Evaluation location: Missouri, United States
Soybean accessions were screened for resistance to Soybean Cyst Nematode race 1 (HG type 2.5.7), race 2 (HG type 1.2.5.7), race 3 (HG type 0), and race 5 (HG type 2.5.7). A SCN bioassay using revised classification system, HG Type (Niblack et al.,2002) was used to evaluate soybean germplasm in the greenhouse. HG Type of classification includes seven indicator lines and a susceptible cultivar for determination of soybean reaction to SCN. The indicator lines included PIs 548402 (Peking), 88788, 90763, 437654, 209332, 89772, 548316 ( Cloud) and, a standard susceptible control cv. Lee-74 ( PI 548658). Lee-74 was replaced with cv.5601T which is also a susceptible variety. The current HG Type classification system can't distinguish Race 1 population from Race 5 but cv. Pickett was included as a differential line which is resistant to Race 1 but susceptible to race 5. Soybean germplasm accessions were screened with near-homogeneous populations of SCN races or HG Types to obtain consistent reaction. The source and method for developing near-homogeneous populations of SCN Races or HG Types used for screening soybean accessions was already described (Qui et al.,1999). The method for SCN bioassay performed in the greenhouse followed established protocols (Arelli et al., 2000) with the modifications described in Arelli and Wang (2008). Each plant was grown in a 7-cm in diameter clay pot filled with steam sterilized soil on greenhouse bench top with an evaporative cooling and under bench heating systems. The duration of lighting, heating and, cooling systems in the green house were regulated during the bioassay for proper growth of soybean seedlings and nematodes. Ten seedlings for each accession, indicator lines and a susceptible control were included in each experiment and maintained at 27+or-2 degrees C. When the seedlings were 4 or 5 days old each plant was inoculated with a given nematode Race or HG Type using eggs and juveniles. Each seedling was inoculated with 5 ml of suspension (de-ionized water) consisting approximately 2000 eggs and juveniles. The inoculum was delivered close to the roots of seedling. Approximately 30 days after inoculation, plant roots were washed individually with a strong jet of water to dislodge the females and cysts. These were counted under a stereomicroscope and, a female index (FI%) was calculated for the number of females developing on each plant (Golden et al.,1970). Female index is the number of SCN females occurring on a soybean plant expressed as the percentage of mean number of females on susceptible Lee-74 or cv.5601T. Ratings of resistant (FI=0 to 9%), moderately resistant (FI=10 to 30%), moderately susceptible (FI=31 to 60%) and, susceptible (FI>60%) used to classify the reaction of plants were based on Schmitt and Shannon (1992). Each experiment was repeated and FI% was averaged for a given race or HG type. The screening procedure with some modifications has been published in Euphytica ( 2009 ) 165: 1-4.