Quantitative measurement of vascular flow in petioles of healthy and Verticillium ‐infected tomato

A Scholander pressure bomb was modified to measure accurately and continuously the flow through excised tomato petioles of sufficient length to contain no open xylem vessels. The contribution of vascular occlusion to water stress induced by Verticillium albo‐atrum was determined by comparing vascular flow through petioles supporting healthy, symptomless or wilted leaves. Invariably, flow was markedly reduced or completely prevented well before visual symptoms were apparent in leaves. In the system described collection of exuded xylem fluids for analysis over extended periods also allows the determination of the fate of fungal products introduced into vessels or the release of products from xylem walls or cells following treatment or infection. The various uses of this system to study xylem occlusion and host‐parasite inter actions in vascular diseases are discussed.