Evaluation location: Minnesota, United States
Trees were purchased from commercial nurseries and were planted in a nursery field at the University of Minnesota, St. Paul campus during the summer of 2014. Trees were spaced 0.9 m apart within rows and 3 m apart between rows. Trees were three and four years old at the time of inoculation. All trees received water as needed during the growing season. In addition, trees were fertilized every 3 months during the growing season with 4.9 ml of Osmocote® Plus (15-9-12) (Everris NA Inc., Dublin, OH). Methods of inoculum preparation and inoculation have been described previously (Beier et al., 2017). In brief, an isolate of Ophiostoma novo-ulmi with known pathogenicity, collected from Minnesota, was used for inoculations. A drill was used to make a single small hole (2.38 mm wide by 4 mm deep) in the tree 0.5 m above the ground. Tape was wrapped tightly around the drill bit to maintain a consistent depth. Immediately following drilling, 25 μl of an O. novo-ulmi spore suspension (1 × 106 spores/ml) was injected into the hole using a micropipette and the wound was subsequently wrapped with Parafilm M® (Bemis Co., Inc., Neenah, WI) to avoid desiccation. To more accurately determine disease susceptibility in the cultivars, additional samples, which were not used for histological assessments, were included in 2015. Trees in the 2015 trial were inoculated on May 28 (43 days after budbreak), while trees in the 2016 trial were inoculated on May 26 (40 days after budbreak). Disease severity ratings (DSR) were made at 90 days postinoculation (DPI) based on the percentage of the crown exhibiting permanent wilt using a 1–12 disease severity scale: 1 = 0%, 2 = 1%–9%, 3 = 10%–19%, 4 = 20%–29%, 5 = 30%–39%, 6 = 40%–49%, 7 = 50%–59%, 8 = 60%–69%, 9 = 70%–79%, 10 = 80%–89%, 11 = 90%–99% and 12 = 100%.