EFFECT OF SOIL SHADING BY SURFACE RESIDUES DURING SUMMER FALLOW ON TAKE-ALL OF WINTER-WHEAT

Artificial inoculum of Gaeumannomyces graminis var. tritici was incorporated 10 cm deep in field plots about 1 mo after harvest of winter wheat and 2 mo before planting of wheat, and plots were either shaded with wheat straw or left bare. Plants grown in shaded plots had more severe take-all and lower grain yields than plants grown in plots left bare. During the oversummering period between wheat crops, soil temperatures at a depth of 5.1 cm were routinely 10 C cooler in shaded plots than in bare plots, and maximum soil temperatures reached 32.5-37.4 and 37.5-42.5 C on more days in bare plots than in shaded plots. Regression models of data from laboratory experiments indicated that the ability of inoculum (infested oat grains) to cause disease would be completely eliminated by 12 consecutive exposures for 6 hr/day to temperatures of 35 C (24 C at other times), four exposures for 6 hr/day to 40 C, or one exposure to 45 C. Thermal inactivation of oversummering inoculum is an important limiting factor to the development of take-all in Kansas. Management practices that increase soil shading (such as volunteer wheat, double-cropping, and no-till) tend to prevent high soil temperatures and may promote inoculum survival and disease.