The effects of Pierce's disease on leaf and petiole hydraulic conductance in Vitis vinifera cv. Chardonnay

In this study, we test the hypothesis that the symptoms of Pierce's Disease (PD) result from the occlusion of xylem conduits by the bacteria Xylella fastidiosa (Xf ). Four treatments were imposed on greenhouse-grown Vitis vinifera cv. Chardonnay: well-watered and deficit-irrigated plants with and without petiole inoculation with Xf. The hydraulic conductance of the stem-petiole junction (k(jun)) and leaves (k(leaf)) were measured, and Xf concentrations were established by quantitative polymerase chain reaction (qPCR). Leaf hydraulic conductance decreased with increasing leaf scorch symptoms in both irrigation treatments. The positive relationship between Xf concentration and symptom formation in deficit-irrigated plants suggests that water-stress increases susceptibility to PD. In field-grown vines, water relations of symptomatic leaves were similar to naturally senescing leaves but differed from green control leaves. Overall, these results suggest that the development of PD symptoms represents a form of accelerated senescence as part of a systemic response of the plant to Xf infection.