Taking Stock Of New Citrus Rootstocks
March 23, 2010 With all the diseases and other problems in citrus production these days, it is easy to get the impression that there isn’t much good news for the grower. To be sure, citrus canker and HLB-greening diseases have increased costs and cloud the future of Florida production with uncertainty. Amid all this gloom, a series of new rootstocks developed by USDA, some already available for use by Florida growers, offer rays of hope for improved production and profitability.
Good Wood
US-812 rootstock, a hybrid of Sunki mandarin with trifoliate orange, was released in 2001 and has already been used for hundreds of acres of established new citrus plantings. US-812 combines outstanding fruit production on a standard size tree with cold hardiness, good fruit quality, and resistance to Phytophthora root rot, CTV, and citrus blight.
Two other rootstocks, US-802 and US-897, were released in 2007 to provide additional rootstock alternatives for production on a range of tree sizes and providing tolerance to the Phytophthora-Diaprepes complex on poorly drained flatwoods soils.
US-802 rootstock is a hybrid of pummelo with trifoliate orange that yields a very vigorous and productive tree on a wide range of soil types. In the face of disease pressure, many growers have focused on US-802 rootstock as a way of establishing large cropping trees quickly, to shorten the time for a new planting to pay back the investment and begin making a profit.
In a rootstock trial with Hamlin in Osceola County, 8-year-old trees on US-802 rootstock were more than 3 feet taller and wider than trees on Swingle rootstock, as well as significantly more productive. Other growers have tuned in to the high density opportunity provided by the dwarfing rootstock US-897, itself a hybrid of Cleopatra and Flying Dragon trifoliate orange. In the same rootstock trial in Osceola County, Hamlin trees on US-897 rootstock remained at about 9 feet tall, even through 18 years of age, while trees on Swingle grew to about 15 feet tall, and trees on US-802 grew to 20 feet tall.
If tree and row spacing is adjusted to optimum, some argue that the very high density possible for trees on US-897 rootstock might make it the hands-down winner for fruit productivity per acre, as well as providing easy harvesting and management with a very short tree.
Coming Soon
US-942, a fourth rootstock in the USDA stable of thoroughbreds, is being released this year. Similar to US-812, the US-942 rootstock is a hybrid of Sunki mandarin with trifoliate orange and combines outstanding fruit production with cold hardiness, good fruit quality, and resistance to Phytophthora root rot, CTV, and citrus blight. The way in which US-942 distinguishes itself is by producing that high volume of fruit on a more compact tree, somewhere between the standard size of US-812 rootstock and the dwarf size of US-897. Among the four good new rootstock options provided by the US series rootstocks, a good fit can be found for most new planting plans or replant situations.
Sour Power
Another USDA research area that has been progressing behind the scenes for several years is the development of an improved sour orange rootstock, or SuperSour. Hybrids to recreate a sour orange-type rootstock without the CTV-sensitivity of the standard sour orange have been produced and are being compared sequentially, to identify the one or several that combine the best traits of sour orange with full resistance or tolerance to CTV. In an ironic good twist of fate, some of the same germplasm that was used to create the SuperSour selections also appears to confer some tolerance to HLB-greening disease.
While the commercial release of a SuperSour rootstock is likely several years away, planting of cooperative field trials with advanced SuperSour selections is expected within the next few years, and will give growers the opportunity to participate in the selection of the ultimate SuperSour themselves.
Rooting For Resistance
Unhappily, none of the rootstocks currently available to Florida citrus growers provide good resistance or tolerance to HLB-greening disease, including the US rootstocks that have so far been released. Fortunately, other new rootstocks under development by USDA, including transgenic versions of US-812, US-802, US-897, US-942, and the new SuperSour rootstocks will provide some tolerance or resistance to HLB-greening disease. A library of resistance genes is being tested to identify those with the strongest ability to improve tree life and productivity in the face of HLB-greening disease. Although the best combination of germplasm and resistance genes is not yet defined, it is certain that resistant or tolerant rootstock varieties will be a critical component of the ultimate solution to the HLB-greening problem.
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