Elm leaf scorch: abnormal physiology in American elms infected with fastidious, xylem-inhabiting bacteria

Abstract

Stem elongation, leaf area, xylem function, water relations, and stem starch reserves were compared between leaf scorch affected American elms (Ulmusamericana L.) infected with fastidious, xylem-inhabiting bacteria and symptomless trees. Stem elongation was significantly lower in elm leaf scorch affected trees than in symptomless trees, while no differences occurred in leaf area. Stem hydraulic conductivity in affected trees decreased from June through September, but increased during the same period in symptomless trees. Dye infiltration rates and number of functional vessels in stems collected from leaf scorch affected trees in September were significantly lower than in symptomless trees. Changes in stem water potential paralleled xylem infiltration rates. No differences in any measured parameters occurred between elm leaf scorch affected and symptomless trees in June prior to symptom onset. Once symptoms developed, stem water potential was more negative in affected trees than in symptomless trees. Mean stem starch levels were significantly higher in symptomless trees than in symptomatic trees. Results confirm that water deficit in leaf scorch affected trees may be attributed to xylem dysfunction possibly caused by bacterial occlusion. Growth reduction can be associated with decreased starch reserves induced by reduced water translocation and reduced effective photosynthetic area caused by the foliar necrosis of elm leaf scorch.