Population dynamics of Gliocladium virens associated with growth of cotton seedlings

Abstract

Selective media were developed for the isolation of Gliocladium virens (GVSM) and G. roseum (GRSM) from nonsterile soil. Gliotoxin and acriflavin were key ingredients in the selective media for G. virens and G. roseum, respectively. Discrimination of colonies of G. virens on GVSM and G. roseum on GRSM from other fungi, including Trichoderma spp., was achieved after three or four days incubation following dilution plating. Recovery of indigenous isolates and four gliotoxin-producing strains of G. virens on GVSM was significantly greater or equal to recovery on three other media which have been used for isolation of G. virens. Recovery of five strains of G. roseum on GRSM from nonsterile soil was greater than or equal to the recovery on a previously developed selective medium for G. roseum. In a defined medium, sources of carbon and nitrogen and carbon:nitrogen (C:N) ratio influenced production of gliotoxin by a strain of G. virens (Gv-wt). When glucose and phenylalanine were used as the carbon and nitrogen sources, respectively, the greatest antibacterial activity against Bacillus cereus was expressed. With glucose and phenylalanine, dynamics of mycelial growth and gliotoxin production were similar at C:N ratios of 18:1, 31:1, and 42:1, but slower at a 80:1. Maximum gliotoxin (140 mg/L) was produced after eight days at all C:N ratios tested except 80:1 where 10 days of incubation were required. In soil infested with Gv-wt, population increases of G. virens were correlated with root length of growing cotton seedlings (r2=0.95). After the shoots were excised (30 days after planting), populations of G. virens declined. Without plants, soil population of G. virens remained constant for the duration of the experiment (60 days). Percentage of roots colonized by G. virens in infested soil increased over the course of the experiment (60 days) and reached 60 percent of root pieces sampled. Colonization of roots by indigenous G. virens was never greater than three percent. Growing cotton seedling roots appear to provide a good basis for the proliferation and survival of G. virens.