Breeding new rootstocks for the Australian citrus industry

Abstract

Rootstocks have shaped the history of world citrus production perhaps more than in any other food or fiber plant used by humans. For example, historically important issues such as phytophthora and Citrus tristeza virus (CTV) at one time appeared insurmountable but have been solved through new rootstocks that offered genetic resistance. There are few other crops where so many important commercial traits are influenced by the choice of rootstock. Within the Australian context, growers already have access to rootstocks with good tolerance to CTV and phytophthora, and there is an increasing availability of new options with salt tolerance and graft compatibility. The challenge for breeders is to develop new hybrids that capture all of these currently available features but offer additional benefits such as increased productivity, low vigour, improved fruit quality and resistance to newly emerging disease threats. This project sought to achieve this by generating hybrids from unique parents and then benchmarking these against recent releases from other international breeding programs and currently available commercial rootstocks. It used a strategy that involved multiple generations of crossing, and the production of bridging hybrids to overcome fertility barriers, followed by a sequence of large field trials with multi-year assessment of tree health, vigour, productivity and fruit quality. Information from these field trials was immediately used to guide the selection of parents in the annual hybridization activities, providing a feedback loop for constant improvement, and has resulted in a rich pipeline of germplasm to help secure the future of Australian citrus production. The conscious decision to detour from the very narrow genetic base of parents (sweet orange, mandarin, pummelo, trifoliate orange) universally used by other rootstock breeding programs is beginning to show benefits, and places the Australian rootstock breeding program in a strong position to deliver ongoing benefits to Australian growers.