Seed vigor, soilborne pathogens, preemergent growth, and soybean seedling emergence

Emergence of soybean [Glycine max (L.) Merr.] seedlings in the field is frequently less than predicted by standard germination, but the causes of this emergence failure are not well understood. This study explored the influence of soil pathogens and seed vigor on soybean seedling preemergent growth and emergence. Seed from six seedlots, representing a range in seed vigor, were planted as pregerminated and as dry seed into sterile and pathogen-infested soil maintained at a constant soil water potential (-0.005 MPa). Final emergence (FE) and emergence rates from two planting depths (25 and 60 mm) were recorded under ambient greenhouse conditions. Once the FE stage had been reached, nonemerged seedlings were exhumed, and classified as stunted, abnormal, or dead. The FE of the high- and medium-vigor seedlots was always higher than the low-vigor seedlots, and the advantage was greatest under stressful conditions (deep planting, nonsterile soil). The FE was always lower in nonsterile than sterile soil. Seedlings emerged more slowly from deep plantings and when pathogens were present, and FE decreased as emergence was delayed in nonsterile soil. This relationship became more pronounced with low vigor seed. Planting pregerminated seeds always resulted in higher FE than dry seeds, but it did not eliminate emergence failure. Abnormal seedlings accounted for most of the emergence failure in sterile soil, but the lack of germination or of growth immediately after germination was also important in nonsterile soil. Lack of germination alone therefore does not account for lack of emergence; postgerminative, preemergent growth may be more important.