Soybean Cyst Nematode, Heterodera glycines, Populations Adapting to Resistant Soybean Cultivars in Kentucky.
Soybean cyst nematode (SCN), Heterodera glycines, is the most limiting biotic factor of soybean (Glycine max) production in Kentucky (KY). Unpublished results of a survey of commercial soybean fields in KY in the late 1980s indicated that H. glycines (HG) type 0 (race 3) was the most common HG type in the state. HG type 0 populations cannot reproduce (female index [FI] <10% compared with reproduction on a standard susceptible cultivar) on PI88788, which is the basis of H. glycines resistance in >90% of the soybean cultivars grown in the United States. Recent reports from Illinois (4), Missouri (3), and North Carolina (2) indicate that most populations of H. glycines in those states are now able to reproduce on resistant soybean cultivars derived from PI88788. Because cultivars derived from PI88788 are grown almost exclusively in KY to manage H. glycines, a limited survey was needed to update information on H. glycines populations. Most soybean fields in KY are grown in a 1-year rotation with corn (Zea mays), a nonhost crop for H. glycines. Therefore, the survey targeted fields that had most recently been in corn. Otherwise, fields were arbitrarily selected for sampling. Composite samples were collected in the fall of 2006 or the spring of 2007 and consisted of 20 soil cores (10 to 15 cm deep × 2.5 cm in diameter) collected following a zigzag pattern. Samples were mixed and stored at 4°C until processing. H. glycines cysts were extracted from soil by a sucrose centrifugation and flotation technique (1). Eggs were liberated by crushing cysts caught on a sieve (250-μm-diameter pores) with a rubber stopper. Liberated eggs were then collected, stained with acid fuchsin, and counted. Up to three samples from each county surveyed were sent to the University of Missouri Nematology Laboratory, Columbia, MO for HG type testing (3). A total of 139 samples, representing 19 major soybean-production counties in KY, were analyzed for H. glycines levels. H. glycines eggs were recovered from 106 (76%) samples and 16 (84%) counties. SCN population densities ranged from 38 to 4,275 eggs per 250 cm3 of soil. HG type tests were conducted on 20 populations from eight counties. HG types 1.2.5.7 (race 2) and 2.5.7 (race 1 or 5) were identified, with HG type 2.5.7 (race 1) being the most common (60% of populations screened). No HG type 0 populations were detected. All populations tested had a FI ≥10% on three of nine indicator lines (PI88788, PI209322, and PI548316). FIs on these indicators ranged from 15 to 80, 11 to 81, and 23 to 88%, respectively. Sixty percent of populations tested had FIs ≥30% on PI88788. Of the populations screened, 25 and 35% had FIs ≥10% on PI548402 (Peking) and Pickett, respectively. All populations tested had FIs = 0 on PI437654. Survey results indicate that cultivars deriving their H. glycines resistance from PI88788 may have reduced effectiveness in suppressing current H. glycines populations in KY. Consequently, producers may need to grow soybean cultivars derived from non-PI88788 resistance sources to successfully manage H. glycines in the future. References: (1) D. E. Hershman et al. Plant Dis. 74:761, 1990. (2) S. R. Koenning. Plant Dis. 88:942, 2004. (3) M. G. Mitchum et al. Plant Dis. 91:1473, 2007. (4) T. L. Niblack et al. Online publication. doi:10.1094/PHP-2008-0118-01-RS. Plant Health Progress, 2008.