Root Heterogeneity And Replant Disease Development On The Fine Root System Of Apple
Root growth and development structure plant interactions with soil-borne pathogens by shaping the spatial and temporal availability of susceptible tissues. To evaluate apple root heterogeneity and susceptibility to apple replant disease (ARD) pathogens, a greenhouse bioassay of two rootstock genotypes, M.26 and CG.210, was conducted. Pathogen abundance was compared across root branching orders and growth trajectories (1st order fine-feeder vs. pioneer) and anatomical development and tissue phenolics were evaluated as mechanisms of tissue resistance. Rootstock growth and defense were then investigated for the two genotypes previously shown to be more (M.26) or less (CG.210) susceptible to ARD. Quantitative PCR estimates of pathogen DNA concentration were lower in pioneer roots and 3rd order roots compared to 1st and 2nd orders, which corresponded to distinct stages of tissue maturation including the senescence of cortical tissues in 2nd order roots and loss in 3rd order roots. Phenolic profiles indicate distinct compounds are preferentially allocated to different root branching orders. Phloridzin (phloretin 2'-O-glucoside) was found in greater concentration in higher branching orders, while defense induction or stress response was only detected in 1st order and pioneer roots. CG.210 roots had lower abundance of Cylindrocarpon spp., Pythium irregulare, and P. sylvaticum DNA in 1st order and pioneer roots compared to M.26, a different phenolic profile, and accumulated two-fold more root biomass than M.26 in both the replant soil and the pasteurized control. These results suggest that apple root maturation controls tissue resistance and response to pathogen infection, and that root branching order can provide a functional classification of fineroots meaningful to investigations of plant-pathogen interactions. The ability of CG.210 to maintain growth in replant soil may be attributable to relative resistance to replant pathogens in distal root branches and tolerance of infection based on increased rates of root growth.
root branching order; apple replant disease; soil-borne pathogens
Bauerle, Taryn L.
Nelson, Eric Bronson; Kessler, Andre
M.S. of Horticultural Biology
Master of Science
dissertation or thesis