Effect of cultural practices on incidence and severity of sudden death syndrome and yield of soybean, and soybean cyst nematode population density and fungal infection

Sudden death syndrome (SDS) is a root and lower stem disease of soybean (Glycine max (L.) Merril) caused by the fungus Fusarium solani f.sp. glycines.

Incidence and severity of foliar symptoms of SDS were assessed in 1996 and 1997 on a susceptible and a moderately resistant cultivar grown in no-till and conventional-till soils with rows spaced 19 cm and 76 cm apart. Root rot severity and yield of soybean were determined also. Cysts of Heterodera glycines were counted from root-associated soil for each treatment to determine cultural effects on population densities and the relationship of SCN with SDS.

Tillage had no effect on incidence or severity of SDS in 1996. In 1997, disease incidence (Dl) was lower in no-till in the first rating and had no effect in the second rating. The interaction of tillage and cultivar was also significant for Dl which was higher with the susceptible cultivar and conventional tillage. Disease severity (DS) was higher in the 76-cm rows than in the 19-cm rows in 1996 and in the second rating in 1997. In 1996, when the resistant and susceptible cultivars were compared separately, row spacing did not affect the DS of the resistant cultivar. However, DS of the susceptible cultivar was higher for the wide-spaced rows. Disease severity was higher in the 19-cm rows in the first rating in 1 997. Disease incidence and severity were both higher in the SDS-susceptible cultivars in 1996 and 1997. Root rot rating was higher in the susceptible cultivar in 1996 and higher in the 76-cm row spacing in 1997. Cyst counts were higher in the 76-cm row spacing in 1996 and in the susceptible cultivar in 1997. Yield was higher in the resistant cultivar in 1996 and in the 19-cm spaced rows in both years.

In 1996, soybean maturity group V cultivars were evaluated and in 1997 maturity group IV, V, and Roundup-Ready® cultivars were tested for disease reaction to SDS. Disease incidence was measured in both years and DS was determined in 1996. The effect of cultivar was significant for Dl and DS in 1996. Among the 52 maturity group V cultivars tested in 1996, Riverside 529 had the highest disease incidence and severity. Four cultivars. Rivers Robin, NK S 52-25, Deltapine 3519, and J77-339 had Dl and DS values of 0. In 1997, disease incidence was significant for both ratings in maturity group IV cultivars. Dixie 478 had the highest Dl and DS ratings. For maturity group V cultivars. The effect of cultivar was significant for the first Dl rating, but not the second. Deltapine DPX 9752 had the highest Dl rating. The effect of cultivar was significant for the second Dl rating of the Roundup-Ready® cultivars. Cultivar SB-HBR-X9746 RR had the highest Dl. Yield was not correlated with incidence or severity of SDS in 1996. Yield was correlated with disease incidence in the Roundup-Ready® cultivars in 1997.

Soybean cysts extracted from soil in the row spacing, tillage, and cultivar study were placed on potato dextrose agar, and fungal colonies were isolated and identified from individual cysts.Fusarium solani f.sp. glycines, the causal agent of SDS was isolated from cysts in both years. In 1996, the percentage of cysts not colonized by fungi was higher in the conventional till treatment and cyst colonization by Gliocladium roseum was higher in the no-till treatment.

Row spacing had no effect on fungal infection of cysts in either 1996 or 1997, and cultivar had no effect in 1996. In 1997 the effect of cultivar was significant. Colonization of SCN by F. solani was higher in the SDS-susceptible cultivar and sterile gray mycelium was isolated from a higher percentage of cysts from the SDS-resistant cultivar. The interaction of tillage and cultivar was significant for the percentage of cysts colonized by Trichoderma spp. in 1996 and the sterile gray mycelium in 1 997. For the resistant cultivar, more cysts were colonized by Trichoderma spp. in conventional till than in no till. Within the conventional tillage treatment, the percentage of cysts colonized by the sterile gray mycelium was higher for the resistant cultivar. The interaction of cultivar and row spacing was significant for percentage of cysts colonized by F. oxysporum in 1997. Within the susceptible cultivar, the percentage of cysts colonized by F. oxysporum was higher for the 76-cm-spaced rows.