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Influence of Semiarid Cropping Systems on Root Diseases and Inoculum Density of Soilborne Pathogens

There is interest in converting the 2-year rotation of rainfed winter wheat with cultivated fallow in the Pacific Northwest of the United States into direct-seed (no-till) systems that include chemical fallow, spring cereals, and food-legume and brassica crops. Eight cropping systems in a low-precipitation region (<330 mm) were compared over 9 years to determine effects of changes on diseases. Fusarium crown rot was more prevalent in wheat following cultivated than chemical fallow, and Rhizoctonia root rot was more severe when winter wheat was rotated with chemical fallow than with no-till winter pea. Take-all occurred even during the driest years and was more severe on annual spring wheat than on annual spring barley. Inoculum density (picograms of DNA per gram of soil) differed (α < 0.05) among cropping systems for Fusarium culmorum, F. pseudograminearum, Gaeumannomyces graminis var. tritici, and Pythium spp. but not for Rhizoctonia solani AG-8. Phoma medicaginis var. pinodella was detected only where winter pea was planted frequently. This is the first report of P. medicaginis as a component of the dryland stem rot complex of pea in north-central Oregon. Results of this investigation will provide guidance for developing crop species with resistance to Fusarium crown rot and black stem of pea.