Variety rotation as a strategy for managing soybean cyst nematode

Results

SCN population densities

Overall, egg population densities on susceptible soybean were greater than that on the SCN-resistant soybean varieties especially in earlier years. The initial SCN egg population density at planting in 2003 was 4,579 eggs/100 cm3 of soil. In the fall of 2003, the average egg population densities after one growing season on S (susceptible ‘Pioneer 92B13’), R1 (‘Latham EX547 RR N’ derived from PI 88788), R2 (‘Pioneer 91M90’ derived from Peking), and R3 (‘Latham AR5084’ derived from PI 437654) were 8,485, 2,055, 934, and 1,208 eggs/100 cm3 of soil, respectively, indicating all three varieties (R1, R2, R3) were resistant to the original SCN population in the field. In later years, data indicated that the level of SCN resistance was highest in R3, intermediate in R2, and lowest in R1. Tillage had little effect on SCN egg population density.

In continuous monoculture of the three resistant varieties, egg population densities increased over years from 2005 to 2014 (Figure 2). In Figure 2, Year 0 was the fall of 2004 when the egg population densities were at their lowest point. R1 is SCN-resistant ‘Latham EX547 RR N’ derived from PI 88788; R2 is SCN-resistant ‘Pioneer 91M90’ derived from Peking; R3 is SCN-resistant ‘Latham AR5084’ derived from PI 437654; S is SCN-susceptible ‘Pioneer 92B13’. Each data point in the graphs is an average of two tillage treatments with six replicates.

The egg population densities in the 2005 fall were estimated at 1206, 70, and 40 eggs/100 cm3 of soil on R1, R2, and R3 monocultures, respectively. After 10 years, the egg population densities in the fall of 2014 were 2203, 1983, and 525 eggs/100 cm3 of soil on R1, R2, and R3 monoculture, respectively. The rate of increase of SCN population densities over years was highest in R2 (1.40/year), followed by R3 (1.29), and lowest in R1 (1.06). No annual increase or decrease of SCN fall egg population density over the 10 years was detected on the SCN-susceptible soybean monoculture (Figure 2).

Virulence of the SCN populations

The effects of tillage and soybean varieties on SCN virulence in the different sequences are presented in Figure 3. Significant effects of PI88788-derived R1, and PI437654-derived R3, and tillage were observed on the FI on PI 88788 (Figure 3A,B). However, a significant effect of R3 was detected only in CT (Figure 3C). Both R1 and R3 selected SCN populations towards increased virulence to PI 88788, and R1 had greater effect than R3. The FI of the initial population on PI 88788 was estimated at 10.

Monoculture of R1 had the greatest selection pressure and after 12 years of monoculture the FI on PI 88788 was estimated at 31 (Figure 3A,B). Monoculture of R3 for 12 years selected the SCN population in CT soil that had FI 21 on PI 88788. Rotation of one or two SCN-resistant soybean varieties with susceptible soybean slowed the change of FI on the PI 88788 (Figure 3A,B). No effect of the susceptible soybean and Peking-derived R2 on the FI on PI 88788 was detected.

There was greater effect of the varieties and tillage on the FI of the SCN populations on Peking than PI 88788 (Figure 3C,D). R2 and R3 had positive effect and R1 and S had negative effect on the SCN FI on Peking. The FI of the SCN population at the beginning of the experiment on Peking was estimated to be only 1.18. There was greater effect of R2 on FI on Peking in CT than NT; monoculture of R2 for 12 years increased FI on Peking to 35 in CT soil and 11 in NT soil (Figure 3C,D). Monoculture of R3 increased FI on Peking to 7.3 in both CT and NT. Rotation of one or two SCN-resistant soybean varieties with the susceptible soybean resulted in only a small increase of FI on the Peking over the 12 years (Figure 3C,D).

The FI on PI 437654 were 0 or less than 1 in most measurements, indicating the soybean line had broad resistance, and no nematodes virulent to PI 437654 were selected by any of the varieties and tillage treatments.

S is SCN-susceptible ‘Pioneer 92B13.’
R1 is SCN-resistant ‘Latham EX547 RR N’ derived from PI 88788.
R2 is SCN-resistant ‘Pioneer 91M90’ derived from Peking.
R3 is SCN-resistant ‘Latham AR5084’ derived from PI 437654.
The model for FI on PI 88788 after a variety sequence is: FI = 11e^{(0.08779R1 + 0.05699T x R3)} -1.
The model for FI on Peking is: FI = 2.18e^{(0.14985R2 + 0.11135R3 + 0.08392T x R2 – 0.08394R1 – 0.14204S)} -1.
In the models, R1, R2, R3, and S are number of years of the soybean varieties in a crop sequence; T is tillage where no-till is 0 and conventional tillage is 1.