Soybean seed deterioration in the tropics. I. The role of physiological factors and fungal pathogens

Abstract

Two field experiments were conducted to determine whether the rapid loss of soybean seed viability in the lowland humid tropics is primarily due to physiological factors or to seed-borne microorganisms, or to both. In both experiments benomyl fungicide was applied (1 g/l) to four cultivars of contrasting storability as a foliar spray weekly from mid-flowering to maturity. The seeds were harvested at maturity or after a delay of two weeks. Plants in Experiment 1 matured under dry conditions and low fungi incidence. Harvest delay and benomyl treatment had little effect on seed pathogen incidence or germination at harvest. In Experiment 2, Which matured in moist conditions, delayed harvest reduced viability by 25%, with significant cultivar × harvest time interactions. Seeds from plants treated with benomyl had higher viability than seed from untreated plants, and significantly lower incidence of Phomopsis species, Macrophomina phaseoli, Fusarium species and Colletotrichum truncatum. Pathogens seem to be important in seed deterioration prior to harvest. To determine the effect of benomyl treatment on rate of decline in seed viability in storage, seeds from Experiment 1 and 2 were stored at 80% relative humidity and 28 or 35°C and sampled periodically. Seeds from benomyl-treated and untreated plants declined in viability at equal rates. Dusting of benomyl on seed of Bossier soybean equilibrated to 10, 13 or 16% moisture content before the start of storage at 28°C completely inhibited surface fungal growth, but did not influence rate of viability decline. Rate of viability decline varied directly with seed moisture content. Extracted embryos from freshly harvested Bossier seed grew on agar and were free from fungal pathogens, while embryos from seed stored for 9 months failed to grow even though little infection (0–5%) was observed. These results strongly suggest that the role of pathogens in seed deterioration during storage may frequently be secondary to that played by physiological factors.