Rootstock genotype and orchard replant position rather than soil fumigation or compost amendment determine tree growth and rhizosphere bacterial community composition in an apple replant soil

Apple replant disease (ARD) is a complex soilborne disease syndrome that often causes problems when renovating old orchard sites. Soil fumigants sometimes control ARD, but biological and cultural alternatives are needed. In this study the growth of two widely used clonal apple (Malus domestica) rootstocks (M7 and M26) were compared to three new rootstocks from the Cornell-Geneva series (CG16, CG30 and CG210 (a.k.a. CG6210)) in an orchard site with a history of ARD, in Ithaca, NY. Trees were planted in two distinguishable positions – the previous tree rows versus the old inter-row grass lanes. Additionally, we compared the effects of compost amendment and fumigation with dichloropropene plus chloropicrin on tree growth on each replant rootstock. Rhizosphere bacteria and actinobacteria were assessed using PCR-DGGE for the rootstocks M7, M26, CG30 and CG210. Tree growth on the rootstocks M7, M26 and CG16 was suppressed in the old tree rows relative to grass lanes, while trees on CG30 and CG210 rootstocks grew equally well in both positions. The species composition of rhizosphere bacteria and actinobacteria differed significantly between the planting positions and between the rootstocks M7 and M26 compared to CG30 and CG210. In contrast, the preplant compost or fumigation soil treatments had no effect on tree growth and little impact on rhizosphere bacterial community composition.