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Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a major soybean (Glycine max) disease in north-central regions of the United States and throughout the world. Current sources of resistance to Sclerotinia stem rot express partial resistance, and are limited in number within soybean germ plasm. A total of 6,520 maturity group (MG) 0 to IV plant introductions (PIs) were evaluated for Sclerotinia stem rot resistance in the United States and Canada in small plots or in the greenhouse from 1995 to 1997. Selected PIs with the most resistance were evaluated for resistance in the United States and Canada in replicated large plots from 1998 to 2000. The PIs in the MG I to III tests in Urbana, IL were evaluated for agronomic traits from 1998 to 2000. The selected PIs also were evaluated with an excised leaf inoculation and petiole inoculation technique. After the 1995 to 1997 evaluations, all but 68 PIs were eliminated because of their susceptibility to Sclerotinia stem rot. In field tests in Urbana, higher disease severity in selected MG I to III PIs was significantly (P< 0.05) associated with taller plant heights and greater canopy closure. All other agronomic traits evaluated were not associated or were inconsistently associated with disease severity. MG I to III PIs 153.282, 189.931, 196.157, 398.637, 417.201, 423.818, and 561.331 had high levels of resistance and had canopies similar to the resistant checks. The resistance ratings from the petiole inoculation technique had a high and significant (P< 0.01) correlation with disease severity in the MG I and II field tests. The partially resistant PIs identified in this study can be valuable in incorporating Sclerotinia stem rot resistance into elite germ plasm.

The soybean cyst nematode (SCN), Heterodera glycines, is a major soybean yield-limiting factor, and the use of resistant cultivars is one of the most effective means to manage the nematode. During the past decade, a number of resistant cultivars in maturity groups I and II have been developed and made available to growers. A total of 47 resistant cultivars and nine susceptible cultivars were evaluated at 15 SCN-infested field sites and two noninfested sites during 1996 to 1998 in Minnesota. As expected, more nematodes developed on susceptible cultivars than on resistant cultivars. Egg density on susceptible cultivars increased by 1.9- to 10.6-fold during the growing season at 12 sites and did not change at the other three sites. Average egg density decreased over time for resistant cultivars at all sites, except where the initial egg density was low (≤455 eggs per 100 cm3 soil). Nematode reproduction factors (Rf = egg density at harvest/egg density at planting) for individual resistant and susceptible cultivars were highly consistent across the eight sites where initial SCN density was more than 1,000 eggs per 100 cm3 soil. Resistance, however, varied among the cultivars, with the average Rf of individual resistant cultivars across the sites ranging from 0.3 to 1.7. Resistant cultivars produced an average yield of 3,082 kg/ha compared with 2,497 kg/ha by susceptible cultivars at eight of 10 sites where egg density at planting was greater than 700 eggs per 100 cm3 soil. In contrast, no difference in yield was observed between resistant and susceptible cultivars at sites where egg density at planting was lower than 500 eggs per 100 cm3 soil. Yield differences between resistant and susceptible cultivars increased with increasing initial SCN egg density. In six fields infested with initial densities of more than 5,000 eggs per 100 cm3 soil, resistant cultivars produced 28.4% (676 kg/ha) more yield on average than susceptible cultivars. Soybean yield increased when cultivars with increasing resistance to the SCN (lower Rf or females formed on roots) were grown in fields infested with SCN. Average relative yield (yield of a cultivar/average yield of all resistant cultivars at a site) of individual resistant cultivars across all SCN-infested sites ranged from 0.76 to 1.10. Yield consistency of soybean cultivars was low among the different sites, indicating that many other factors affected yield. Our results suggest growing resistant cultivars is an effective method to manage SCN in Minnesota while minimizing yield loss due to SCN.

