FACTORS INFLUENCING THE INCIDENCE OF EMBRYO INFECTION BY PYTHIUM SPP DURING GERMINATION OF WHEAT SEEDS IN SOILS

The incidence of embryo infection (EI) by Pythium spp. in soil held at a matric potential (psi(m),) of -10 J/kg was a function of inoculum d ensity (ID) at lower IDs and reached a plateau (EI(max)) at higher IDs . In field soils, EI(max) (69.9-89.0%) was achieved at 150-200 propagu les per gram. In pasteurized soil reinfested with P. ultimum var. spor angiiferum, EI(max) (congruent to 100%) was achieved with only 50 prop agules per gram. The incidence of EI was greatest at -10 J/kg, progres sively less as soil was drier, and negligible at drier than -200 J/kg. Both ID and psi(m), could be limiting for EI at lower IDs, whereas on ly psi(m), affected EI when soil contained enough ID for EI(max). When the pH levels of two natural soils were adjusted to 4.3-7.6 with H2SO 4 Or Ca(OH)(2), the incidence of EI was greatest at pH 5.1-6.7. The tr eatment of soil with chloramphenicol resulted in a higher incidence of EI in soil adjusted to alkaline pH, suggesting that antagonistic soil bacteria had some involvement in the low incidence of EI in alkaline soil. The incidence of EI was higher as seeds aged and lower when seed s were soaked in water before planting.