Genetic variation in resistance to Uromycladium acaciae fungus, growth, gummosis, and stem form in Acacia mearnsii populations

Abstract

A new epidemic disease caused by the rust fungus Uromycladium acaciae emerged in 2013/2014 causing defoliation, branch, and stem deformations in plantations of Acacia mearnsii (black wattle) in South Africa. This study investigated the level of genetic resistance to the rust fungus Uromycladium acaciae in six progeny tests from two different trial series. A total of 269 open pollinated families of different levels of improvement, ranging from wild selections to fourth cycle generations were included in the study. Families originated from land race subpopulations of South Africa and from Australian provenances. Rust severity was assessed across all the trials at 6 months of age. In the rust screening trials, different measures of rust incidence were assessed at 6, 12, and 17 months, as well as tree height and mirid damage at 12 months. In the progeny trial series, tree height, stem defects, and gummosis were measured at 36 months. Results suggested that rust incidence is under moderate genetic control at the family level. The coefficient of additive genetic variation was large (59.4%) for the disease incidence. The family mean heritability for rust disease was $$ {h}_{fm}^2=0.48\pm 0.16 $$ in the progeny trial series, whereas it was $$ {h}_{fm}^2=0.33\pm 0.06 $$ in the rust screening trials. Among the rust assessment methods, 6-month rust incidence (yes/no) assessment produced higher heritability estimates. With some exceptions, Australian provenances from low altitude sites along the eastern coast of Victoria and the south east coast of New South Wales showed high tolerance to rust, better growth, low gummosis incidence but poor stem form. All the South African sources showed high rust susceptibility and had average or below average growth, gummosis incidence, and stem form. For rust disease improvement, the South African material should be avoided and the best existing Australian families should be used for further breeding. In addition, new seed sources from Australia should be infused to the breeding program in order to increase the genetic diversity while increasing genetic gain for the rust, gummosis, and growth.