Florida citrus faces new threat: Black spot

HAINES CITY, Fla. -- Florida citrus growers already struggling with the costs of control-ling the fatal citrus greening disease face a new economic threat from black spot, a fungal disease discovered last month in Immokalee.

HAINES CITY, Fla. -- Florida citrus growers already struggling with the costs of control-ling the fatal citrus greening disease face a new economic threat from black spot, a fungal disease discovered last month in Immokalee.

Once black spot spreads across Florida -- which appears inevitable, although it could take many years -- growers face increased production costs and smaller harvests because the disease leads to premature fruit drop. The disease could also lead to another ban on sales of fresh Florida citrus fruits in other citrus-producing states, notably California, and other countries in an effort to keep the disease out.

"It seems like we can't catch a break right now," G. Ellis Hunt Jr., president of Hunt Bros. Inc., a family citrus grower and fresh fruit shipper in Lake Wales, told The Ledger of Lake-land. "Most growers are up against the wall on the cost of production. Nobody can stand additional costs right now."

An unidentified Immokalee grower reported unusual black spotting on his late-season Valencia oranges last month, said Denise Feiber, a spokeswoman for the Florida Department of Agriculture and Consumer Services.

A March 28 "tentative analysis" done by the state Agriculture Department confirmed black spot, she said, and it is waiting for the U.S. Department of Agriculture to confirm that finding before releasing an official announcement. The USDA results could be available later this week.


But tests done by the University of Florida's Citrus Research and Education Center in Lake Alfred and the Gulf Coast Research and Education Center in Balm also found the fruit was infected with black spot, said Megan Dewdney, assistant professor of plant pathology at Lake Alfred who performed one of those tests.

"The chances of a false positive are very low," said Jim Graham, a soil microbiologist at Lake Alfred. "This is not a gray area."

Dewdney and Natalia Peres, the assistant professor of plant pathology at the Balm center who also tested the infected fruit, agreed black spot has likely established itself in Florida even though only one grove has tested positive.

"I don't think this arrived last month," Peres said. "It takes time for (the disease) to build up and to see symptoms. Now that we know what we're looking for, I think we'll find other cases."

The disease gets its name from the black lesions it leaves on oranges, grapefruit, tanger-ines and other citrus fruit. Although the lesions don't affect the juice, they make it impossi-ble to sell the fruit on the fresh market.

Juice processors buy 95 percent of Florida's oranges, the state's largest crop, and about 60 percent of grapefruit, but 70 percent or more of tangerines are sold as fresh.

Polk County annually leads the state in citrus production, including oranges and tangerines. In the 2008-09 season, it produced more than 30.2 million boxes of citrus, or 16 percent of Florida's total citrus production.

Although no black spot cases have surfaced in Polk, the county's growers may be especially vulnerable because the disease spreads mostly through dead leaves that fall to the ground, Dewdney said. When the leaves get wet, they eject thousand of spores that get blown onto fruit in the tree.


A single dead leaf can contain 50,000 spores, Peres said, but infected leaves usually don't have lesions, so it's impossible to distinguish them from normal citrus leaves.

Every truckload of just-picked citrus invariably contains dead leaves and other debris as part of the harvesting process. And Polk is home to three large citrus processing plants in Lake Wales and Auburndale and several of the state's largest packing houses in Dundee and Haines City.

As trucks travel along the state's highways to these Polk destinations, dead leaves inevitably blow off as they pass by a large portion of the county's 82,629 acres of commercial citrus groves, the largest in the state.

"That's what I see as the problem," Peres said. "That's why it's going to be difficult to contain."

As with citrus canker, also spread by infected plant material, the risk could be lessened by requiring tarps over the truckloads of citrus, Graham said. But he acknowledged that would not eliminate the risk entirely.

Because of the time needed to build up a big enough population of spores in a commercial grove before symptoms become obvious, black spot may have arrived in Florida five, or even 10, years ago, Peres said.

In its early stages, perhaps only a single piece of fruit out of 100 may show one or two black spots, Dewdney said. As spore populations grow, the symptoms become more visible.

Black spot first arose in Australia in the 1890s and spread across the globe, Dewdney said. It surfaced in Brazil, the world's largest citrus grower in 1980.


The Brazilian experience shows how slowly the disease spreads, said Peres, a Brazilian native. It took 12 years for it to spread from the first outbreak in Rio de Janeiro to the coun-try's main citrus region in Sao Paulo.

The researchers agreed they can't say how fast black spot will spread in Florida until state agriculture officials determine how far it has spread already.

Surveyors have inspected groves within a three-mile radius of the Immokalee grove, Feiber said. It hopes to complete five miles by the end of the week.

The researchers said they don't have enough scientific data on black spot in other countries and that the disease may act differently in Florida.

Losses from premature fruit droppage were reported as high as 80 percent in Brazilian groves that did nothing to fight the disease, Dewdney said.

The main defenses include spraying with a copper compound, which blocks the spore from penetrating fruit, or a class of fungicides called "strobilurins," she said.

Because continued growth opens up new areas on the fruit peel for spore penetration, growers may have to spray for black spot from four to six times between the time the fruitlets appear in the spring until harvest from October to June, Dewdney said.

Growers who spray that often may be able to reduce premature fruit drop to 5 percent to 10 percent above normal, she said.


Peres expressed more optimism, saying an aggressive spraying program could eliminate additional fruit drop from black spot.

But in rainy years, when spores become more numerous, growers could lose an additional 20 percent of their crop even with spraying, Peres said.

Late-season Valencia oranges, harvested from March to June, are particularly vulnerable because they stay on the tree the longest, Peres said.

Valencias are also the growers' most profitable oranges because they are the main variety in not-from-concentrate orange juice, the most widely sold and expensive juice product.

Even with higher Valencia prices, Hunt said, "at this point I don't think anybody can afford four additional sprays just for that."

Graham, who has worked with copper compounds in his canker research, gave a "seat of the pants" estimate that additional spraying for black spot would cost growers $200 to $300 per acre each season.

But Mike Murphy, CEO of Cooperative Producers Inc., an Immokalee grower, said he and other growers already use copper and strobilurins to control canker and other fungal infections, so the additional sprays for them would cost only about $100 a year.

Murphy expressed more concern black spot would close U.S. and international markets to fresh fruit growers.


The USDA only last fall lifted its 4-year-old canker quarantine in Florida, which barred shipments of citrus to 10 other U.S. states and territories, including California, previously one of the largest markets for fresh Florida citrus. The quarantine was lifted after scientists showed canker-infected fruit could not spread that bacterial disease.

Murphy said he expects California and the European Union to press to reinstate the ban on Florida citrus to keep black spot out.

"We just backed up 10 paces," Murphy said. "We're back to square one. I don't know how we're going to handle the marketing problem."

What To Read Next
Get Local