Heritability of Ulmus minor resistance to Dutch elm disease and its relationship to vessel size,but not to xylem vulnerability to drought

Dutch elm disease (DED) is a vascular wilt disease that causes the occlusion and cavitation of xylem vessels. Therefore, it is hypothesized that those elms that are less vulnerable to cavitation by drought might be more resistant to DED. To test this hypothesis, the relationship between xylem vulnerability to cavitation and susceptibility to DED was examined in progenies of crosses between susceptible and resistant individuals of Ulmus minor. Hydraulic conductivity and xylem vulnerability curves were evaluated and anatomical features such as vessel size, length and grouping were measured. Next, elms were inoculated with Ophiostoma novo‐ulmi, the cause of DED, and pre‐dawn and midday water potentials, stomatal conductance and wilting percentages were assessed. Progenies of R × R crosses showed significantly lower mean wilting percentages (30–50%) than the progeny of S × S crosses (75%). Fifty percent conductivity loss was reached at c. ?1 MPa, pointing out a high vulnerability of this species to drought‐induced cavitation. Crown wilting percentage as a result of inoculation and xylem vulnerability to cavitation by water stress did not show any significant correlation. Nevertheless, significant differences in theoretical hydraulic conductivity and vessel size parameters (diameter, length and size distributions) were found among the tested progenies. Susceptible trees had significantly wider and longer vessels. Xylem structure of resistant elms seems to restrict pathogen spread rather than prevent cavitation.