By Jude Grosser
The ultimate solution to the HLB problem is having good rootstocks that can mitigate or eliminate the disease’s impacts in any grafted commercial scion. With this, growers could profitably grow any scion, including grapefruit, Hamlin or even Murcott. Thus, rootstock breeding efforts focus on directly screening new rootstock hybrids for their ability to confer HLB tolerance or perhaps even resistance to grafted scions.
After the arrival in 2005 of HLB in Florida, making a profitable rootstock decision became more complicated. Previously, one could choose a rootstock based almost solely on yield, fruit quality and soil type. Now, despite increasing evidence suggesting that the choice of rootstock and scion variety may have a favorable impact on grove performance and financial outcomes in the presence of HLB, tree survival is also critical.
To date, approximately 18,000 hybrid rootstock candidates have been evaluated in the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) high-throughput ‘gauntlet’ screening process. Researchers have identified several promising hybrids showing the ability to transmit HLB tolerance across the graft union into the infected Valencia scion.
Most of the promising new rootstock candidates are from the ‘gauntlet’ screening, but researchers have also identified a few from field trials, including apparent deletion mutants of x639 and UFR-1. One of the most reliable scion/rootstock combinations that is performing well at multiple locations (under good nutrition programs) is OLL-8 sweet orange on UFR-5 rootstock. This combination can make 2.5 boxes per tree with 7 pounds solids per box on 7-year-old trees.
A few of these rootstock candidates show no Candidatus Liberibacter asiaticus(CLas) replication in the roots along with suppression of CLas in the scion. These rootstock candidates are being propagated by rooted cuttings and tissue culture micropropagation at Agromillora Florida Inc. and Phillip Rucks Nursery for Stage 2 trials. Several of the most promising selections also have good genetics to battle other rootstock issues like high salinity and citrus blight.
Growers can increase the potential for success in growing citrus by paying attention to the rootstock progress in research trials, particularly in the region where they are growing citrus. They can find this data here. Growers can also find information about rootstocks in the Florida Citrus Rootstock Selection Guide, 4th Edition. They can access a rootstock data table which is an easy-to-use reference to 20 characteristics of 48 rootstocks. It provides information on rootstock tolerance to salinity, pH, clay and wet soils, drought, temperature, HLB incidence, phytophthora, and other diseases and pests.
Jude Grosser is a professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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