Interaction of Brassicaceae Seed Meal Soil Amendment and Apple Rootstock Genotype on Microbiome Structure and Replant Disease Suppression.

Preplant soil application of a Brassica juncea-Sinapis alba seed meal formulation (SM) at a rate of 6.6 t ha alters composition of the orchard soil microbiome in a manner that yields sustainable long-term suppression of soilborne pathogens in apple production systems. However, the cost of SM amendment has hindered the adoption of this tactic to manage apple replant disease in commercial orchards. Greenhouse trials were conducted to assess the effect of reduced SM application rates in concert with apple rootstock genotype on structure of the rhizosphere microbiome and associated disease control outcomes. At all application rates assessed, SM treatment increased tree growth and reduced disease development relative to the control. In general, total tree biomass and leader shoot length were similar in soils treated with SM at 4.4 or 6.6 t ha regardless of rootstock genotype. Equivalent increase in tree biomass when cultivated in soil treated at the lowest and highest SM amendment rate was attained when used in conjunction with G.41 or G.210 apple rootstocks. Suppression of Pythium spp. or Pratylenchus penetrans root densities was similar at all SM application rates. When cultivated in nontreated replant orchard soil, Geneva rootstocks (G.41 and G.210) exhibited lower levels of Pythium spp. and P. penetrans root colonization relative to Malling rootstocks (M.9 and MM.106). For a given rootstock, structure of the rhizosphere microbiome was similar in soils treated with SM at 4.4 and 6.6 t ha. G.41 and G.210 rootstocks but not M.9 or MM.106 cultivated in soil treated with SM at 2.2 t ha possessed a rhizosphere bacterial community structure that differed significantly from the control. Findings indicate that effective control of apple replant disease may be attained at lower SM amendment rates than employed previously, with lower effective rates possible when integrated with tolerant rootstock genotypes such as G.41 or G.210.