Development and comparison of symptom indices for quantifying grapevine resistance to Pierce's disease
Symptoms of Pierce's disease (PD) were assessed under greenhouse conditions on field-resistant and field-susceptible grapevines in order to characterize the PD resistance phenotype in the genus Vitis. A cane maturation index (CMI) was developed to quantify the green-islands symptom, which was measured at 12 weeks post-bacterial inoculation, along with leaf scorch and percentage of xylem vessels blocked by occlusions. Canes of resistant genotypes matured normally and had a significantly lower CMI score of 0.9 (on a 0-to-6 scale) compared with 5.1 for the susceptible genotypes. The CMI scoring method had a high correlation (R-2 = 0.91) with previously characterized field performance, whereas leaf scorch had only a moderate correlation (R-2 = 0.51) with field performance. Average scorched area on leaves of the susceptible and resistant genotypes was 80 and 48%, respectively, demonstrating that leaf scorch can be extensive in resistant genotypes under the presented screening conditions, and suggesting that systemic infection can occur in all evaluated genotypes. Occlusions within both stem and petiole vessels were composed principally of tyloses and were significantly higher in petioles than in stems of either resistant or susceptible back.-rounds. Susceptible genotypes displayed a higher level of stem tylose occlusions relative to resistant genotypes, but correlation to field performance was low (R-2 = 0.13). Ease of use and high correlation to field performance makes CMI scoring a better choice for PD resistance evaluations relative to other phenotypic symptom assessments.