Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: 4/1/2008
Publication Date: 7/1/2008
Citation: Bhat, R., Baumgartner, K., Fujiyoshi, P.T. 2008. Characterizing resistance to infection by the root pathogen Armillaria mellea in tolerant and susceptible grapevine rootstocks.. Phytopathology. 98:S20.
Technical Abstract: Grapevine rootstocks that are resistant to the basidiomycete fungus Armillaria mellea have not been identified, mainly due to lack of a rapid and reliable inoculation technique. The aim of our research was to develop an in planta assay for inoculating grapevines and tracking root infection. We propagated tolerant (Freedom) and susceptible (3309C) rootstocks from green cuttings rooted in tissue culture medium in a growth chamber at 25C. After 6 weeks, surface of the culture medium of each plant was inoculated with four mycelial agar plugs of a strain virulent on grape (n=25 per rootstock). Within days, hyphae proliferated the medium, and by week 2, were in direct contact with roots. At 0, 2, 4, 6, and 8 weeks post-inoculation, plants were harvested (n=5 per rootstock), roots were washed of culture medium, and pathogen growth in root tissue was confirmed by treatment with wheat germ agglutinin conjugated to a fluorophore (AlexaFluor 488) and confocal microscopy. Hyphae and roots were scanned on separate channels; hyphae appeared green, roots appeared red. Percent colonization of root area by hyphae was quantified (ImageJ, v1.37). Analysis of variance indicated significant differences in root colonization between rootstocks; 3309C had significantly higher root colonization than Freedom starting at week 4 and continuing throughout the experiment (P < 0.0001). The pathogen was detectable in roots, via microscopy, only two weeks after inoculation. This is a dramatic improvement over the previous inoculation technique, which resulted in earliest pathogen detection months to years after inoculation. With the ability to infect plants by Armillaria in a much shorter amount of time and on a reliable basis, we are now able to investigate the mode of root penetration by the pathogen, to identify susceptible portions of the root system, and to determine when foliar symptoms form in response to infection.