Final report for OS19-126
Georgia has a vital role in the national vegetable production with a farm gate value of $1.14 billion (Wolfe and Stubbs, 2016). The state is a leading producer of fresh market vegetables including cucurbits, brassicas, bell peppers, onions, sweet corn, and tomato. In Georgia, more than 66% of vegetable growing areas are infested with at least one species of root-knot nematodes (Meloidogyne spp.) at the level of one nematode per 100 cm3 of soil where management is needed to achieve economically reasonable yields (Marquez et al., 2021). The nematodes most associated with vegetable production in Georgia are three species of M. incognita, M. arenaria, and M. javanica that cause severe yield loss particularly in small-scale farms that may not have the resources necessary to utilize fumigants (Marquez et al., 2019).
The phase-out of methyl bromide has caused a void in vegetable production that forces growers to use new fumigants and non-fumigants nematicides to control nematodes. However, new regulations regarding the application of fumigants, the difficulty of application, and the high costs of fumigants are compelling growers to use sustainable and environmentally friendly yet effective options for PPN control. In southern Georgia, despite long growing seasons, the practice of growing two vegetable crops on the same agricultural land often creates a narrow window for growers to use cover crops. Hamid and Hajihassani (2020) evaluated several cultivars of oilseed radish (Carwoodi, Cardinal, Final, Image, Concorde, Control, Eco-Till, Karakter, and Cannavaro), white (Tachiibuki), and black (Pratex) oats. They reported that the Control and Carwoodi radish and Tachiibuki oat were resistant to M. incognita. Field research conducted in Georgia reported an increase in M. incognita population levels in the rhizosphere of some Brassica species, but the incorporation of the crop residues into soil suppressed the nematode (Monfort et al., 2007). Nematode management can be improved by gaining more information on nematode population changes as influenced by nematode-suppressive cover crops and the efficacy of non-fumigant nematicides on cash crops grown after cover cropping.
The objectives of this project were to determine the suppressive effect of oilseed radish, white mustard, cereal rye, and oats on root-knot and stubby-root nematode populations and to evaluate the subsequent damage caused by these nematodes to cabbage grown after cover crops and treated with a non-fumigant nematicide, fluensulfone.
The experiment was established in a grower’s field naturally infested with root-knot nematodes in Tifton, Georgia. Off-season treatments were oilseed radish cv. Control (resistant to M. incognita) and Image (susceptible), cereal rye cv. Wrerens Abrussi (susceptible), oat cv. Tachiibuki (resistant), black oat cv. Protex (susceptible) and mustard cv. Caliente 199 (susceptible). A fallow with weeds treatment was also included for comparison. In-season treatments were an untreated control and the application of Fluensulfone. Plots were 30 ft long and 6 ft wide, with 6 ft alleys in a completely randomized design with ten replicates for each cover crop (two adjacent rows were grown for each cover crop). Oilseed radish and mustard were seeded at a rate of 12 lbs/acre, and oats and rye were seeded at 60 lbs/acre, respectively, on nonraised beds on October 24, 2019. Nematode samples (1 composite soil sample consisting of 6 cores/plot) were collected randomly on a zig-zag pattern three days before planting. Nematodes were extracted from a 100-cm3 sub-sample taken from each composite sample using the centrifugal-flotation technique (Jenkins, 1964), and numbers of root-knot and stubby-root (Paratrichodorus spp.) nematodes were enumerated. Cover crops were terminated on January 7, 2020, and plants were mowed down with a 6-ft flail mower, and then a moldboard plow was used to incorporate plant material 4-6 inches into the soil. Crops were left in the soil to decompose until February 5, 2020, when the rows were refilled with a moldboard plow and formed into 30 inch raised plastic mulch beds with a single line of drip tape.
