The effects of soybean maturity and determinacy on the host-parasite relationships of Heterodera glycines were investigated in a field microplot study over 2 years. Determinate and indeterminate isolines of the maturity group (MG) III cultivar Williams 82 and the MG V cultivar Essex were grown in microplots artificially infested with a race 3 isolate of H. glycines at three initial population (Pi) densities (0, 300, and 3,000 eggs/100 cm³ soil). Soybean seed yields, nematode final population (Pf) densities and reproductive index (Pf/Pi), and root colonization by Macrophomina phaseolina, the causal agent of charcoal rot, were monitored in each year. Seed yields were reduced (P = 0.05) in the presence of H. glycines in both years, but losses were greater in 1996 in the absence of drought stress. Yield loss was lower (P = 0.06) for the determinate isoline of Essex than for the other cultivar-isoline treatments across years. Nematode reproduction was density-dependent in the more conducive environment of 1996 but was unaffected by soybean maturity or determinacy traits. Root colonization by M. phaseolina increased (P = 0.05) in the presence of high H. glycines densities on determinate, but not indeterminate, isolines. Differences in H. glycines-induced yield loss among cultivar-isoline treatments were not related to nematode reproduction, M. phaseolina colonization, or environmental stresses. These results indicate that the effects of soybean maturity and determinacy on H. glycines-soybean interactions are not independent and that their combined effects must be considered in geographic regions where both traits vary.
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