Variation in Eucalyptus globulus LABILL. and E. nitens DEAN and MAIDEN in Susceptibility of Adult Foliage to Disease Caused by Mycosphaerella cryptica (COOKE) HANSF

Severity of disease caused by Mycosphaerella cryptica (COOK) HANSF. was assessed on the adult foliage of Eucalyptus globulus LABILL. in two provenance trials (encompassing all four subspecies) and a progeny trial of E. globulus ssp. globulus LABILL. located in Victoria, Australia. Disease was relatively low in all trials (most trees with less than 15% crown severity), except for two provenances at one trial, Judbury (E. globulus ssp. globulus) and Mansfield (E. globulus ssp. bicostata), that had mean crown severities of approximately 25% and 40%, respectively. Eucalyptus globulus ssp. bicostata (MAIDEN et al.) KIRKPATR. was significantly (P < 0.01) more susceptible than E. globulus ssp. globulus, E. globulus ssp. pseudoglobulus (NAUDIN ex MAIDEN) KIRKPATR. and E. globulus ssp. maidenii (F. MUELL.) KIRKPATR., with subspecies maidenii significantly less diseased than all other subspecies. There was significant variation between provenances within subspecies globulus (P < 0.01) but not within subspecies pseudoglobulus, maidenii or bicostata. Subspecies globulus also showed significant (P < 0.01) variation between families. There was a moderate to high genetic correlation between disease of the adult foliage and disease of the juvenile foliage (caused by both M. cryptica and M. nubilosa (COOKE) HANSF.) assessed several years earlier, both at the provenance (r_G = 0.67) and family (r_G = 0.33) levels. Narrow sense heritability of disease of the adult foliage (M. cryptica) was low (h² = 0.17), compared to that of the juvenile foliage (h² = 0.35) and juvenile defoliation (h² = 0.45) assessed previously. Selection for overall disease resistance (both adult- and juvenile-phase foliage) can be carried out more quickly and accurately at the juvenile stage when trees are 2-3 years old, potentially reducing the time required for resistant trees to be selected and deployed in the field. Mycosphaerella leaf disease on adult E. nitens (DEAN and MAIDEN) MAIDEN was also assessed in two provenance trials; however, there was very little disease observed and no significant differences were found between provenances.