The dual inoculation with Rhizobium sp. and cyanobacterial extracts enhances the common bean (Phaseolus vulgaris L.) responses to white rot disease caused by Sclerotinia sclerotiorum

Abstract

In this study, laboratory and greenhouse experiments were conducted to investigate the suppression effect of two biocontrol agents caynobacterial (Spirulina sp., Nostoc linckia and Anabaena variabilis) extracts, along with Rhizobium leguminosarum biovar phaseoli, against Sclerotinia sclerotiorum in common bean plants. Four isolates of S. sclerotiorum were tested for pathogenicity in bean plants, and all isolated fungi proved to be pathogenic and caused white rot symptoms. Results of in vitro studies showed that algal extracts significantly inhibited the mycelial growth of the pathogen when compared to the untreated control. N. linckia gave the highest reduction (56.29%), followed by A. variabilis (51.85%) and Spirulina sp. (45.93%), respectively compared to control (0%). In greenhouse experiments, the combined effect of Rhizobium sp. and cyanobacterial extracts significantly reduced disease incidence and severity under artificial infection with S. sclerotiorum. The treatments showed the maximum effects for controlling disease incidence and severity caused by S. sclerotiorum, which were in the range of 13.33 to 26.67 % and 1.24 to 1.82, compared to 73.33 and 4.50 % in infested control, respectively. In addition, these treatments increased number of nodules, plant height, root length, fresh and dry weight of shoots, N2 % and total nitrogen compared to control. The effects were similar to those of the fungicide Vitavax, which reduced the disease incidence and severity but adversely affected Rhizobium sp. and the symbiotic N2 fixing parameters. Considerable increases in activity of oxidative reductive enzymes (peroxidase and polyphenol oxidase) were recorded in plants grown from treated bean seeds.