Abstract:
In South Africa, Eucalyptus grandis is an important species due to its fast growth and general suitability of its timber for a range of products. However, E. grandis is susceptible to fungal diseases such as Crysoporthe austroafricana and Coniothyrium sp. cankers in the subtropical region of Zululand and is therefore mainly planted as a parental species in a hybrid combination with E. urophylla in this region. The current strategy is to maintain large breeding populations of both parental species in order to provide improved elite selections for hybrid crosses. In order to develop the best interspecific hybrid breeding strategy for E. grandis, it is important to first determine estimates of genetic parametersof the pure species parents. Estimating the genotype by environment interaction (G×E) is also necessary in proposing the basis for setting up breeding populations and selecting environmentally stable genotypes. With this in mind, two E. grandis full-sib progeny trials were planted in Zululand and one in the KwaZulu-Natal Midlands region. The aims of this study were firstly to determine the magnitude of G×E of E. grandis across the three sites; secondly, to estimate the genetic parameters for growth of the E. grandis parents selected for intraspecific crosses; and lastly, to identify the best parents to use for intra- and interspecific crosses in future hybrid breeding programmes. Results of our study indicated that G×E would be practically negligible for growth in Zululand and one group of elite parents can be used for hybrid crosses in this region. In general, growth traits were under low to moderate genetic control, and the variation in additive geneticsenabled us to identify E. grandis parents that could be utilised for intraspecific crosses and deliver progeny with genetic gains of 28.4%. Our study also highlighted that a relatively large portion of the genetic variation was explained by dominance geneticvariation and a strategy to capture this non-additive variation needs investigation. Although our study achieved the stated aims, it must be kept in mind that E. grandis is mainly used as a hybrid parent with E. urophylla in Zululand. A study to investigate whether the parents with good general combining ability values from our study are also good general combiners in interspecific hybrid combinations with E. urophylla needs to be conducted.
