Pierce's disease symptoms: Comparison with symptoms of water deficit and the impact of water deficits
The pathology of diseases such as Pierce's disease (PD) of grapevine (Vitis vinifera L.) that are caused by the xylem-limited bacterium Xylella fastidiosa (Xf) is widely attributed to vessel occlusion and subsequent water deficits. Grapevines (Vitis vinifera L. Chardonnay) were exposed to water deficits, stem inoculation with Xf, and combinations of both to evaluate whether symptoms of PD were a consequence of water deficits. When vines were inoculated with Xf and exposed to water deficits, more extensive PD symptoms developed throughout the plant than when infected vines were well watered. However, vines infected with Xf exhibited symptoms unique to PD that included inhibited periderm development in stems (green islands), leaf blade separation from the petiole (matchsticks), and irregular leaf scorch. Vines exposed to water deficits and not Xf displayed accelerated periderm development, basal leaf abscission at the stem/petiole junction, and uniform leaf chlorosis. Water deficits induced the development of an abscission zone at the stem/petiole junction, but PD did not. Pierce's disease symptoms could not be produced with any of several water-deficit treatments, including severing all but one secondary vein near the leaf tip. Results indicate that factors other than water deficits are involved in producing the symptoms of PD. The widely accepted hypothesis that PD-infected plants develop water deficits that cause green islands, matchsticks, localized leaf scorch, and eventual death of vines should be reevaluated.