In 1974, Dr Lilian Fraser presented the inaugural Daniel McAlpine lecture on ‘Diseases of citrus trees in Australia – the first hundred years’. On the 50th anniversary of this lecture, I will reflect on recent progress in citrus pathology, highlighting some of the success stories and the remaining challenges. The ongoing collaboration between plant pathologists and industry is critical given diseases often change the course of industries and can lead to their demise when solutions are not found. Historically, Australian citrus growers have faced widespread tree death due to Phytophthora root rot and citrus tristeza virus (CTV) with effective management achieved by changes in rootstock varieties. In 1928, a forward-thinking group of citrus nurserymen in NSW formed the Cooperative Bud Selection Society to manage the buying and selling of citrus budwood from trees designated as ‘healthy’, although the testing procedures were not available to support these decisions. Citrus trifoliata rootstock reduced the impact of both CTV and Phytophthora, but severe tree stunting due to citrus exocortis viroid required careful selection of budwood from disease-free trees to produce healthy nursery trees. This reinforced the need for a bud selection committee, which has now evolved, with government support, into the Auscitrus propagation scheme. To provide high-health status budwood and rootstock seed to industry, it has been necessary to develop rapid and sensitive molecular diagnostic assays to an ever-growing range of graft-transmissible pathogens. Methods of eliminating graft-transmissible pathogens from germplasm have also been refined.
The development of a clean planting material program for the citrus industry has been an important step forward in plant disease management. However, citrus trees quickly become infected with CTV in the field through the action of aphid vectors and the extreme diversity of the virus means that we are not dealing with ‘one beast’. CTV stem pitting in grapefruit is now managed by mild strain cross protection, although this strategy is not successful in all grapefruit varieties and in more tropical climates. Fungal diseases are primarily an issue in higher rainfall regions and development of disease management solutions has required fundamental research on the life cycle of the pathogens.
Australia has successfully eradicated bacterial citrus canker several times over the past century from both production and urban areas. The biggest challenge currently faced by the global citrus industry is the devastating bacterial disease huánglóngbìng (HLB). While still exotic to Australia, HLB is present in the island nations to our north. Keeping HLB out is paramount as a solution to this problem is not available, despite tremendous global research investment. The risk of entry of HLB into Australia through natural means (e.g., cyclone dispersal of psyllids) or in illegally imported budwood gives HLB the status of being one of Australia’s greatest biosecurity threats. While we have managed to find effective management options for several diseases, and successfully eradicated other pathogen invasions, the big question remains if we are ready and able to eradicate an incursion of HLB?