In vitro inhibition of phomopsis phaseoli by microorganisms acquired from the environment of the soybean

Stem canker of soybean is caused by Diaporthe phaseolorum var. caulivora and its conidial stage, Phomopsis phaseoli. Stem canker is currently controlled by fungicides, crop rotation, resistant soybean cultivars, and tillage practices. Biocontrol agents such as Bacillus cereus, Gliocladium roseum, and Trichoderma species have been used as successful controls of plant diseases related to Diaporthe and Phomopsis.

Microorganisms isolated from the phylloplane, rhizosphere, and seed pods were reinoculated onto soybean leaves, stems, and seed; those not pathogenic to soybean were tested against P. phaseoli mycelium and spores in vitro. Cross culture methods were used to examine the effects of temperature, C/N ratios, and hyperparasitism. Zones of inhibition were measured to determine if anti biosis occurred in the temperature and C/N studies. Culture filtrates of the isolates were used in spore germination tests.

Twelve microorganisms were used in experimentation with one of the organisms having been collected a previous year. Isolates produced their largest zones of inhibition at 22 and 24 C. The largest zones of inhibition produced by the isolates during the carbon/nitrogen study developed when carbon was absent or deficient or when nitrogen was absent. Significantly fewer P. phaseoli spores germinated at a concentration of 5,000 spores per ml, when placed in isolate filtrates. Hyperparasitism did not occur between any isolate and P. phaseoli. Three

Bacillus pumilus isolates produced results as potential biocontrol agents. Isolates D.l and D.T collected from the phylloplane produced significantly larger zones of inhibition from the P. phaseoli control in the temperature and C/N studies. Isolate D.T also significantly reduced the number of P. phaseoli spores that germinated in the germination study. Isolate FSII, Bacillus pumilus, from the rhizosphere, produced significantly larger zones of inhibition in the temperature and nutrition studies, however, did not significantly reduce the germination of P. phaseoli spores in the germination study. These isolates need to be further examined to determine their potential as future biocontrol agents and as a part of integrated systems.