Role of rhizobitoxine in protecting soybean roots from Macrophomina phaseolina infection.

Bacterization of soybean seeds or roots with Rhizobium japonicum significantly reduced charcoal rot disease caused by Macrophomina phaseolina . Rhizobium japonicum inhibited the growth of M. phaseolina on both liquid and solid media. Replacement of nutrient medium with culture filtrate of R. japonicum significantly reduced mycelial growth of M. phaseolina . Whole culture extracts of R. japonicum yielded a toxic substance which was identified as rhizobitoxine after chromatographic, ultraviolet, and infrared spectrophotometric analyses. This compound also was detected in the roots of soybean inoculated with either R. japonicum alone or in combination of R. japonicum and M. phaseolina . Dosage response curves with rhizobitoxine showed it to be antifungal. The possible role of rhizobitoxine in protecting soybean roots from M. phaseolina infection is discussed.