SEVERITY OF PHOMOPSIS SEED DECAY, SEED QUALITY EVALUATION, AND FIELD PERFORMANCE OF SOYBEAN

The ability of seed quality tests to predict field performance of soyb ean seed lots with different incidences of infection by the causal age nts of phomopsis seed decay (PD), was evaluated. Different PD seed inf ection levels were induced in one 'Corsoy 79' soybean seed lot by expo sing naturally infected pods to 0, 3, 4, or 5 d at >95% relative humid ity and 25 degrees C. Seeds were then removed from pods and dried. See d quality was evaluated by germination tests on creped cellulose waddi ng (Kimpac), rolled towel, and sand and by cold, accelerated aging (A. Aging), tetrazolium (TZ), and conductivity tests. Field emergence of t hese seeds was evaluated at two locations and two 1991 planting dates in Iowa. Differences between laboratory germination test results for s eeds with low (6.4 and 11.5%) infection levels were minor. At higher i nfection levels (18.2 and 32.4%), test differences were as large as 23 %. Phomopsis-diaporthe infection was located primarily in the seed coa t with lesser infection levels in cotyledons. Cotyledonary infection w as not associated with variation in test re suits, but seed coat infec tion was, as evidenced by greater numbers of infected-abnormal seedlin gs in Kimpac and towel tests than in other tests. Field emergence decl ined with increasing PD seed infection at all sites and planting dates . Kimpac, towel, sand, A.Aging, and cold tests gave good estimates of field emergence at seed infection levels to 18.2%, but at 32.4% infect ion, Kimpac and towel tests underestimated field emergence. Tetrazoliu m and conductivity tests overestimated seed quality at low and high in fection levels.