Soybean cyst nematode (SCN) is a very important soil-borne disease for soybean production in North Dakota (ND) and can cause 15-30% yield loss without obvious above-ground symptoms. Managing SCN becomes crucial to reduce economic losses for farmers. Enhancement of soil-based natural suppression could be an alternative means to manage SCN or as a part of an integrated pest management program. Nematode-suppressive soils are often recognized or suspected when population densities of the nematode decline greatly over time after initial establishment. Field observations of suspected nematode-suppressive soils must be verified by greenhouse tests and specific soil suppressiveness is transferable. Soils suppressive to SCN have been reported in a number of locations in the U.S. and other areas in the world. However, soil suppressiveness to SCN and biocontrol agents associated with it have not been investigated in ND. During our previous work for increasing SCN populations for HG type tests, we found 15 field soil samples with low SCN densities produced zero or very low SCN cysts whereas the other samples at the similar levels produced high numbers of cysts. We suspect some of the fields may have suppressiveness to SCN, which is transferrable to conducive soil. Thus, we propose to sample and assay soybean fields for SCN population densities, and select and evaluate ten soybean fields with no reproduction of SCN or declined SCN populations to identify suppressive soil to manage SCN in ND.

1. Sample and assay soybean fields with no reproduction of SCN or declined populations from previous work to select ten soybean fields still having low SCN densities.
2. Evaluate these soybean fields for SCN reproduction to identify suppressive soil to manage SCN in North Dakota.

1. The fields with no reproduction of SCN or declined SCN populations will be identified.
2. The levels of soil suppressiveness in these fields to SCN will be disclosed.
3. The information will be made available to soybean farmers.

Updated November 30, 2021:
Identification of Suppressive Soil for Managing Soybean Cyst Nematode in North Dakota

PI: Guiping Yan, Ph.D.
Collaborators: Drs. Sam Markell and Berlin Nelson

Objectives of the research

1. Sample and assay soybean fields with no reproduction of SCN or declined populations from previous work to select ten soybean fields still having low SCN densities.
2. Evaluate these soybean fields for SCN reproduction to identify suppressive soil to manage SCN in North Dakota.

Completed work

A total of 23 fields were sampled in May 2021 and soybean cyst nematodes (SCN) were extracted and quantified from the soil. From these soil samples, 10 fields with SCN egg numbers between 0 and 640 eggs per 100 cc of soil from Cass, Dickey, Lamoure, Nelson, Richland, and Trial counties in North Dakota were selected for subsequent experiments. These fields were previously reported to have high SCN egg numbers, but were found to have low numbers at the time of soil collection. Table 1 showed the reduction of SCN eggs in those selected field samples compared with the data collected from 2016.

After selecting 10 field soil samples, an experiment was set up in August, 2021, with four treatments; T1: field soil was autoclaved at 121°C for 2 hours, and 2,000 SCN eggs were inoculated, T2: 90% autoclaved-field soil mixed with 10 % non-autoclaved field soil and inoculated 2,000 SCN eggs, T3: non-autoclaved field soil inoculated with 2000 SCN eggs, and T4 (control): non-autoclaved field soil with no inoculation. The soil was mixed thoroughly for each treatment, and a SCN population from Richland county was used for the inoculation. Susceptible soybean cultivar Barnes was used for the host of SCN. All treatments were replicated four times and were planted in plastic cone-containers each containing an average of 100 cc of soil in a growth chamber maintained at 27 °C. Those entries were terminated 60 days after planting in October, 2021. After the termination of the experiment, plant parts above the soil surface were removed, and cysts were extracted from plant roots and soil in each container using the sieving and decanting method. White females were identified and counted under the dissecting microscope.

To confirm the results from the first experiment, the soil was collected again in October 2021 from the 10 fields selected. The second experiment was started in November 2021 by using the new soil sampled from the same location as before. A hatching experiment was conducted to hatch SCN eggs into 2nd stage juveniles (J2) in an incubator as inoculum, and an experiment with three field soils was set up in the growth chamber (Figure 1). Each of the soybean seedlings (cultivar Barnes) for treatments 1-3 was inoculated with 700 J2 after six days of planting.

Preliminary results

After the harvest of the first experiment, for the total of 160 cone-containers, white females were extracted from plant roots and soil in each of the containers and then counted. Five field samples (HG 21-1A, HG 21-1B, HG 66, HG 119-2, HG 21-3) were found to have high numbers of white females ranging from 160 to 421 per 100 cc of soil in the non-inoculated treatment (T4) with 100% natural field soil, indicating that they supported SCN reproduction and might not possess suppressiveness against SCN. Egg and juvenile counting that is being conducted is necessary to confirm the results based on the numbers of white females.

Out of the other five field samples, one of the field samples from Richland County (HG 21-2A) showed a significant reduction in SCN white females in 100% field soil (T3 and T4) as compared to 10% field soil mixed with autoclaved soil (T2), indicating the potential to be suppressive. For the other four soil samples, the non-inoculated field soils had very little numbers of white females ranging from 0 to 2 per 100 cc of soil but no significant difference was found in other treatments of these soils. This might be due to the possibility of error in counting white females because some of the white females changed the color to brown during the counting period and it is difficult or almost impossible to distinguish the newly changed brown females from old brown females already present in the field soils. Counting of eggs and juveniles that is underway is essential for confirming the results based on the numbers of white females.

Work to be completed

SCN cysts from the first experiment are stored in the suspension vials and some of them were already crushed and SCN eggs and juveniles are being quantified under a microscope. After getting all the data from the first experiment (SCN white females, eggs, and juveniles), reproductive factors comparing the initial and final populations of SCN eggs and juveniles will be obtained, which is a major factor for determining suppressiveness in the experiment. For the second experiment, we carefully examined the possible errors from the first experiment and tried to minimize them. Those five field soils that showed the higher number of white females in the first experiment will be discarded from the second experiment if the egg and juvenile counts support the results from the white females.

The second experiment started in the middle of November. It is the repetition of the first experiment with some changes. To prevent white females from changing color and make counting more consistent after the harvest, the second experiment is being conducted on weekly basis (2-3 field soils per week). We will hatch SCN eggs into second-stage juveniles (J2) and inoculate 600-700 J2 per plant instead of SCN eggs. For the second experiment, we may add more field samples from the initial 23 field soils which had fewer SCN eggs to increase the chance to identify suppressive soils. The second experiment will be harvested after two months of planting and in a similar way as in the first experiment. White females, eggs, and juveniles will be identified and quantified, and compared among the treatments to find the suppressiveness. The data from the second experiment will be compared with the results of the first experiment to evaluate the efficacy of the experiments. Enhancement of soil-based natural suppression could be an alternative means to manage SCN or as a part of an integrated pest management program.

View uploaded report

SCN is a destructive pest on soybean. Managing SCN is crucial to reduce economic losses for farmers. Crop rotation, cultivar resistance and chemical nematicides are common management strategies. However, crop rotation does not allow soybean farmers to produce sufficient soybean every year. Resistant cultivars could be short-lived as SCN is adapting to the cultivars and new virulent fortns can develop and overcome the existing resistance. Most effective chemical nematicides have been restricted because of their harmfulness to the environment or cost. This proposed project for identifying suppressive soil will allow for biological control of SCN in infested soybean fields. Developing biocontrol agents and bionematicides has become increasingly attractive as they are highly specific to the target nematodes and friendly to the environment. Enhancement of soil-based natural suppression could be an alternative means to manage SCN or as a part of an integrated pest management program.