EARLY PLANTING OF EARLY-MATURING SOYBEAN - EFFECTS ON SEED-GERMINATION AND PHOMOPSIS INFECTION

The use of early-maturing soybean [Glycine max (L.) Merr.] cultivars h as been suggested as a means of avoiding drought stress and extending harvest maturity in the southeastern USA. This experiment determined t he effect of cultivar maturity on seed germination and Phomopsis longi colla T.W. Hobbs seed infection following early planting. Six cultivar s ranging from Maturity Group (MG) 00 to IV were planted in late April , mid-May, and early and late June in 1992 and 1993 at Lexington, KY. The May planting was sprayed at weekly intervals between growth stages R5 and R7 with the foliar fungicide benomyl [methyl 1-(butylcarbamoyl )-2-benzimidazolecarbamate] to control seed infection. A single, recom mended application of benomyl was also made at growth stage R6 for all planting dates in 1993. Seed was harvested at harvest maturity (HM, d efined as the time when seed first dried to <140 g kg(-1) moisture) an d 3 wk after that date. Higher level (>30%) of P. longicolla seed infe ction frequently occurred for the April and May plantings, which often resulted in lower (<85%) seed germination for earlier maturing cultiv ars. As harvest maturity was delayed due to later planting dates of ea rly-maturing cultivars (MG 00-II) or the use of full-season cultivars (MG III, IV), seed quality improved. Weekly benomyl applications to th e mid-May planting reduced seed infection to low levels and increased seed germination to >90% at HM in both pears for all cultivars. Thus, high-quality soybean seed can be produced on early-maturing cultivars grown outside the range of adaptation, if seed infection by P. longico lla is controlled. Cultural methods are discussed for reducing seed in fection and improving seed quality of early-maturing cultivars.