The mechanisms of plant root defences against soilborne necrotrophic pathogens are poorly understood, particularly in perennial tree fruit crops such as apple. Pythium ultimum is a component of the pathogen complex that incites apple replant disease (ARD). Previous field evaluation has demonstrated that apple rootstocks have different levels of ARD tolerance, but the underlying molecular mechanisms are unknown. Genotype responses in tissue cultured apple plants to P. ultimum infection were investigated at the biological, microscopic and molecular levels using ARD tolerant G.935 and susceptible B.9 apple rootstocks. Distinct differences in survival rate, root biomass, shoot biomass and maximum root length were observed between these two genotypes. Microscopic observations indicated contrasting dynamics in symptom development. The B.9 rootstock demonstrated an inability to limit pathogen progression, resulting in rapid progression of necrosis across the entire root system. In contrast, effective constraint of pathogen progression was shown in roots of G.935, as indicated by a limited or localized root necrosis. Apple genes, including MdCHS, MdCHIB, MdBIS and MdGLUC, which correspond to annotated functions in plant defence responses, showed differential expression profiles during infection between these two genotypes. These observations suggest that discrete molecular and biochemical processes exist in the two apple genotypes in response to P. ultimum infection.
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