Influence of paired-row spacing and fertilizer placement on yield and root diseases of direct-seeded wheat

Root diseases, namely take-all caused by Gaeumannomyces graminis (Sacc.) Arx & D. Olivier var. tritici J. Walker, Rhizoctonia root rot caused by R. solani Kuhn AG8, and Pythium root rot caused by several Pythium Pringsh. species, become yield-limiting to wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) in the inland Pacific Northwest when these crops are grown without adequate rotation, especially in direct-seed (no-till) systems. We tested whether yields of wheat could be increased by (i) pairing the rows rather than spacing them uniformly, so as to keep the crop canopy open longer into the growing season, and (ii) placing fertilizer within rather than between the rows, so as to make nutrients more accessible to diseased roots. All experiments were done in fields with a recent history of intensive wheat. Direct seeding was used to further intensify pressure from root diseases, and soil fumigation and/or stubble burning were used in some experiments to reduce pressure from root diseases. Mixed effect models were used to analyze the yield results, with location (observation) treated as the random effect. When all yield data for four winter wheat experiments were used, and spacing, fumigation, and their interaction were the fixed effects in the model, spacing and fumigation but not their interaction were significant. When the yield data were limited to fertilizer placed within each row (directly beneath the seed), spacing, fumigation, and their interaction all were significant. However, when the yield data mere limited to fertilizer placed between the rows, or to yields from plots without straw on the soil surface, neither spacing, fumigation, nor their interaction were significant. Similar results were obtained in a spring wheat experiment where fumigation, row spacing, and fertilizer placement were tested is all combinations. Moreover, the amount of Rhizoctonia root rot, take-all, or both were less in response to paired-row spacing in two winter wheat experiments. The higher yields can be explained by the microenvironmental benefits of a more open canopy,vith paired rows together with the concentration of soil disturbance and row cleaning with paired shanks (presumably resulting in more warming and drying of the top soil where the pathogens are most active), and placement of fertilizer directly beneath each seed row, all of which have the potential to reduce the effects of root diseases.