Developing rapid evaluations for resistance to Xylella fastidiosa, the causal agent of Pierce's disease
Pierce's disease (PD) of grapevines is caused by Xylella fastidiosa (Xf), a xylem limited, gram-negative bacterium that plugs xylem vessels leading to vine death. Studies on the multiplication and movement of Xf in Vitis and Muscadinia species have found that Muscadinia species to have high levels of Xf resistance leading to its use in breeding programs. Seedlings from AT00-76 (F2-7 (V. vinifera) x 8909-08 (V rupestris x M. rotundifolia), expected to segregate for Xf resistance, and open-pollinated (O.P.) seedlings from V. vinifera cv. Chardonnay, expected to be uniformly susceptible to Xf, were needle inoculated with a 20 PI drop of 2x10(8) cfu/ml Xf suspension at the 3-, 4- and 5-Leaf (3L, 4L, 5L) stages. O.P. Chardonnay seedlings were sampled 7 weeks-post-inoculation (wpi) at 10 cm above the inoculation point (15 seedlings each). Plants from the AT00-76 population were sampled at two locations (15 seedlings each): (i) the inoculation point; and (ii) 10 cm above the inoculation point 2 and 7 wpi. Water inoculations served as negative controls. ELISA tests were used to evaluate the levels and presence of Xf. The O.P. Chardonnay seedlings had high Xf populations 10 cm above the inoculation point. Although at least 30% of the AT00-76 seedlings were expected to be resistant, Xf was detected in almost all tested seedlings. The highest levels of Xf among the AT00-76 seedlings were detected when these seedlings were inoculated at the 3L stage. However, Xf populations were lower at the inoculation point compared to the first 2 wpi sampling, but Xf concentrations increased over time at the 10 cm sampling point. Xf populations did not increase between the two sampling periods in the 4L seedlings with three exceptions where Xf was detected at low levels. The 5L AT00-76 seedlings had Xf concentration that decreased or remained at the initial inoculation level between the two sampling dates. The first symptoms of PD on the O.P. Chardonnay seedlings appeared 6 to 7 wpi; these plants displayed leaf symptoms and irregular cane lignification. Symptoms on AT00-76 seedlings were first noticed 7 wpi on 3L and 4L plants and 6 wpi on 5L plants. These plants had typical leaf symptoms, but did not display the uneven stem lignification symptoms.