Current UC Viticulture Research

Viticulture Consortium West

By micropropagating plants in agar medium that also sustained the infectious form of the pathogen, the mycelium, we found that roots were infected in days instead of months (Fig. 1). Inoculation with homogenized mycelium rather than artificial ‘residual roots’ ensured equal challenge of replicate plants (Fig. 2). We used confocal microscopy to quantify infection of root tips, and developed the first quantitative PCR (Q-PCR) method for Armillaria. Both methods were then compared for detecting differential root colonization in a resistant grape rootstock (Freedom) versus a susceptible grape rootstock (3309C) at 2, 4, 6, and 8 weeks postinoculation, in two independent experiments.

On the day of inoculation, percent root colonization, as measured by confocal microscopy, and fungal biomass, as measured by Q-PCR, was 0 in both rootstocks. Armillaria was detected by microscopy and Q-PCR starting two weeks post-inoculation. At each consecutive harvest interval following inoculation, Freedom had significantly lower root colonization than 3309C. This was consistent in both replications of the experiment. Q-PCR showed similar differences: Freedom had higher fungal biomass than 3309C. We also found a significant positive correlation between percent root colonization and fungal biomass, demonstrating that our two separate methods give similar results. There were no escapes, based on fact that both methods detected the pathogen in all inoculated plants at all four harvests and in both replicate experiments.

The new assay results in faster infection than old assay, based on fact that pathogen was detected at 2 weeks post-inoculation. The new assay equally challenges all inoculated plants; no escapes. Also, the new assay detects differences in host susceptibility, based on our findings that 3309C had higher root colonization and higher fungal biomass at each of 4 harvests and in both replicate experiments. We are currently adapting this infection assay to Prunus crops and Juglans, all of which are more susceptible to Armillaria root disease than grape.

Click to enlarge items below

Figure 1 (includes both photos):

Tissue culture medium beneath the agar plugs taken from a 7-d culture of A.mellea (see arrows) is visibly clouded by the mycelium at 5 d postinoculation.

Non-inoculated plant with agar plugs taken from sterile media (see arrows), for comparison.

Figure 2 (includes both photos):

For the new infection assay, inoculum can be produced in 7 d

In comparison to the old infection assay, production of artificial ‘residual roots' takes 6 months.

Supplemental Data - Figure of confocal microscopy of grape root tips colonized by A. mellea.

Supplemental Data - Figure of temporal changes in A. mellea biomass.

Kendra Baumgartner

Baumgartner K, Coetzee M, Hoffmeister D. 2011. Secrets of the subterranean pathosystem of Armillaria (pdf). Molecular Plant Pathology DOI: 10.1111/J.1364-3703.2010.00693.X. 20pp.

Baumgartner K, Bhat R, Fujiyoshi P. 2010. A rapid infection assay for Armillaria and real-time PCR quantitation of the fungal biomass in planta (pdf). Fungal Biology 114:107-119.

Baumgartner K, Rizzo DM. 2006. Relative Resistance of Grapevine Rootstocks to Armillaria Root Disease. American Journal of Enology and Viticulture 57:4:408-414. Abstract

Baumgartner K, Warnock AE. 2006. A Soil Inoculant Inhibits Armillaria mellea In Vitro and Improves Productivity of Grapevines with Root Disease. Plant Disease 90:439-444.

Baumgartner K. 2004. Root Collar Excavation for Postinfection Control of Armillaria Root Disease of Grapevine (pdf). Plant Disease 88:1235-1240.

Baumgartner K, Rizzo DM. 2002. Spread of Armillaria Root Disease in a California Vineyard. American Journal of Enology and Viticulture 53:3:197-203. Abstract