ORIGINAL PAPER Sap flow-based quantitative indication of progression of Dutch elm disease after inoculation with Ophiostoma novo-ulmi Josef Urban • Milon ˇ Dvor ˇa ´k Received: 5 January 2014 / Revised: 29 July 2014 / Accepted: 4 August 2014 / Published online: 15 August 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Key message The rate of progression of Dutch elm disease can be continuously and quantitatively esti- mated from sap flow measurements. Abstract Response of sap flow to inoculation with Oph- iostoma novo-ulmi, a causal agent which causes vascular mycosis called Dutch elm disease, was studied in a field experiment comprised of 4-year-old wych elm trees (Ulmus glabra). Sap flow was measured on inoculated trees using the trunk heat balance method with external heating (EMS 62, Czech Republic) throughout the experiment. The first detectable symptoms of reduction in sap flow occurred 6 days after inoculation and all inoculated trees died within 16 days. Our experiment confirmed the ability of O. novo- ulmi to quickly kill young elm trees. The disease pro- gressed faster than in previous experiments utilizing O. ulmi. To the best of our knowledge, this is the first experiment using sap flow measurements on trees inocu- lated by O. novo-ulmi. The trunk heat balance sap flow method is an effective non-invasive tool for continuous quantitative monitoring of the progression of vascular tree diseases, and show increased potential for field and greenhouse studies on changes in xylem hydraulic conductivity in a wide range of broadleaved and coniferous tree species. Keywords Dutch elm disease  Sap flow occlusion  Elm dieback  Xylem dysfunction  Xylem hydraulic conductivity  Trunk heat balance method Introduction Ophiostoma novo-ulmi Brasier (1991) is a causal agent which causes vascular mycosis called Dutch elm disease (DED). It is the most important disease of elm trees (Ulmus spp.) throughout the Northern hemisphere. DED was first observed in the early 1900s in the area of Northwestern Europe (Spierenburg 1921; Brasier 1991). DED was caused solely by O. ulmi (Buism.) Nannf. until the 1940s but this species lost its aggressiveness (Brasier 1988) and was replaced by a new form of pathogen. The newer, more aggressive form was divided into two races—an Eurasian (EAN), probably originating in the area of Moldavia and Ukraine, and a North American race (NAN), (Brasier 1979). The more aggressive form of O. ulmi, O. novo-ulmi was described by Brasier (1991) as a new species. Ten years later, Brasier and Kirk (2001) designated races EAN and NAN as subspecies; Ophiostoma novo-ulmi ssp. novo- ulmi (EAN) and Ophiostoma novo-ulmi ssp. americana (NAN). The conidia of Pesotum ulmi (anamorph of O. ulmi/ novo-ulmi), transmitted on the body surface and digestive tract of Scolytidae beetles, germinates in the trachea of the last tree ring of the twig and starts to grow within and against the direction of sap flow. Consequently, the xylem vessels are blocked up and sap flow declines. Many pro- cesses decrease the hydraulic conductivity in the xylem, Communicated by A. Bra ¨uning. J. Urban (&) Department of Forest Botany, Dendrology and Geobiocenology, Faculty of Forestry and Wood Technology, Mendel University in Brno, Zeme ˇde ˇlska ´ 3, 61300 Brno, Czech Republic e-mail: josef.urban@mendelu.cz M.Dvorˇa ´k Department of Forest Protection and Wildlife Management, Faculty of Forestry and Wood Technology, Mendel University in Brno, Zeme ˇde ˇlska ´ 3, 61300 Brno, Czech Republic 123 Trees (2014) 28:1599–1605 DOI 10.1007/s00468-014-1068-0