Nematicides have been used to control different nematodes including SCN (Schmitt et al., 1983;Noel, 1987; The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, Sasser and Uzzell, 1991; Smith et al., 1991). Nematicides, is a destructive pest of soybean, Glycine max (L.) Merr. Field research was conducted at Waseca and Lamberton, MN, from 1996 through however, are not acceptable for control of SCN because 1999 to evaluate the effect of crop sequence on SCN population they are not cost effective and potentially have negative density and on crop yields. Cropping sequence treatments were (i) environmental impacts. monocultures of corn (Zea mays L.), SCN-resistant soybean, and Crop rotation is an effective strategy for SCN man-SCN-susceptible soybean; (ii) susceptible soybean rotated with 1, 2, agement, especially in the southern USA. A number of or 3 yr of corn; (iii) resistant soybean rotated annually with corn; and studies have demonstrated that growing nonhost crops (iv) rotation of corn-resistant soybean-corn-susceptible soybean. Egg reduced SCN population densities and increased soydensity was determined at sowing and harvest, and crop yields were bean yield (Young and Hartwig, 1992; Koenning et al., determined each year. In general, yields of resistant soybean were 1993;Trevathan and Robbins, 1995;Weaver et al., 1995; higher than susceptible soybean. Resistant soybean in annual rotation Young, 1998;Howard et al., 1998). Two or more years with corn produced the highest yield, and susceptible soybean in monoculture produced the lowest yield among all treatments. A longer of a nonhost crop resulted in low or undetectable SCN period of corn in rotation resulted in higher yield of subsequent densities and acceptable soybean yields (Francl and susceptible soybean in most instances. Yields of corn following corn Dropkin, 1986;. Koenning et al. (1993) were lower than following soybean. Egg density at the start of the study demonstrated that 1 yr of a nonhost crop was sufficient was 6994 and 14 000 eggs 100 cm Ϫ3 soil at Waseca and Lamberton, to control SCN with no additional benefit beyond 2 yr respectively. Average Pf/Pi (egg density at harvest/egg density at of growing a nonhost crop. Soybean cyst nematode sursowing) was 0.59 (0.23-0.86) for corn, 0.49 (0.21-0.73) for resistant vival rate is higher in northern region than in southern soybean, and 3.3 (0.74-9.91) for susceptible soybean for all treatments regions of the USA (Riggs et al., 2001). Consequently, at the two sites across the 4-yr period. After 3 yr of corn, egg density The experiments were initiated at two field sites located in southwest (Lamberton) and south central (Waseca) Minne-Senyu Chen, Univ. of Minnesota Southern Research and Outreach sota in 1996. The Waseca site had been in an annual corn-Center, 35838 120th Street, Waseca, MN 59093; Paul M. Porter, Dep. soybean rotation prior to 1996 and with susceptible soybean in 1995. At Lamberton site, susceptible soybean had been Univ., Brookings, SD 57007; and Ward C. Stienstra, Dep. of Plan...

Heterodera glycines, commonly known as the soybean cyst nematode (SCN), has become a major factor in soybean production in the Midwest United States. The influence of five tillage treatments and two treatments of row spacing on SCN population dynamics and yield of SCN-resistant and -susceptible soybean cultivars was investigated in a corn-soybean rotation system in southern Minnesota from 1993 to 1996. No effects of tillage and row spacing were observed on nematode population density. As expected, the susceptible cultivar Sturdy consistently supported higher nematode densities than did the resistant cultivar Bell in 1993 to 1995 and Freeborn in 1996. Nematode reproduction varied among years. Predicted nematode density at equilibrium was 3,800, 13,000, 12,000, and 27,000 eggs per 100 cm3 of soil in plots with the susceptible cultivar and 480, 240, 430, and 700 eggs per 100 cm3 of soil in plots with the resistant cultivars in 1993, 1994, 1995, and 1996, respectively. The effects of tillage and row spacing on soybean yield were inconsistent. The resistant cultivars yielded 653, 195, and 435 kg/ha more (P < 0.05) than the susceptible cultivar in 1994, 1995, and 1996, respectively, but no yield difference between susceptible and resistant cultivars was observed in 1993. Planting resistant cv. Bell increased the yield of the following susceptible cv. Sturdy compared with continual planting of the susceptible cultivar. A sequence with continued resistant cultivar or cultivars, however, produced a higher overall yield and lower nematode density at the end of the 4-year rotation cycle than any sequence in which the susceptible cultivar was included. Yield of resistant and susceptible cultivars was negatively related to the SCN initial population density.