Biological control of damping-off caused by Pythium ultimum and Rhizoctonia solani with Gliocladium virens in soilless mix

Gliocladium virens controlled damping-off of zinnia, cotton, and cabbage caused by Pythium ultimum or Rhizoctonia solani in nonsterile soilless mix. This antagonist most effectively controlled disease among 50 isolates of bacteria and fungi, including species of Pseudomonas, Bacillus, Trichoderma, and Penicillium. Twenty isolates of G. virens varied in their efficacy in controlling P. ultimum and R. solani. Some isolates controlled P. ultimum but not R. solani, and vice versa. This range of activity suggests a complex mechanism of action that might apply to one pathogen but not the other. Inoculant of G. virens routinely was preincubated in the soilless mix before contamination of the mix with pathogen inoculum. Control of P. ultimum was effective when sporangial inoculum of the pathogen was introduced at the time of planting the host seed; however, control of R. solani required prior contact of G. virens with inoculum of R. solani. Disease control efficacy lasted for at least 2 mo when G. virens was introduced with the pathogen inoculum and the mix was planted with zinnia seeds at intervals. The number of colony-forming units of G. virens remained high during the testing period, but the number of pathogen propagules was greatly reduced. The efficacy of the isolates tested, however, was not correlated with the number of colony-forming units of G. virens. Sodium alginate formulations of isolate G20 of G. virens, selected for control of both pathogens, maintained a high population density in a dry formulation when stored for 2 mo at 4 and 20 C, but not at 30 C. Storage of an alginate formulation at these same temperatures in air-dried soilless mix was not successful. Alginate formulations of G. virens added to soilless mix before planting seed show promise as a control for damping-off in the greenhouse production of bedding plants.