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Interaction of Brassicaceous Seed Meal and Apple Rootstock on Recovery of Pythium spp. and Pratylenchus penetrans from Roots Grown in Replant Soils

Pythium spp. and Pratylenchus penetrans are significant components of the diverse pathogen complex that incites apple replant disease in Washington State. The structure of the Pythium population differs among orchard soils but is composed of multiple pathogenic species. Studies were conducted to determine the effect of brassicaceous seed meals and apple rootstock on the activity and composition of these pathogen populations. Brassicaceous seed meals differed in capacity to suppress Pythium numbers and apple root infection, as well as differentially transformed composition of the population recovered from apple roots. Brassica juncea seed meal (SM) was the sole seed meal examined to suppress Pythium numbers and root infection; however, a persisting population was always detected in which Pythium irregulare existed as the dominant or co-dominant species. In general, the Geneva series rootstocks were less susceptible to root infection by native populations of Pythium, whereas M26, MM106, and MM111 were highly susceptible. Apple rootstocks from the Geneva series consistently supported lower populations of P. penetrans than did Malling or Malling-Merton rootstocks. B. juncea SM was superior to Brassica napus SM or Sinapis alba SM in suppressing lesion nematode populations. Significant rootstock × seed meal interaction was detected, and nematode suppression in response to B. napus or S. alba SM was only observed when used in concert with a tolerant rootstock, while B. juncea SM suppressed lesion nematode root populations irrespective of rootstock. These findings demonstrate that utilization of brassicaceous seed meal amendments for replant disease suppression must employ an appropriate rootstock in order to achieve optimal disease control.