There are many challenges that the tomato industry faces, from the fragility of the crop itself in a changing climate, to its susceptibility to damage from insect and soilborne pests, and to its production techniques that add stress to natural resources and ecosystem services (Lange and Bronson, 1981; Boulard et al., 2011; Pathak and Stoddard, 2018). Tomatoes are one of the most widely grown and valuable crops produced in the United States and take the lead in consumption per capita of processed vegetables (Gould, 2014). Tomatoes contain high levels of antioxidants important for human consumption, including vitamin C, polyphenols, and carotenoids, levels of which vary depending on variety, ripeness, and cultivation (Hallmann, 2012). Prior to the 1780s, tomatoes were produced and consumed mainly in European countries and did not peak in popularity in the U.S. until 1830-1840 (Gould, 2014). Of the 170 million tons of tomatoes produced worldwide, the United States contributes significantly to worldwide production, as the second top producer of fresh and processing tomatoes, after China (Guan et al., 2018).
The northern root-knot nematode (NRKN),
Historically, the leading management process to combat nematode infestations was chemical control, such as the fumigant methyl bromide (Cetintas and Yarba, 2010; Desaeger et al., 2020). Due to the increasing regulations and push for less toxic chemicals, scientists and farmers alike are seeking economically feasible and environmentally responsible alternatives for root-knot nematode management. Many available chemically-based nematicides do not provide long-term solutions for nematode suppression, and use is often restricted due to environmental and human health concerns. However, there is some evidence to suggest that plant-parasitic nematode management can be aided by cultural practices, biological control agents, and screening for naturally occurring nematicidal compounds in plants (Perez et al., 2003). Biologically-based nematicides can be an important tool for organic or farmers seeking to become more environmentally friendly, due to their ability to reduce chemical pressure in crop fields. Additionally, few existing nematicides are both biologically-based and consistently effective, leaving organic growers with limited options for management (Desaeger and Watson, 2019). Utilizing natural pest control strategies can both improve soil quality and human health by reducing harmful residue or contamination (Hulot and Hiller, 2021). To keep up with the demand of tomato production in the United States and the rest of the world, nematode populations can be monitored and managed through the use of a variety of integrated management strategies, when possible.
In recent years, an influx of biologically based nematicides have been developed and become commercially available to growers (Desaeger et al., 2020). Currently, there are a number of biologically based nematicides registered for use on tomatoes. However, product effectiveness varies greatly between growing season, moisture, soil composition, cultivar in field efficacy trials (Desaeger and Watson, 2019). The products involved in our experiments described below are developed by Microbes Inc. (MN21 and MN11) and are uniquely composed of lysed cell material (consisting of bacteria, yeast, and amino acids), with the goal of reducing plant-parasitic nematode populations.
To evaluate the efficacy of two new organic products (MN11 and MN21; Microbes, Inc.) at reducing
A greenhouse trial was established in 2021, to evaluate the potential efficacies of four product solutions against the plant-parasitic nematode,
The experiment was repeated as previously described with slight modifications. The optimal formulations of each product were selected to repeat (MN11.2 and MN21.2). Inoculated, nontreated controls (positive controls) and noninoculated controls (negative controls) treatments were included for comparison. Each of the five treatments (positive control, MN11.2, MN21.2, Velum Prime, and the negative control) had twelve replicates. Six of the replicates were deconstructed at five weeks post-application (5 WPA), and the other six were concluded at ten weeks post-application (10 WPA). At both takedowns, the plant parameters (above ground height and root mass) and nematode parameters (juvenile nematodes/100 cm3 soil, mature female galls/1 g roots) described above were collected from each pot.
Soil within each bag was homogenized and a 100 cm3 volume subsample was collected for processing. Immediately following the trial conclusion, we used the sugar floatation centrifugal method to elucidate vermiform nematodes (Jenkins, 1964). Root-knot nematode juveniles were enumerated using a counting dish and an inverted Nikon TMS microscope at x200 and x1000 magnification, within five days of processing.
Roots were gently cleared of soil, rated on the Gall Indices Rating developed by Hussey and Janssen (2002): 0 indicating no galls and up to 5 indicating over 75% of the root system contains galls. Root systems were then weighed and recorded. Following evaluation, 1 g of lateral roots was collected from each plant. Each root sample was individually cut into 1 cm pieces and stained using the NaOCl acid fuchsin-glycerin procedure (Bybd et al., 1983). Samples were then temporarily stored in plastic petri dishes with glycerol until reading. From there, we used a dissecting scope at 35X to quantify galls within three days of staining (Fig. 1).
Data analysis was completed via RStudio version 1.3.1093 (R Core Team, 2020). For each variable, a one-way ANOVA followed by Tukey’s HSD post-hoc analysis was conducted to determine significance between treatments for each plant and nematode parameter collected. Unless otherwise stated,
During both trials, we did not observe any notable phytotoxic defects to plant coloration or growth for any of the treated plants. In the first trial, the positive control (inoculated, nontreated) possessed the lowest root mass of the five treatments. All of the positive control plants were heavily galled with lower masses (Figs. 2A,B). Root masses of plants treated with Velum Prime and MN21.2 were 5.76x and 5.72x heavier than the positive (inoculated, nontreated) control plants, respectively (
In the first greenhouse trial, plants with a higher Gall Index Rating (GIR) indicated more severe infestations and presence of galling on roots (Fig. 3A). Untreated (positive control) plants were significantly more galled than treatments M11.2, MN21.1, MN21.2, and Velum-applied plants (
In the first greenhouse trial, all four treatments (MN11.1, MN11.2, MN21.1, and MN21.2) were lower than concentrations in the positive control plant roots. Both versions of MN21 were lower than MN11.1 (
In the first greenhouse experiment, treatments MN21.1 and MN21.2 were both effective at reducing symptoms and signs of root-knot nematode infestations, similar to the application of Velum Prime (active: fluopyram; Bayer CropScience). The product MN11.2 showed potential efficacy in the first trial, but in the repeated greenhouse trial, plants treated with this product had higher galling than the untreated positive control plants. Studies on root knot nematode infestations at high concentrations on tomatoes are associated with increased root mass due to galling (Olthof and Potter, 1977; Gugino et al, 2006). However, our heavily galled positive control plants possessed root masses lower than plants treated with effective products and a negative control. Untreated, inoculated positive control plants also possessed visibly necrotic root tips along with their weight reduction. This could indicate that root losses in our infected plants may have occurred due to root tip necrosis, more than tissue increases from gall formation.
In the second experiment, our noninoculated, negative controls were contaminated with low rates of root-knot nematodes. This could have been due to droplet splashing during routine watering, or small amounts of soil transport on gloves during daily care. It is notable to mention that for the last two weeks of the repeat trial, the plants both from the positive control and MN11.2 pots were notably wilted.
Overall, our results indicate promising control of the northern root-knot nematode by the new nematicide, MN21.2. Results during both trials showed a similar reduction of mature female galls in roots and juveniles in soil populations to the plants treated with Velum Prime. These findings also consistent with prior studies in tomato plants that fluopyram is an effective product for reducing