Agronomic practices and common root rot in spring wheat: Effect of tillage on disease and inoculum density ofCochliobolus sativusin soil

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Abstract
The effect of tillage, primarily zero tillage, on common root rot in spring wheat was studied at three locations in Saskatchewan. Generally, disease intensity was lower under zero than conventional tillage. It was significantly lower in 6 of 10 years at Scott and in 3 of 7 years at Swift Current. Differences occurred in wheat in oilseed - wheat - wheat, fallow - oilseed - wheat, and fallow - wheat rotations and in continuous wheat. On average, the disease intensity in minimum tillage plots at Swift Current was intermediate between that in zero and in conventional tillage. Effect of depth of seeding usually was significantly greater under conventional than zero tillage. Covariance analysis indicated that seeding depth influenced the disease intensity: shallow seeding was associated with the reduced disease under zero tillage. Isolations from subcrown internodes showed Cochliobolus sativus to be the dominant cause of common root rot at the three locations, with a positive association between the frequency of its isolation and disease intensity. C sativus was obtained more frequently from plants under conventional than zero tillage. The inoculum density of C sativus in the top 8 cm of soil usually was significantly less under zero tillage than under conventional tillage. However, there was no indication that the inoculum density of C sativus directly influenced the disease intensity. The former was nonsignificant as the covariance in covariance analysis of intensity. Possibly inoculum density always exceeded the threshold density required for maximum disease under the test environments.