1a. Objectives (from AD-416):
Determine effect of rootstock genetic background (+/- PD resistance) on disease severity and X. fastidiosa population levels in PD-susceptible scions following challenge inoculation of scions with X. fastidiosa.
1b. Approach (from AD-416):
The basic experimental design will evaluate PD symptom development and X. fastidiosa population levels in PD-susceptible scions grafted onto rootstocks that are either resistant or susceptible to PD. Source of PD susceptible plant material will be the wine grape variety ‘Chardonnay’, known to support high populations of X. fastidiosa and exhibit severe PD symptoms. Source of PD resistant material will be a backcross generation 2 (BC2) raisin selection (referred to here as PDR-BC2) with PD resistance introgressed from 89-0908 (V. rupestris X V. arizonica).
3. Progress Report:
Results of this study are in support of objective 4 of the parent project. The basic experimental design evaluated Pierce’s disease (PD) symptom development and X. fastidiosa (Xf) population levels in PD susceptible scions grafted onto rootstocks that are either resistant or susceptible to PD. The first year (FY12)experiment was conducted in a greenhouse at the USDA-ARS SJVASC facility in Parlier, California. Source of PD-susceptible plant material was the wine grape variety ‘Chardonnay’, known to support high populations of Xf and exhibit severe PD symptoms. Source of PD-resistant material was a modified backcross generation 2 raisin selection (referred to here as PDR1) with PD resistance locus PdR1 introgressed from 89-F0908 (V. rupestris X V. arizonica). Each treatment consisted of ~10 plants (replicates). The Xf strain Stag’s Leap was used as challenge inoculum, as this strain was recovered from Pierce Disease (PD-symptomatic) grape in California and is known to cause severe PD symptoms in inoculated plants. Plants were mechanically inoculated above the graft union (e.g., scions) and evaluated visually for PD symptoms after 14 weeks. PD severity was visually assessed using a nominal 0-5 rating scale, where 0 corresponds to no visual symptoms and 5 corresponds to death of the plant. In all cases, the response of the scion to PD remained unaltered, regardless of rootstock genotype. ‘Chardonnay’ scions expressed similar levels of PD severity on both PD-susceptible (mean severity rating 3.2) and PD-resistant rootstocks (mean severity rating 3.6), whereas PDR1 scions expressed only mild symptoms on PD-susceptible (mean severity rating 1.0) or PD-resistant (mean severity rating 1.4) rootstocks. PD symptom severity ratings for mock inoculated plants of all scion/rootstock combinations had means of less than 1.0, but greater than 0, presumably due to water stress at some point post-inoculation. Real-time PCR was used to quantify bacterial titers in stems and petiole samples collected 25 cm above the point of inoculation. DNA samples were extracted from lyophilized tissue and used as template. Real-time PCR reactions were run in triplicate to determine technical variability and standard curves were included in each plate to facilitate normalization. Real-time PCR data were converted to the equivalent number of Xf genomes; mean population levels were compared among scion/rootstock combinations for both stem and petiole samples. Xf population levels in ‘Chardonnay’ scions were similar regardless of rootstock genotype (~10,000,000 cells) in both stem and petiole samples. In contrast, Xf population levels were substantially lower in PDR1 scions (~10,000-100,000 cells), but no significant differences were noted for PDR1 scions based on rootstock genotype. Collectively, the results of the first year experiment indicate the tentative answer to the question posed in the project title is “No”. Interestingly, PDR1 scions grafted onto ‘Chardonnay’ rootstocks remained resistant to PD. This observation indicates that PD-susceptibility factors present in a rootstock do not result in alteration of PdR1-mediated response to PD in the scion.