Formation of sporangia by Phytophthora drechsleri in soil at high matric potentials

Abstract

The formation of sporangia by Phytophthora drechsleri was examined in soil at constant matric potentials (ψm) between 0 and −0.3 bar, corresponding to soil water contents between saturation and field capacity. At ψm = −0.3 bar, P. drechsleri required 1.5 days to form a significant number of sporangia in soil. After 3–11 days in soil, when maximum numbers of sporangia were found, all ψm values from −0.025 to −0.3 bar were optimum for the formation of sporangia in both sterilized and unsterilized soil. In contrast, few sporangia were formed in soil at ψm values of 0 and −0.01 bar. Mycelia on the soil surface at ψm = 0 formed many sporangia, indicating that aeration at a depth of 5 mm in saturated soil was inadequate for sporangia to form. Viable sporangia were recovered from soil as long as 35 days after mycelial disks were placed in soil. Evidence is presented for the release of zoospores by sporangia in soil at all ψm values tested.