were evaluated for root galling. For root gall evaluations, about 100 carrots from each plot were randomly sampled and were visually rated for the severity of root galling on a scale of 0-10, as shown in Fig. 1 (0=no galls, 10=completely galled roots). The average galling on these roots was used to give a galling index for each plot. Carrots with a gall rating of 3 or lower were considered marketable. Data on the percent marketable carrots was also recorded. Data on root galling and marketable carrots was analyzed using statistical analysis software.
Results
In 2022, significant levels of phytotoxicity, poor germination and loss of plant stand were observed in the plots treated with a higher rate of Nimitz. Moderate to severe galling and root-knot nematode-induced forking and galling were observed in the trial. The severity of root galling was assessed at harvest (Nov. 7, 2022). The treatments DP1 and Salibro had significantly less galling on the roots compared to the untreated control plots (Fig. 2). The treatments Nimitz1 (low rate) and the organic product MB were intermediate in performance and had numerically lower galling than the untreated control and the Nimitz2 (high rate) but were not statistically different. Likewise, the percentage of marketable carrots was significantly higher in the treatments Salibro and DP1 (Fig. 3)
In 2023, loss of plant stand and some phytotoxicity was observed in the plots treated with Nimitz. Again, moderate to severe galling and root-knot nematode-induced forking and galling were observed in the trial. The treatments DP, Velum+Watermaxx (VW) and Salibro had significantly less galling on the roots compared to the untreated control plots (Fig. 4). The two Nimitz treatments were intermediate in performance and had numerically lower galling than the untreated control and the treatments MB1 and FMC. However, there was no difference in the percentage of marketable carrots among treatments.
In these trials, there was some treatment effect on root galling throughout the season, with DP, Salibro and the Velum+Watermaxx treatments having lower root galling index compared to the untreated control and other products. The research trial demonstrates that the application of some of these products under experimental conditions in carrots provided acceptable control of root-knot nematodes compared to the untreated control. Some of these products have the potential to be considered as an alternative, viable and safe option, but further evaluation and optimization are needed to better determine the efficacy of these products as sole treatments and in combination with other products and their continued use by the carrot industry.