Abstract
Genetic variation in frost tolerance, resistance to the rust fungus Uromycladium acaciae, growth, stem form, and gummosis were evaluated in 110 open-pollinated families of black wattle (Acacia mearnsii De Wild). Families were tested at six frost-prone sites in northern KwaZulu-Natal and southeastern Mpumalanga, South Africa. Frost-hardy provenances were susceptible to rust disease and had poor growth. Locally grown F1 seed sources that originated from cold-hardy Australian seed sources had better growth and were tolerant to rust, but at a cost of lower frost tolerance. Considerable genetic variation was observed between families within seed sources for frost damage (hfm2 = 0.77), rust incidence (hfm2 = 0.89), and height (hfm2 = 0.80). The corresponding narrow-sense heritabilities (hi2) were 0.30, 0.80, and 0.32. Genotype-by-environment interaction levels were low for most traits. Except for a strong genetic correlation between tree height and diameter (0.90), all the pairs of traits had weak to moderate genetic correlations. Recurrent selection will be successful in improving frost tolerance and rust incidence. However, the current population comprises limited germplasm that is both tolerant to frost and resistant to rust. Thus, we recommend infusing germplasm from known cold-hardy Australian provenances into the current population to increase genetic variation for frost tolerance, rust, and growth.
Study Implications
Black wattle (Acacia mearnsii De Wild) is widely planted in South Africa for bark extract and woodchip exports. The species is prone to frost damage and susceptible to rust fungus Uromycladium acaciae, two major limitations to its cultivation. In this study, 110 open-pollinated families of black wattle originated from its native range in Australia and local South African seed sources were studied for frost and disease tolerance. The results from this study suggest that there is considerable variation within species for frost tolerance and fungal disease. We suggest a tandem selection strategy for black wattle farming in South Africa to increase resistance to frost damage and fungal diseases.