Effect of crop rotation on take-all of wheat in double-cropping systems

Take-all of wheat (Triticum aestivum), caused by Gaeumannomyces graminis var. tritici, became a serious problem with the widespread adoption of wheat:soybean double-cropping and minimum tillage farming systems in the southeastern United States during the past 30 years. A longterm crop rotation study was initiated in 1994 with 12 double-cropping sequences incorporating wheat, rye, or canola as the fall-planted crop and soybean or grain pearl millet as the summer crop. Cotton and fallow were included in some summer rotations during the last 2 years of the study. The purpose was to identify sustainable alternatives to the continuous wheat:soybean system that would provide acceptable management of take-all. G. grammis var. tritici cultured on autoclaved oats was incorporated into soil prior to planting the first season's crop. Take-all was severe in rotations with continuous wheat each year. Pearl millet was compatible with the cropping system but did not affect incidence or severity of take-all in a following wheat crop. Soybean or pearl millet had little effect on yield loss due to take-all in a subsequent wheat crop. A 1-year rotation with canola significantly reduced take-all incidence and severity. At the end of the second and third seasons, in those rotations where wheat followed I year of canola, wheat grain yield was the same as that in control plots that bad little or no take-all. Two consecutive years of canola did not suppress take-all or improve wheat yields any more than a single year of canola between wheat crops. Seedling assays for take-all incidence and severity in growth chambers were conducted using soil collected twice each year near the end of each crop's growing season. Results were similar to those observed in the field. However, canola in the rotation had a greater effect in suppressing disease severity than disease incidence. Canola can be a valuable rotational crop for management of take-all in wheat in the southeastern United States.