After laying the plastic, one row was treated with the nematicide fluensulfone at 5 pt per acre using a CO2 pressurized tank. A pre-mixed solution of fluensulfone and water in a 3-L bottle was injected directly into the drip tape on February 14, 2020. The drip tape irrigation system was turned on for approximately 30 minutes to clear the drip tape off the remaining nematicide and water it into the root zone. The remaining one row was left untreated. The nematicide-treated and untreated subplots were alternated in a randomized complete block design. Seedlings of cabbage cv. Savoy were transplanted 18 inches apart (12 plants per plot) in all plots on February 24, 2020, and plants were managed for foliar diseases, insect and weed infestations following growers’ practices for vegetable production. Soil samples were collected before nematicide application and at cabbage harvest on September 1, 2020, from each treated and untreated subplot by taking six soil cores. Soil cores were mixed thoroughly, and nematodes were extracted from 100 cm3 of soil and counted. Five root samples of cabbage were randomly collected from each plot and assessed for severity of root galling using a scale 0 to 5 where 0= no galls and 5= >75% galling. Cabbage heads were harvested and weighed for evaluating marketable yield.
The difference in nematode counts at different sampling times was compared in a generalized linear mixed model with negative binomial distribution in SAS. A two-way ANOVA was used in the GLIMMIX procedure under the residual-normality assumption when considering nematicide treatment, and the residual plots were all bell-shaped. Means were separated using the Tukey-Kramer multiple comparison test at P =0.05.
The effect of cover crop treatment (variety) was not significant at different sampling times, indicating that all the cover crops had a similar impact on both the root-knot and stubby-root nematodes. In contrast, sampling time significantly affected the nematode population density in soil (P <0.0001). After termination and incorporation of cover crop residues into the soil profile, the root-knot nematode population density was reduced (P <0.05) only in plots grown with Tachiibuki oat. Oilseed radish and mustard are cool-season annual crops that can be planted in the fall and harvested in the spring in regions with mild winter temperatures, including Southeast US. However, cultivation of these crops for fifteen weeks in the present study did not result in sufficient plant biomass for the production of biocidal compounds (e.g., isothiocyanates). This study indicates the potential of Tachiibuki oat as a trap crop for suppressing root-knot nematode populations in soil.
Growing mustard, rye, and Control radish significantly increased populations of Paratrichodorus spp., suggesting that these crops are susceptible to stubby-root nematodes. Our data support the hypothesis that the Pratex oat can suppress stubby-root-nematode populations (USDA-NRCS. 2014). Drip application of fluensulfone on cabbage significantly reduced the root-knot nematode populations in plots previously grown with Tachiibuki (white) and Pratex (black) oats, and mustard compared to untreated plots. Similarly, the nematicide reduced the stubby-root nematode numbers on white and black oats, mustard, and rye treatments (P <0.05). The efficacy of fluensulfone on root-knot nematode numbers in the soil was not consistent across all treatments.
The effect of nematicide treatment on gall severity was significant only for untreated plots with plots grown with Image radish having a significantly lower gall severity than the mustard, rye, and fallow treatments. Application of fluensulfone reduced the root gall severity of cabbage in plots grown with oilseed radish, oat, and rye cover crops compared to untreated control. Nematicide application in plots previously grown with cover crops did not affect cabbage yield compared to untreated plots. A long-term study will be required to make further recommendations for growing a winter cover crop species with the potential to suppress root-knot nematode populations in vegetable production systems.
We could not repeat the trial in the same grower’s field because it was under the production of a vegetable crop in October 2020. We also did not find another field site with an adequate population density of root-knot nematodes.
Educational & Outreach Activities
Due to Covid-19 restrictions in 2020, we did only a few in-person consolations and many phone calls to share the outcome of this project with extension agents and growers. We discussed several factors such as selecting nematode-suppressive cover crops, appropriate timing of cover crop cultivation in the winter to achieve adequate plant biomass production, and best practices of termination of cover crops to increase the maximum level of nematode control. During the workshops for Extension agents and Ag professional, this project's results were shared, and feedbacks were collected from the participants to use in the future studies. In addition, several virtual grower meetings were used from January to March 2021 to communicate the results with vegetable growers.
This project results in some preliminary information on the use of winter cover crops for suppression of root-knot and stubby-root nematodes; however, we could not establish the repeat trial as planned to confirm the data obtained in the first year of trial. We obtained useful information on cover crop species that could be used either as a “trap” or biofumigant” crop by farmers for nematode control. We determined that oat could act as a trap crop to reduce the populations of root-knot and stubby-root nematodes in the soil. We were able to address questions concerning improved production of vegetables when oilseed radish or mustard cover crops are used in specific situations. In addition, we did gain some valuable information on the planting time of cover crops in the winter to obtain adequate biomass production.