Effect of Temporal Deployment of Different Sources of Resistance to Soybean Cyst Nematode

Crop rotation is an important management practice to reduce disease losses caused by soybean [ Glycine max (L.) Merr.] cyst nematode (SCN, Heterodera glycine Ichinohe). Thirteen rotation treatments consisting of ‘Bedford’, a soybean cultivar resistant to SCN races 3 and 14, ‘Forrest’ resistant to races 1 and 3, susceptible ‘Essex’, and a nonhost crop ( cotton [Gossypium hirsutum L.]) were evaluated in an SCN infested field for 4 yr. Yields of soybean cultivars in continuous production systems were compared with those in rotation with the nonhost, resistant, and susceptible cultivars. Bedford and Forrest yields, over the 4 yr, were less variable than Essex yields. Forrest and Essex produced similar yields in 3 out of 4 yr, and yields of Essex were significantly lower than Bedford yields in 2 out of 4 yr. Beneficial effects of different rotations on cultivar performance were not very conspicuous, however, yields of Essex were increased with rotation in five of the six comparisons compared with continuous Essex, whereas Bedford in rotation was significantly higher than continuous Bedford in two of the five comparisons. Grain yields of Forrest were not affected by rotation systems. The nematode density, after 4 yr, was lowest in cotton‐Forrest‐Bedford‐cotton system and greatest in Bedford‐cotton‐Essex‐Forrest system. The preceding crop had a greater influence on the nematode population density than the rotation system per se. Treatments with cotton as the preceding crop had lower nematode population densities than resistant or susceptible soybean. Rotating cultivars caused changes in SCN race structure. SCN race 14 was present in almost all rotations including Essex and Forrest. Bedford tended to increase the population with genes for parasitism on PI 88788, resulting in a shift to race 4. The shift to highly pathogenic race was slower in rotations that included susceptible Essex and Forrest as a component crop. Inclusion of susceptible soybean lines in rotations may delay, but not prevent, the development of highly virulent races of SCN. Rotation of soybean cultivars was found to be a useful practice in the management of SCN‐caused losses. Research Question The soybean cyst nematode (SCN) is an important pest of soybean in the USA. Rotation, resistant cultivars, and cultural practices are used to manage the population density of this nematode and associated soybean yield losses. Rotations of resistant soybean with susceptible soybean, with or without a nonhost crop, have been recommended for management of this pest. This research evaluated the effects of selected rotational sequences on (i) the population densities of SCN, (ii) the race structure of SCN populations, and (iii) the effects of these rotations in stabilizing soybean yields in the presence of this pest. Literature Summary The deployment of resistant cultivars for management of soybean cyst nematode is problematical due to the genetic diversity of this pest in field populations. Resistance breaking biotypes or races of this nematode are selected when resistant cultivars using Peking or PI 88788 as a source of resistance are deployed. Several schemes for minimizing selection pressure on this pest, which will also result in higher soybean yield, have been proposed. The basis of these strategies is: (i) a nonhost crop or a resistant cultivar are used to minimize SCN density so that a susceptible soybean cultivar can be grown with minimal yield loss, (ii) inclusion of the susceptible cultivar should reduce selection pressure on the pathogen population, and (iii) reducing the frequency with which a particular source of SCN resistance is deployed will also prevent the development of populations of the pest that can parasitize the resistant cultivar. Research to demonstrate the benefits of the approach are generally lacking. Some researchers have suggested that, while these rotations may delay development of resistance‐breaking biotypes of SCN, a monoculture of the resistant cultivar resulted in superior yields. Study Description The experiment was constructed from 1983 to 1986 at the Lee Farm of the University of Missouri Delta Center near Portageville, in a field infested with race 14 of the soybean cyst nematode. Thirteen rotational sequences were evaluated for their effects on soybean grain yield, for SCN population density at the end of the soybean growing season, and for their effect on the race structure of the SCN populations. Rotations included the susceptible cultivar Essex, the race 1‐ and 3‐resistant cultivar Forrest, and the race 3‐ and 14‐resistant soybean cultivar Bedford in rotations with or without a nonhost crop cotton. Soybean yield and final SCN cyst densities were determined every year. The races of the nematode populations were determined in 1983, 1984, and 1985 using the soybean differentials Pickett, Peking, PI 88788, and PI 90763, and susceptible Lee 68. Applied Questions Is the inclusion of a nonhost in the rotation necessary to prevent SCN race shifts? A nonhost in the rotation will help delay a shift in the SCN population but probably not prevent it. Similarly, inclusion of a susceptible soybean cultivar tended to retard selection of SCN races that can damage Bedford soybean. Growing a cultivar resistant to the SCN population in a particular field as infrequently as possible is the best way to preserve the usefulness of resistant cultivars. Which rotations are superior for maintaining soybean yields at high levels—those that include a nonhost crop or rotated cultivars with different types of SCN resistance? Soybean yield was generally improved following a cotton crop, but the SCN cyst density increased to greater than 200 cysts/pt when a susceptible soybean cultivar was grown after cotton. This often resulted in lower yields for the second soybean crop following cotton. Growing a nonhost crop every other year could prevent this yield loss. The time at which the rotation is started may also determine the best rotation to use. The resistance of Forrest soybean was no longer useful against the SCN population in these fields. The Essex‐Forrest‐Bedford rotation would probably provide better yields than Bedford monoculture if race 3 or race 1 were present, whereas with race 14, a rotation with a nonhost should be included every second or third year.