Characterizing offspring of Dutch elm disease-resistant trees (Ulmus minor Mill.)

Abstract

Abstract Populations of Ulmus minor in Europe were severely damaged by Dutch elm disease (DED) pandemics. However, elm breeding programmes have permitted selection of resistant elm varieties currently used for reforestation. In restored elm forests, resistant (R) and susceptible (S) trees interbreed, but little is known about resistance in their offspring. In this work, growth, DED resistance and xylem anatomy in the offspring of two resistant U. minor trees (R1 and R2) were studied. To verify whether transmission of traits in offspring is determined by maternal or paternal trees, a complete randomized plot was established with clonal material from controlled crosses (R1 × S and R2 × S) and parent trees (R1, R2 and S). Trees were inoculated with O. novo-ulmi firstly at age 4 years and again at 5 years. Growth, susceptibility to DED and vessel size in offspring were closer to the traits of maternal than of paternal trees. This association disappeared after the second inoculation when symptoms increased. The more resistant trees in R1 × S and R2 × S had wide and narrow earlywood vessels, respectively, suggesting that water-conducting strategies and resistance mechanisms vary in offspring. Tylosis formation was related to resistance only in R2 × S offspring, possibly due to the narrow earlywood vessles of trees. Latewood vessels were normally narrower in the more resistant trees. This study sheds light on anatomical resistance mechanisms of elms against DED: (1) offspring exhibit high variability in responses among individuals, (2) narrow earlywood vessels are not a prerequisite for DED resistance and (3) barrier zones are not fully associated with tree resistance in offspring.