The dynamics of root diseases of wheat and barley in the transition from conventional tillage to direct seeding

Direct seeding is the planting of a crop into the previous stubble without tillage. Direct seeding can reduce soil erosion, but few growers in the Pacific Northwest have adopted this practice, partially due to fears of increased root diseases caused by Gaeumannomyces graminis var. tritici, Rhizoctonia spp., and Pythium spp. However, the impact of direct seeding on these diseases is not well understood. To examine the effect of transitioning from conventional tillage to direct seeding, two field plots were established. One was managed with direct seeding for 12 years and the second was conventionally tilled. From 2001 to 2004, a portion of each plot was tilled or direct seeded, and planted to wheat or barley. There were consistently more crown roots on plants in tilled than in direct seeded plots. In 2001 and 2002, Rhizoctonia root rot and yield did not differ between tillage types. However, in the third and fourth years of the transition to direct seeding, a higher incidence of Rhizoctonia root rot, increased hyphal activity of R. solani, and reduced yields were observed in direct seeded plots. Populations of R. oryzae and Pythium spp., and incidence of take-all were the same for both management practices. A disease survey was conducted, comparing fields managed with direct seeding to adjacent tilled fields. Rhizoctonia root rot was consistently higher in wheat fields direct seeded for 3 to 5 years compared with neighboring tilled fields, but in fields direct seeded for 14 to 21 years, the incidence of disease did not differ or was higher with direct seeding. In greenhouse assays, the relationship between added inoculum of Rhizoctonia spp. and disease was the same in soils collected from tilled or direct seeded fields. The rate of growth of Rhizoctonia spp. through intact soil cores from direct seeded and tilled fields was examined. R. solani AG-8 grew further through tilled soils than direct seeded soils, but the growth of R. oryzae did not differ. A new technique for identification and quantification of Pythium spp. directly from soil was developed using real-time PCR and species-specific primers designed from variable regions of rDNA.