Diseases of wheat in long-term agronomic experiments at Pendleton, Oregon

Diseases of winter wheat were evaluated over 3 years in four long-term (27- to 60-year) cropping system experiments. Disease incidence and severity were evaluated with respect to seasonal precipitation and soil chemical and microbiological parameters. Take-all and eyespot were associated with increasing precipitation, and Rhizoctonia root rot and Fusarium crown rot were favored by drought. Eyespot and crown rot increased with rate of applied nitrogen and were inversely proportional to soil pH. Surface residue from previous crops had variable effects on diseases. Crown rot increased with amount of surface residue and was directly correlated with soil organic nitrogen and carbon. Surface residue also had a variable effect on Rhizoctonia root rot, depending on the magnitude of soil microbial respiration; root rot increased directly with amount of residue in a wheat-summer fallow rotation and was unaffected by residue or tillage in a wheat-pea rotation. Repeated burning of wheat stubble caused variable disease response, depending on precipitation and nitrogen rate. At high fertility, burning suppressed Pythium root rot and Rhizoctonia root rot, and enhanced eyespot and take-all. Effects of crop rotations on diseases appeared related to soil microflora effects on pathogen survival or virulence. Rhizoctonia root rot was most damaging in wheat-fallow rotation, Pythium root rot in wheat-fallow and annual wheat, and eyespot and crown rot in annual wheat. Diseases were collectively least prevalent where nitrogen in a wheat-fallow rotation was applied as pea vines or manure, rather than as inorganic fertilizer. Diseases also were generally less damaging in a wheat-pea rotation than in an annual wheat or wheat-fallow rotation. Soilborne plant pathogenic fungi appeared to suppress wheat yield by 3 to 12%. Long-term experiments provided insights to crop management and seasonal effects that are unlikely to be identified in short-term experiments.