Jude Grosser, a University of Florida professor of plant cell genetics at the Citrus Research and Education Center, Lake Alfred, is part of a research group searching for proverbial needles in a rootstock haystack.
Not only do the rootstocks have to tolerate or resist huanglongbing, but they also have to do well when challenged by blight, citrus tristeza, the diaprepes weevil/phytophthora complex and calcareous soils.
But that’s not all, Grosser told attendees of the recent Citrus Industry Conference in Bonita Springs. The rootstocks need to produce strong yields of highquality fruit and produce enough of the right type seeds to aid nursery propagation.
“There’s a long list of things you need to have in a rootstock,” he says.
The research group includes representatives from the university and U.S. Department of Agriculture as well as 87-year-old grower and citrus breeder Olie Lee.
USDA Agricultural Research Service researchers in Fort Pierce also are working to develop improved citrus rootstocks, Grosser says.
He and the group first reviewed rootstocks in development before citrus greening appeared in Florida to see if any might have natural disease resistance.
Grosser says a new breeding technique that creates tetraploids allows him to cross two hybrids and incorporate characteristics from all four parents.
The technique does not involve genetic engineering and is more akin to creating a test-tube baby. One of the group’s goals is to create a better sour orange rootstock.
Already, Grosser says he’s identified a Nova mandarin-Hirado Buntan pummelo + Argentine trifoliate orange hybrid that he’s used in a cross with a Cleotrifoliate hybrid.
Some of the resulting tetrazyg rootstocks—such as Orange #14 and Orange #19—have performed well in an initial field trial near Dundee under heavy HLB pressure, he says.
More than 75 rootstocks were budded with either Vernia or Vaquarius scion and planted in one of three tree spacings. When compared to traditional diploid rootstocks, such as Swingle and Cleopatra, the tetrazyg rootstocks generally produced fruit with higher pounds solids and more boxes per tree. Fewer trees also showed HLB symptoms than the traditional diploid hybrids.
On the fast track
The group will identify the most promising rootstocks to include in the new Fast Track citrus evaluation program.
Varieties released this way haven’t been evaluated through replicated field trials in various locations but do show significant promise.
Growers pay a nominal fee to register and agree to plant at least a 1 acre solid planting.
Participants then provide feedback to guide release and commercialization decisions. The program is designed to reduce the time needed to evaluate and release new varieties.
Putting rootstocks to the test
To determine whether candidates possess a strong disease package, Grosser says they put them through a series of screenings that weeds out all by the fittest.
The rootstocks must pass greenhouse trials where HLB-infected budwood is grafted onto them and the movement of the bacteria monitored.
In addition, technicians plant the rootstocks in calcareous soil that has a pH of 8 and then measure nutrient uptake. Rootstock candidates also are planted in soil infested with Diaprepes weevils.
After the insects feed on the roots, the plants are moved to soil infected with phytophthora and monitored for disease.
The weevil feeding sites on the roots provide entryways for the pathogen. Plants with vigorous root growth can withstand the one-two punch and remain viable.
Other screenings include exposing the rootstock to all three citrus tristeza strains found in Florida as well as irrigating them with high saline water.