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 s everity were evaluated with respect to seasonal precipitation and soil chemical and microbiological parameters. Take-all and eyespot were as sociated with increasing precipitation, and Rhizoctonia root rot and F usarium crown rot were favored by drought. Eyespot and crown rot incre ased with rate of applied nitrogen and were inversely proportional to soil pH. Surface residue from previous crops had variable effects on d iseases. Crown rot increased with amount of surface residue and was di rectly correlated with soil organic nitrogen and carbon. Surface resid ue also had a variable effect on Rhizoctonia root rot, depending on th e magnitude of soil microbial respiration; root rot increased directly with amount of residue in a wheat-summer fallow rotation and was unaf fected by residue or tillage in a wheat-pea rotation. Repeated burning of wheat stubble caused variable disease response, depending on preci pitation and nitrogen rate. At high fertility, burning suppressed Pyth ium root rot and Rhizoctonia root rot, and enhanced eyespot and take-a ll. Effects of crop rotations on diseases appeared related to soil mic roflora effects on pathogen survival or virulence. Rhizoctonia root ro t was most damaging in wheat-fallow rotation, Pythium root rot in whea t-fallow and annual wheat, and eyespot and crown rot in annual wheat. Diseases were collectively least prevalent where nitrogen in a wheat-f allow rotation was applied as pea vines or manure, rather than as inor ganic fertilizer. Diseases also were generally less damaging in a whea t-pea rotation than in an annual wheat or wheat-fallow rotation. Soilb orne plant pathogenic fungi appeared to suppress wheat yield by 3 to 1 2%. Long-term experiments provided insights to crop management and sea sonal effects that are unlikely to be identified in short-term experim ents.