Penetration and Persistence of Commercially Formulated Beauveria bassiana Conidia in Soil of Two Tillage Systems

Abstract

Penetration and persistence of commercially formulated conidia of Beauveria bassiana (Balsamo) Vuillimen in the profiles of long-term conventional and no-tillage plots were measured by recovery of colony-forming units (CFUs) at selected depths over 235 d after application of an aqueous spray of conidia to the soil. High percentages of fungal propagules recovered from the conventional and no-tillage plots remained in the upper 5 cm of the profiles of the Cecil sandy clay loam soil. The conidia (2–3 by 2.0–2.5 μm) apparently were filtered from the aqueous suspension by the soil matrix as the suspension passed through the soil profile. Penetration of small percentages of conidia to the depth of 10.1 to 15.0 cm in both tillage systems occurred before 8 h after application. Rate of penetration of propagules to these lower depths was faster in conventional plots than in no-tillage plots. Infiltration rates of water were consistent with the differential rate of penetration in the two systems. Although only 300 and 1,800 CFUs/cm3 of dry soil remained in the 10.1- to 15.0-cm depth of the conventional and no-tillage plots, respectively, after 12 d, 90 and 75% of Galleria mellonella (L.) larvae exposed to soil from this depth of the respective tillages were infected. Recovery of propagules from the 0- to 5-cm depth of the conventional and no-tillage plots decreased 95 and 86%, respectively, within 12 d of application of conidia. By day 235, the conventional tillage plot contained only 18 CFUs/cm3 of dry soil at this depth. A 10-fold decrease in the number of CFUs remaining in the no-tillage plots occurred by day 49 (4.8 × 104 CFUs/cm3 of dry soil) with no propagules detected by day 200. Persistence of granular and wettable powder formulations of B. bassiana in the Cecil series soil was compared in a separate study. A 10-fold decrease in viable CFUs in plots treated with aqueous suspensions of the wettable powder occurred by day 21. By day 202, no CFUs remained in those plots. The number of CFUs recovered from plots treated with the granular formulation was 10 times higher than the number recovered from plots treated with the aqueous spray. After 202 d, 111 viable CFUs/cm3 of dry soil remained in the plots treated with granular formulation.