Effect of time of harvest, storage duration and fungicide treatment on soybean health

Globally, one of the most recognized problems in soybean production is loss of seed viability. Similarly, In Uganda, this problem was noticed in the 1970s soon after the introduction of the crop in the country. Although several interventions were put in place, loss of soybean seed viability continued to be a serious problem. The present study examined the effect of harvesting and storage duration on levels of fungal infection and subsequent seed germinability and whether seed treatment with fungicide would reduce fungal seed infection and significantly improve seed viability. The first part of this study examined the effect of time of harvesting time of harvesting time on incidences of seed-borne fungal infection and subsequent seed germination on soybean seed crop. The field study was conducted in Kasese district, and the laboratory work at the National Seed Testing Laboratory (NSTL), Kawanda Agricultural Research Institute (KARI). Six Uganda Seed Project (USP) contract growers were randomly selected to grow either Nam 1 or Nam 2 soybean varieties (3 per variety). At harvest maturity (R8 stage),a portion of the field, sufficient to yield 15kg of seed was randomly selected and demarcated. Each of these portions was divided into three parts and randomly allocated to first, second and the third harvesting, corresponding to R8 (90 days after planting) (DAP) and 7 and 14 days later. The seeds from each portion were hand harvested, sun-dried to 9.5% and 12% moisture contents ar recommended by International Seed Testing Association (ISTA) and Uganda Seed Project (USP), respectively threshed and cleaned by hands. Seeds were then subjected to seed health and germination assays. The Same seed samples were used to study the effect of storage period and seed treatment with fungicide on levels of seed borne fungal infection and seed germinability. For each moisture content group the seeds were either treated with (Vitavax 200FF) at a rate of 3ml/kg of seed, or untreated. Each of these sub-samples was put in a plythene bag and stored at room temperature at (22¬_+ 3 degrees) for 30,60, and 90 days and sampled for seed health and germinability. The results of this study have shown that soybean germinability is strongly influenced by time of harvesting and Fungicide treatment. The early harvested seeds showed higher germinability and most cases lower fungal flora infection compared to the late harvested seeds. Furthermore, high seed moisture content encouraged high level of fungal flora in seeds even when treated with fungicide. Time in storage did not influence seed germinability, implying that seed germinability and storability mostly depends on the quality attained before harvesting and post handling efficiency. The study identified 17 fungal pathogens associated with soybean seeds both in the field and storage. Fungicide treatment significantly reduced the levels of fungal flora occurrences and improved soybean germinability and it is therefore probable that the low germinability in soybean after long-term is due to the effects of these pathogens. The practical implications of the study therefore is that in order to produce high quality seeds capable of long term storage, soybean should be harvested at about 90 DAP, depending on maturity period, soybean should be promptly dried to 9.5% moisture level and treated with appropriate fungicide.