Soil Matric Potential Effects on Changes in Wall Morphology, Germination, and Lysis of Oospores ofPythium ultimum

Thick-walled oospores of Phythium ultimum in agar films on glass slides were incubated in soil for 84 days. In soil with matric potentials of −0.03 to −0.30 MPa, almost all of the thick-walled oospores converted to thin-walled oospores, and 91% of these lysed. In soil near saturation (0 MPa) and in much dryer soil (−1.50 MPa), 82 and 62% of the oospores remained thick-walled, respectively. Occasionally, oospores with attached germ tubes were observed but not in soil with matric potentials of 0, −0.7, or −1.5 MPa. Oospores were germinated in corn meal agar films on glass slides and then were killed by fumigation with propylene oxide, aerated, and placed in soil (...)