INFLUENCE OF WHEAT CHAFF AND TILLAGE ON PYTHIUM POPULATIONS IN SOIL AND PYTHIUM DAMAGE TO WHEAT

The role of Pythium spp in the injurious (allelopathic) effect of wheat straw on growth of wheat plants, such as associated with wheat planted into wheat residue managed by some form of conservation tillage, was investigated experimentally in both the field and growth chamber. The population of Pythium spp in the top 10 cm of soil in plots cropped the previous 7 yr in a wheat-pea rotation was consistently and significantly (P = 0.05) higher during the next 2 yr (years 8 and 9 of the study) in plots tilled all 9 yr so as to leave maximum wheat (or pea) residue near or on the soil surface (minimum tilled) than in plots mold-board plowed (clean tilled) annually. The Pythium population was intermediate where all crop residue was left on the soil surface (no tillage). In growth chamber studies, the Pythium population in a natural Thatuna silt loam increased ca 10-fold within the first 4 days after fresh wheat chaff was added to the soil at 1% (w/w); and wheat sown in this chaff-amended soil failed to emerge or the plants were stunted, spindly, and produced fewer tillers (typical of the residue-related injury) compared with plants grown in the same soil without fresh chaff. However, the injurious effect of fresh chaff was greatly reduced in the natural soil treated with metalaxyl (to selectively inhibit Pythium spp), less evident with weathered (and moldy) chaff in natural soil, and not evident with fresh chaff added to heat-treated (60°C/30min) soil unless the soil was reinfested with Pythium ultirnum var. sporangiiferum (500 propagules g-1 soil). The addition of either NH4Cl or KNO1 resulted in greener seedlings but did not prevent the injury, whether in natural or Pythium-reinfested pasteurized Thatuna silt loam. The results show that the injurious effect of the wheat residue on wheat growth, previously attributed to "phytotoxicity", can be due to increased Pythium damage to wheat. © 1990.