Madar, Z., Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel
Solel, Z., Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel
Sztejnberg, A., Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Rehovot, Israel

The rate of water flow through segments of cypress branches infected by either Diplodia pinea f.sp. cupressi or Seiridium cardinale was markedly lower than in healthy branches. Xylem flow resistance was enhanced more by diplodia canker than by seiridium canker. Water flow rate in wounded control branches was similar to that in healthy unwounded ones. Histological study of branches infected by these pathogens revealed that hyphal colonization of tracheids and parenchyma rays of sapwood tissue was sparse and could not be related to water blocking. Neither tyloses nor resin deposition were observed in the affected zone. Tannin accumulation was occasionally observed in tracheids and parenchyma rays of sapwood. Scanning electron microscope examinations of tracheids from branches affected by seiridium or diplodia cankers, revealed heavy plugging of pit membranes with dense material, while pit membranes of intact wounded or healthy branches were well perforated. We suggest that mortality of cypress due to these pathogens results from obstruction of water flow in the xylem, caused by plugging of pit membranes. © 1990.