http://www.theledger.com/article/20120410/NEWS/120419953?Title=Scientist-Remedy-for-Citrus-Greening-Disease-Fails-to-Improve-Productivity

Growers Debate Greening Fix
Scientist: Remedy for Citrus Greening Disease Fails to Improve Productivity

By Kevin Bouffard
THE LEDGER
Published: Tuesday, April 10, 2012 at 9:36 p.m.
Last Modified: Tuesday, April 10, 2012 at 9:36 p.m.

AVON PARK | Enhanced nutrition programs, the most widely used method in Florida to combat citrus greening, do not sustain the health or productivity of trees infected by the disease.

Or maybe they do.

"The tree looks better, but productivity does not recover," Jim Graham, a soil microbiologist at the Citrus Research and Education Center in Lake Alfred, told about 325 growers Tuesday at the annual Florida Citrus Growers' Institute at South Florida Community College in Avon Park.

The good news is that enhanced nutrition programs appear to stabilize the productivity loss a citrus tree sustains soon after greening infection, Graham told The Ledger after his presentation. The decline in fruit yields could range from 20 to 40 percent compared to a healthy tree.

"This is not about taking shots at enhanced nutrition programs. It's about balancing resources to maintain tree health," he said.

Enhanced nutrition cost growers more than three times the expense of standard fertilizing programs, Graham said.

Graham also cited research by U.S. Sugar, also one of Florida's largest citrus growers, that shows 43 percent of the state's 62.5 million orange trees were infected by greening as of last year. The infection rate continues to double each year, he said.

Greening is a bacterial disease that causes the tree to produce sour, deformed fruit. It could destroy commercial citrus production in Florida unless a current research effort now up to $60 million comes up with effective control measures.

For the past several years, most Florida growers have used an enhanced spray fertilizer program pioneered by Winter Garden grower Maury Boyd. Called "enhanced nutrition," it appears to counteract greening's effects on citrus fruit by replacing minerals and other nutrients the disease robs from the tree.

Enhanced nutrition promotes healthier leaves and fruit on a citrus tree's canopy, but it also promote excessive root growth, Graham said.

"Excessive root growth is not necessarily a good thing. It has unintended consequences," he said.

More roots than the tree needs to produce a crop supplies a breeding ground for a root-damaging fungus called "phytophthora," Graham said. The fungus also exists across Florida's citrus belt.

Greening and the fungus interact to damage roots further, he said. Graham's research shows infected trees in Florida show damage on 20 to 40 percent of their root systems, mostly from greening and secondarily from phytophthora.

"Root damage has a direct effect on tree productivity," he added. "Losses below the ground affect losses above the ground."

Some research in Florida showed yield losses in the range of 20 to 40 percent on greening-infected trees even with enhanced nutrition, Graham said.

Yield losses eventually stabilize around 30 percent with no additional declines over time.

Boyd, who also attended the Avon Park seminar, didn't buy it.

At his Felda grove, where he developed the enhanced nutrition system, Boyd saw no additional phytophthora, he said. Moreover, trees in that grove have shown no yield declines attributable to greening in more than 10 years, both before and after the disease surfaced in Florida in 2005, Boyd said. It surfaced in Boyd's Felda grove a year later.

Boyd got support from Robert Rouse, a citrus horticulturalist at the Southwest Florida Research and Education Center in Immokalee. Rouse has been researching enhanced nutrition at Boyd's Felda grove since 2008.

"I don't see a lot of difference (in yields) pre-greening and post-greening," said Rouse, referring to harvesting data from the grove since the 1998-99 season. "The roots under the trees receiving these (enhanced nutrition) treatments look very well."

Rouse added he didn't see a phytophthora problem there.

[ Kevin Bouffard can be reached at kevin.bouffard@theledger.com or at 863-422-6800. Read more on Florida citrus on his Facebook page, Florida Citrus Witness, http://bit.ly/baxWuU. ]