Dimensions, Density, and Settling Velocity of Entomophthoralean Conidia: Implications for Aerial Dissemination of Spores

Abstract

The epizootic behavior of entomopathogens is determined, in part, by their capacity for spatial spread in a host population. Among the entomophthoralean fungi, this may be accomplished through various mechanisms, including passive aerial dissemination of conidia. In this study we measured the size, shape, density, and settling velocity in still air of conidia of three species of entomophthoralean fungi: Zoophtora radicans, Conidiobolus obscurus, and Conidiobolus thromboides. In addition, the aerodynamic behavior of Z. radicans conidia was observed. Size and shape, measured by light microscopy, were consistent with previously reported values. Conidial densities ( 1.10 g cm -3 for Z. radicans and 1.06 g cm -3 for Conidiobolus spp.) were determined from the refractive index of spores measured by interference microscopy; these are the first reports in the literature of the density of entomophthoralean conidia. Settling velocity was measured by allowing fungal cultures to sporulate into a glass chamber, while falling conidia were observed by video microscopy. Images of falling spores were analyzed by a computerized motion analysis system, and settling velocities were determined. The system was calibrated by measuring the settling velocity of polystyrene microspheres of sizes similar to those of the spores and comparing observed velocities to those predicted by Stokes' Law. These are the first reports of settling velocity for entomophthoralean conidia. The nearly spherical spores of C. thromboides fell at velocities expected from Stokes' Law (median, 1.4 cm sec -1), while the spherical, but larger, spores of C. obscurus fell more slowly than expected (median, 4.6 cm sec -1), for reasons that are unclear. Conidia of Z. radicans fell at a velocity equal to that of spheres of the same volume and density (median, 0.4 cm sec -1), with little or no correction needed for their ellipsoidal shape. The settling velocity of ellipsoidal particles depends on their orientation. The falling orientation and aerodynamic behavior of conidia of Z. radicans were observed by high-resolution video microscopy; the spores consistently fell in a stable orientation with their major axis vertical. Our measurements of conidial size, shape, density, and settling velocity in still air provide information needed for models of particulate transport, which can be used to predict spore dispersal under field conditions involving wind and turbulence above and within plant canopies.