Efficacy of soybean seed treatments in Iowa and the effect of cold stress on damping-off caused by Pythium sylvaticum

Abstract

<p>Soybean (Glycine max (L.) Merr.) production is negatively affected by the occurrence of seedling diseases. In Iowa, the occurrence of seedling diseases is commonly associated with cool and wet weather soon after planting, and several Pythium spp. have been found as the predominant pathogens causing seedling disease. Increasing cost of the seed, risk of seedling diseases reducing stand in cold, wet soils, and the yield benefit sometimes provided by early planting have motivated an increase in the use of fungicide seed treatment. However, a positive effect of seed treatments on soybean plant stand and yield has not been clearly demonstrated in Iowa. This study was conducted to: (i) compare the effect of commercially available soybean seed treatments on plant stand and yield; (ii) assess the effect of cold stress at planting on the incidence of Pythium seedling disease and efficacy of a commercial seed treatment; and (iii) evaluate the effect of chilling temperatures after planting on the Pythium – soybean interaction.</p> <p>Each year, three small plot field studies were conducted at different locations in Iowa in 2014, 2015 and 2016. Fifteen seed treatments were evaluated in 2014, and 13 seed treatments in 2015 and 2016. The experimental design was a randomized complete block with four replications. Seeding rate was 140.000 seeds per acre in 2014, and 120.000 seed per acre in 2015 and 2016. Periods of cool and wet weather soon after planting were observed in seven of nine trials within 4 to 7 days after planting. Enhancement of plant stand and yield by seed treatment was inconsistent among treatments and locations, even in trials where adverse weather conditions were observed soon after planting. Moreover, no bean leaf beetle (Cerotoma trifurcate) injury was observed, low soybean cyst nematode (SCN) (Heterodera glycines) populations were measured at the trial sites, and sudden death syndrome (SDS) (Fusarium virguliforme) foliar disease index was in the trials was also low. Based on these findings, seed treatment may not be indispensable to achieve high plant stands in Iowa and should be regarded as a preventive control measure to protect soybean stand from seedling disease. Even though our results were inconclusive, as suggested by other authors farmers should consider the use of seed treatments in field has a history of SDS, SCN, or seedling disease particularly in early planting.</p> <p>The effect of cold stress on efficacy of a seed treatment containing metalaxyl and ethaboxam (Intego SuiteTM) on soybean emergence was evaluated in a growth chamber experiment. A five-way factorial design with two cold stress temperatures (4à  à ºC and 10à  à ºC), two times for the initiation of cold stress (24 and 96 hours after planting), three cold stress durations (24, 48 and 96 hours), and two levels of seed treatments (Intego SuiteTM and untreated) was conducted in cups inoculated with Pythium sylvaticum or a non-inoculated control. Emergence was reduced when the pathogen was present, and longer periods of cold stress further reduced emergence. No differences where observed between times for the initiation of cold stress. The cold stress had no effect on emergence when the pathogen was absent. Seed treatment protected soybean seedlings subjected to periods of cold stress. Seed treatment also resulted in improved shoot weight and reduced root rot severity. The results from this study demonstrated that seed treatment is a useful management tool to protect seedlings from damping-off caused by P. sylvaticum when soybeans are planted under suboptimal temperatures.</p> <p>To improve understanding of the effect of cold stress on Pythium damping-off, growth chamber and laboratory experiments were performed. A growth chamber study was conducted to test the effect of a 96-hour period of cold stress at different times after planting on soybean damping-off. The experimental design was a three way factorial with two cold stress temperatures (4à  à ºC and 10à  à ºC); seven levels of timing of the initiation of cold stress (no cold stress, 0, 1, 2, 4, 6, 8 days planting); and inoculation with P. sylvaticum-infested millet or sterile millet (non-inoculated control). Increased susceptibility to damping-off was observed when soybeans were subjected to cold stress. Emergence was assessed 21 days after planting and was particularly low when cold stress occurred 2 or 4 days after planting in inoculated cups. In the non-inoculated controls, no effect of cold stress on emergence was observed. Also, emergence and seedling growth was delayed when P. sylvaticum was present. In a laboratory experiment, mycelial growth of P. sylvaticum on diluted V8 media was assessed at different temperatures (4à  à ºC, 10à  à ºC and 18à  à ºC). Low temperatures delayed mycelial growth but the pathogen was still able to grow at 4à  à ºC. In another experiment, seed exudation was assessed by measuring electrical conductivity. Greater seed exudation was detected when soybean seeds were imbibed at 4à  à ºC compared to 10à  à ºC and 18à  à ºC. Sporangia of P. sylvaticum germinated in response to seed exudates, and sporangial germination increased when cultures were exposed to seed exudates from seeds imbibed at 4à  à ºC. These results demonstrate that the timing of cold stress during seed germination play an important role in the occurrence of damping-off caused by P. sylvaticum. Moreover, low temperatures increase seed exudation and may enhance activity of P. sylvaticum.</p> <p>The results from this study have improved our understanding of the soybean-Pythium interaction and conditions that favor disease development. These data will aid soybean farmers in Iowa in making their decisions whether or not to use a seed treatment.</p>