http://www.theledger.com/article/20120326/NEWS/120329433

Citrus Black Spot Continuing to Spread in Florida
The fungal disease causes a citrus tree to shed its fruit prematurely.

By Kevin Bouffard
THE LEDGER
Published: Monday, March 26, 2012 at 11:04 p.m.
Last Modified: Monday, March 26, 2012 at 11:04 p.m.


Megan Dewdney, a plant pathologist
at the Citrus Research and Education
Center in Lake Alfred, has been studying
black spot since its arrival.
ERNST PETERS | THE LEDGER (2011)

IMMOKALEE | Many other plant diseases, notably citrus greening and canker, rank among the top concerns Florida citrus growers, but black spot is moving up the list.

Growers in Brazil, the world's largest orange grower, have a few years more experience with citrus black spot, and they will advise against taking it lightly, said Ricke Kress, president of Southern Gardens Citrus Processing Corp. in Clewiston, one of the state's largest growers, which is currently dealing with the disease.

"They're as concerned about black spot as we are about greening. It's spreading through that country like wildfire," Kress told about 80 growers at a March 13 seminar on the disease at the Southwest Florida Research and Education Center in Immokalee. "The future is in front of us. We have to face it. We have to deal with it."

The fungal disease, which causes a citrus tree to shed its fruit prematurely, was first discovered in March 2010 in an Immokalee grove, and 17 other cases arose in Collier and Hendry counties in the next months.

The most recent data from the Florida Department of Agriculture and Consumer Services shows 141 confirmed black spot cases, a growth of almost eight times in two seasons, but they are still confined to Collier and Hendry.

Those figures don't represent the full extent of black spot's spread because not all suspected cases were sent for lab analysis, Paul Mears, an Agriculture Department official supervising the hunt for black spot, said at the seminar.

Scientists in Brazil and the U.S. have no way to arrest the wind-borne release of black spot spores, which live on fruit, leaves and branches.

"Once the disease is established, it's there. There's no way to eradicate it," said Jim Snively, vice president for grove operations at Southern Gardens, who visited Brazil in January with Kress to study the disease. "Black spot is going to spread across Florida. It may take 20, 30 years. Who knows?"

Indeed much about black spot remains unknown, said Megan Dewdney, a plant pathologist at the Citrus Research and Education Center in Lake Alfred, who has been studying the disease since its arrival.

Among the key unanswered questions are what causes spore ejection; what temperature, rain and other climate factors in Florida are favorable or unfavorable for black spot; and how much premature fruit drop can be expected on infected Florida trees, she said.

Scientists in Brazil have reported 16 percent premature drop even in groves with good control of the disease, Dewdney said.

"That's scary. That's a pretty staggering amount from drop where there's good control," she said. "There are no easy solutions to this disease. It's going to be a long, hard grind, just like greening."

Scientists do know leaf litter from infected trees left on the ground will become a major source of infection spread, Dewdney said. That means more time, effort and money in groves to rake up fallen leaves, especially from April to late September when the fruit is especially susceptible to infection.

The fruit can be protected by traditional fungicides, such as copper and a class of chemicals called "strobilurins" (sold under product names including "Abound" and "Headline").

But Dewdney expressed concern the fungus would eventually evolve a resistance to strobilurins because they are used so frequently to fight other fungal diseases. Moreover, nobody is close to developing and getting regulatory approval for new fungicides, she said.

"Once we lose strobilurins, we have nothing coming down the pipe. That's a big concern," Dewdney added.

In addition to surveying for black spot in Hendry and Collier, state Agriculture Department workers are surveying along the major highway routes between those counties and juice processing plants and packinghouses, Mears said. That includes U.S. 27 leading to Polk County's major juice and packing facilities.

Florida citrus officials fear infected leaves blowing off delivery trucks will land in groves along those routes, hastening black spot's spread. Trucks carrying fruit from groves within a quarantine area surrounding existing black spot cases must cover their loads with a tarp to minimize leaf spread.

[ 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. ]