Promising new citrus varieties for greening tolerance

University of Florida scientists and Tropicana are teaming up to test promising new UF/IFAS-bred citrus varieties for greening tolerance and better-tasting juice. Tolerance means trees are infected and remain infected, but they still perform adequately to make a profit, UF/IFAS researchers say. Resistance means the trees fight off the infection -- either reducing the infection to a very low level or getting rid of it completely.

"I've been out to the site twice in the past year and some of the rootstock-scion combinations just jump out at you with the lush green foliage and, more importantly, a large number of nice-sized fruit," said UF/IFAS CREC Director Michael Rogers.

"What's more, there are multiple large plots for each rootstock combination that allow us to really get a good idea of how these trees are holding up under greening," Rogers said. Greening, also known as huanglongbing, or HLB, has impacted more than 80 percent of Florida's citrus trees, according to UF/IFAS research. Rogers added: "From what I've seen, we've got some reasonably tolerant scion/rootstock combinations that growers should be taking a look at as short-term solutions to living with greening until true HLB-resistant trees are developed."

Tropicana leases 14.5 acres for testing new selections including UF/IFAS-developed citrus cultivars and rootstocks in a commercial grove about 4 miles from the UF/IFAS Citrus Research and Education Center, said Jude Grosser, a UF/IFAS professor of horticultural sciences and a faculty member at CREC. UF/IFAS researchers have a long history of working with Tropicana, Grosser said.

"This trial is providing strong evidence that the combination of improved scion genetics, improved rootstock genetics and optimized nutrition programs is the ticket for the immediate future," Grosser said.

For example, the combination of UF sweet orange OLL-8 cultivar on UFR-4 rootstock looks exceptional for both yield and tree health, Grosser said.

"These clones are a good immediate-term solution to help us until resistant clones are available," he said. "We are continuing to screen rootstock hybrids for HLB tolerance that can be transmitted to a grafted scion. So far, we have screened more than 10,000 hybrids, and we have identified a few truly promising selections. Citrus breeding is a continuum -- we are constantly building on previous progress."

The relationship between Tropicana and UF/IFAS goes back many years.

"They became interested in our sweet orange improvement work through research interactions and presentations made at trade shows and seminars," Grosser said. "We began sending them fruit samples from promising selections for sensory analysis."

Rocco Simonetta, who's now retired from Tropicana, spearheaded the UF/IFAS-Tropicana Project Field trials -- with the first two trials focusing primarily on sweet orange selections in our pipeline that might be good additions to their Not-From-Concentrate (NFC) portfolio, Grosser said. The current trial focuses more on the addition of new rootstocks as necessary to deal with HLB.

Cheryl Nagle of Tropicana worked with Simonetta at the beginning, and she passionately followed up on building the relationship after Rocco's retirement, Grosser said. She now is in charge of the trials.

"This is all truly a team effort," he said.

Tropicana and UF/IFAS started their collaboration on the current project in June 2013, said Allison Drown, who manages the grove near Lake Alfred for Tropicana.

"We monitor tree health, fruit yield and juice quality," Drown said.

Grosser commended Tropicana's interest in testing many new IFAS selections in their citrus trial.

"Tropicana has taken no shortcuts -- they are employing state-of-the-art management of the trial," said Grosser, who led a tour of the private groves at the Nov. 29 centennial celebration of the UF/IFAS CREC.

Tropicana and UF/IFAS breeders started with about 30 scion selections -- or descendants of other varieties, Grosser said, but because of citrus greening, Tropicana decided to look at new and improved rootstocks as well, he said.

Citrus scientists use traditional plant grafting to put together combinations of new scions and rootstocks created in the breeding program.

In plant grafting, scientists call the upper part of the plant the scion, while the lower part is the rootstock. Grosser said he sees promise in about half a dozen scion-rootstock combinations in this trial, most of which are releases from UF/IFAS CREC.

Based on UF/IFAS data, UF/IFAS citrus breeders suggest new oranges, orange-like hybrids and rootstocks from the CREC citrus improvement program that they think Tropicana might be interested in for future use in its business, Grosser said.

"With our advice, Tropicana makes the final decisions on which scions and rootstocks are in the trial, and also the decisions on how to manage the trees," he said. "We are currently working with Tropicana to develop a plan where we can work together to get good yield data this year."