Influence of preseason weed management and in-crop treatments on two successive wheat crops. 2. Take-all severity and incidence of Rhizoctonia root rot

Abstract

The effects of cultivation and herbicide use to control weeds in wheat on wheat growth, the severity of take-all, and the incidence of rhizoctonia root rot were studied for 2 seasons. Preseason treatments were no weed control, paraquat (0.20 kg a.i./ha), glyphosate (0.18 kg a.i./ha or 4 applications of 0.72 kg a.i./ha), and heavy grazing. In-crop treatments were cultivation plus trifluralin, direct drilling plus chlorsulfuron, and direct drilling alone. At the site, take-all was the main disease while rhizoctonia root rot was relatively minor. Glyphosate applied 4 times at 0.72 kg a.i./ha over the previous spring and summer led to greater wheat dry matter (DM) production, significantly (P<0.05) less severe take-all, and a lower incidence of rhizoctonia root rot in the first year than the other preseason treatments. Spraytopping with glyphosate (0.18 kg a.i./ha) or paraquat (0.20 kg a.i./ha) and heavy grazing reduced take-all severity but not the incidence of rhizoctonia root rot. Conventional cultivation resulted in more wheat DM, significantly less severe take-all, and a lower incidence of rhizoctonia root rot than direct drilling. Grain yields reflected the trends of the DM production despite severe yield loss due to head frosting. Plots were split for cultivation and direct drilling in the second year. The highest wheat DM and grain yields were in the cultivated treatments but the effects of cultivation on take-all did not carry over from the first year. In both years, take-all was most severe in the control treatment and least severe in the treatment with the high rate of glyphosate (P<0.05). In the second wheat crop, however, take-all severity was similar in the 2 glyphosate, paraquat, and grazed treatments. The effect of a weed-free fallow obtained by use of a high rate of glyphosate was nullified in the second wheat crop because of a high carryover of volunteer wheat seedlings during the intervening wet summer. There was also a greater incidence of rhizoctonia root rot in the control than in the other treatments, and cultivation again reduced disease incidence compared with direct drilling.