ABSTRACT
Four isolates of Trichoderma (Gliocladium) virens (G-45, G-65, G-85, and G-93) and two isolates of binucleate Rhizoctonia spp. (BNR621 and P9023) were evaluated for biocontrol of preemergence damping-off of Catharanthus roseus (vinca) caused by Pythium ultimum. Putative biocontrol agents were amended to a soilless mix 1, 3, or 6 days prior to seeding and pathogen infestation to determine if colonization of the mix before infestation was important for biocontrol efficacy. Biocontrol of preemergence damping-off of vinca with the four isolates of T. virens was variable. Only isolate G-93 gave control of preemergence damping-off (10 to 18% disease) regardless of the length of time the mix was amended prior to seeding and infestation compared to the infested control (43% disease). In contrast, preemergence damping-off was 10 to 15% with SoilGard (based on isolate GL-21 of T. virens). For isolate G-65, preemergence damping-off of vinca was 0% in lots of mix amended 1 day prior to seeding, but over 60% in lots of mix amended 6 days prior to seeding, compared to 43% in the infested control. With the exception of isolate G-65 in the lot amended 6 days before seeding, the isolates of T. virens were as effective as metalaxyl (19% damping-off) for control of P. ultimum in lots of mix amended 1 to 6 days before seeding. In contrast to T. virens, biocontrol efficacy of isolates BNR621 and P9023 of binucleate Rhizoctonia spp. in a Pesta formulation improved as lots of mix were amended up to 6 days before seeding and infestation. As length of initial amendment increased from 1 to 6 days, preemergence damping-off decreased from 37 to 16% for BNR621, and from 42 to 22% for P9023. Preemergence damping-off was observed in vinca in control treatments with only the putative biocontrol agents (BNR621, 14% disease and P9023, 19.6%); therefore, additional bedding plant species were evaluated for susceptibility to the BNR isolates. In the absence of P. ultimum, isolates BNR621 and P9023 in a Pesta formulation caused an average 82.5, 56.5, and 5.8% damping-off of snapdragon, petunia, and impatiens, respectively. Our results suggest that binucleate Rhizoctonia isolates, although effective for biocontrol of P. ultimum on vinca, should be evaluated for pathogenicity on a crop by crop basis before use on other crops.
