New South Wales DPI research horticulturalist, Graeme Sanderson, prepared this article on Cox rootstock, which features in the winter edition of the Australian Citrus News.
Cox rootstock was bred by the NSW Department of Primary Industries (DPI) at Gosford Horticultural Research station in the early 1960s. A large number of hybrids were produced by crossing Scarlett mandarin and Poncirus trifoliata between 1959 and 1962.
An earlier program at Gosford also produced Benton citrange which is a hybrid of Ruby blood sweet orange and Poncirus trifoliata and was released in 1985. Cox rootstock was released for commercial use in 1995.
The high demand for Benton citrange seed for both Eureka lemon propagation and more recently Imperial mandarin has led to a shortfall of supply. Benton citrange fruit, for the extraction of seed, typically have a low fruit and seed count whilst Cox is productive and seedy.
Benton can also have a bushy, branching habit as a nursery seedling unless grown at high density to encourage upright growth. Cox is uniform, upright with good seedling vigour in the nursery.
Cox was selected on the basis of its performance under several lemon varieties in trials at Gosford and showed resistance to Phytophthora root and collar rots as well as compatibility with Eureka lemon.
Trials were conducted on Cox as a rootstock for Eureka lemon during a Horticulture Innovation Australia project ‘Production of Quality Lemons for the Premium Local and Overseas Markets” CT97005, with the final report presented in August 2004.
One of the recommendations states that “Benton citrange tended to produce high yields of good quality fruit from both virgin and replant soils from the Gosford trials and it is now recommended for future plantings in both soil situations”.
It also said that Cox showed sufficient promise to be recommended for planting in virgin and replant soils in the Gosford region and tested in a wider range of soil types.
The seedless Eureka lemon (Eureka SL) from South Africa became available for evaluation in Australia in 2004 with small numbers of trees established on Benton and Cox at regional evaluation and demonstration sites in 2005.
This was part of the ‘New citrus varieties program’ supported by HIA and coordinated from NSW DPI Dareton Research Station. Under drip irrigation and in a deep sandy loam soil type at Dareton Research Station, Cox and Benton performed well as rootstocks for Eureka SL. Internal fruit quality was similar at full colour maturity in mid-June with Juice%: 42, °brix: 8, acid%: 5.6 and ratio: 1.4. Yield over 3 seasons was generally higher on Cox from 2011-2013.
A West Australian evaluation site was sprinkler irrigated and on a bleached, coastal plain sand soil type which is low in organic matter and has poor water holding capacity.
The Cox hybrid rootstock had stronger growth than Benton rootstock in this situation, most likely due to the mandarin component of its parentage being more adapted to ‘poorer’ soil conditions.
Cox is currently under evaluation as a rootstock for mandarin in both Queensland and at Dareton along with several other Scarlett mandarin x Trifoliata hybrids from the former Gosford breeding program.
Auscitrus is the source of supply for Cox rootstock seed in Australia.
For more information:
Graeme Sanderson