Biocontrol potential of Bradyrhizobium japonicum against soybean sudden death syndrome
Soybean [Glycine max (L.) Merr.] is the leading oilseed crop which accounts for 60% of the global oilseed production. Soybean sudden death syndrome (SDS) is among the most important soil-borne fungal diseases that threaten soybean production in US. Rhizobia play an important role as a nitrogen source for soybean via the fixation of atmospheric nitrogen and its subsequent conversion to useable forms. Slow-growing Bradyrhizobium japonicum (BJ) is the best known species nodulating soybean. There are currently a few reports of the use of Bradyrhizobium spp. in row crop diseases management. In literature, Bradyrhizobium isolates successfully reduced symptoms of diseases caused by Macrophomina phaseolina, Fusarium solani, and Fusarium spp. However, to the best of our knowledge, no information on the role of Bradyrhizobium japonicum in the management of SDS is established. In view of this knowledge gap, this study was designed to: (i) determine the effects of B. japonicum seed treatments and/or soil inoculant on the incidence and severity of the SDS under controlled conditions; (ii) evaluate the effects of B. japonicum seed treatments on SDS under field conditions (irrigated and non-irrigated)
To accomplish the first objective, two independent experiments were conducted twice under growth chambers and greenhouse conditions using a BJ strain from Iowa State University. Treated seeds (germinated or non-germinated) planted in clay pots or foam cups were exposed to three different growth chamber temperatures (15°C, 20°C and 25°C). Ten days later, treated seeds were shifted to a greenhouse at 25°C for the rest of study period. Treatments with BJ inoculated on the seeds planted in FV infested soil were found significantly effective in reducing SDS incidence and severity compared with untreated controls. In addition, BJ treatment of non-germinated or germinated seeds had significant influence on SDS incidence at low temperature. No significant effects on plant biomass were detected.
To address the second objective, field trials were conducted during 2015 to 2017 cropping seasons using four BJ strains in seed treatments. At R6 (full seed set) growth stage, BJ treated plots showed lower SDS disease index compared with untreated plots in 2015 and 2016. Although no significant effects of treatments on nodule count and plant biomass, higher nodules count at V5 (fifth trifoliate) were observed in BJ treated plots compared with untreated plots. In addition, the BJ-treated plots showed yield advantage compared with untreated controls across three years.
The results from these studies showed that B. japonicum can reduce SDS. Current findings also suggest that B. japonicum can be a potential biocontrol agent, which is environmentally friendly, and as one of the alternatives to soybean growers in Iowa