*Rootstock selection is one of the most important factors to consider when planning a citrus development. Rootstocks affect yield, fruit size and quality.

They also vary in their adaptability to different soil types, tolerance of adverse soil conditions, soil pests and diseases and their suitability for use on replant sites. Different rootstocks are appropriate for different varieties and planting systems.

The most appropriate rootstock will depend on site characteristics, intended planting system, the likelihood of specific pest, disease or salinity problems, the variety to be grown, planned market outlets and marketing strategies for the fruit. Certain rootstocks have resistance/tolerance to Phytophthora, CTV, nematodes and different diseases.

It is now well established that Huanglongbing (HLB) (Candidatus liberibacter asiaticus) or ‘Greening disease’ is devastating and has caused damage to the citrus crops around the world. HLB is a bacterial disease which is vector-transmitted by Psyllid insects that spread it through the tree canopy, causing decline and then death.

The citrus Psyllid is like the citrus leaf miner in requiring new flush of leaves for reproduction. However, unlike the citrus leaf miner, adult Psyllids can survive on hardened leaves, and if not controlled will move to a new flush as it becomes available. Therefore, suppression of adult Psyllids during the winter will be useful before they reproduce prior to the spring flush.

HLB disease is not present in Australia, however, this disease could make it to Australia through the importation of infected plants or budwood, cuttings and rootstock. Therefore, there is an urgent need for the citrus industry to safeguard itself against HLB in case it occurred in Australia in future.

Australia is also under threat, since HLB is present in neighbouring countries to our north and the islands of Torres Strait could provide a potential pathway for its spread into Australia.

Dr Tahir Khurshid is leading a Hort Innovation funded project “Evaluation of new rootstocks for the Australian Citrus Industry” at the NSW Department of Primary industries at Dareton.

One of the key components of the current national rootstock evaluation project is to import HLB tolerant rootstocks and to test them with different scion varieties in different citrus growing regions of Australia.

Currently there is no rootstock available in Australia which is effectively proven tolerant against HLB. Therefore, we are relying on the rootstocks which are bred overseas to have tolerance against HLB.

Dr Khurshid has established relations with scientists overseas who are working on the HLB tolerance and other aspect of rootstocks. He has also been in a close contact with the overseas scientists to negotiate the import of HLB tolerant rootstocks.

The first set of seven rootstocks were arrived from United States Department of Agriculture (USDA), Fort Pierce Florida in December 2020 as a result of the long-term negotiations. They are US-802, US-812, US897, US-942, US-1283, US-1284 and US-1516.

These rootstocks are hybrids of Trifoliata and other citrus types such as grapefruit, mandarin and oranges. The new rootstocks are now underway, and seeds have been germinated to develop the rootstocks seedlings to be budded with different scion varieties in near future.

These imported rootstocks are bred by Dr Kim Bowman from USDA, Florida who has published a few articles on the efficiency of these rootstocks against HLB. However, these rootstocks are not tested in Australia. In Australia these rootstocks will be tested in replicated trials in citrus growing areas for yield, tree growth and fruit quality with different scion varieties across different growing regions in Australia.

Since there is no HLB in Australia, but it is present in the neighbouring Indonesia, therefore, these rootstocks will be tested for HLB in Indonesia under a new project “Preparedness and management of huánglóngbìng (Citrus greening disease) to safeguard the future of citrus industry in Australia, China and Indonesia” funded by Australian Centre for International Agriculture Research (ACIAR) and Hort Innovation in Indonesia and Australia.

In this project, the efficiency of new rootstocks will be quantified for their tolerance to HLB with different scion varieties. The project is led by the citrus team of NSW Department of Primary Industries and Citrus Australia.

Because Indonesia has got HLB in citrus orchards, the new project will provide a wonderful opportunity to test these rootstocks with different scion varieties in HLB infested regions of Indonesia to quantify the tolerance of these rootstocks to HLB.

The information obtained from the Indonesian research work will be brought back to the citrus growers in Australia. It will also help us to identify the most effective rootstock/scion combinations for the HLB tolerance and for yield and quality.

NSW Department of Primary Industries has recently signed a collaborative agreement with the University of Florida, Lake Alfred. The negotiations took a long time to obtain the new rootstocks from the University of Florida. However, Nathan Hancock (CEO, Citrus Australia Limited) played an important role and made it possible to obtain these rootstocks for the Australian citrus industry.

A new set of 20 rootstocks were provided by Dr Fred Gmitter (University of Florida) which arrived in Australia in April 2021. There were seven rootstocks from UFR series, six rootstocks from B11R series, four rootstocks from N40R series and three from AMB+CZO, Orange14 and Orange 16 series respectively.

These rootstocks are all hybrids of different citrus species. The rootstocks were recently germinated to develop rootstock seedlings. The trial sites will be established in the citrus production regions of Australia in sandy loam and clay soils in the next year.

*Written by Dr Tahir Khurshid a research physiologist at Agriculture NSW Department of Primary Industries Dareton. tahir.khurshid@dpi.nsw.gov.au