Florida growers fight citrus greening

Photo courtesy of Dr. Phil Stansly, UFL Southwest Research and Education Center.
Phil Stansly discusses with growers the preliminary results of UFL citrus greening field trials on insecticides and micronutrients.

The Asian citrus psyllid, Diaphorina citri, is a culprit wreaking havoc in Florida citrus groves. The insects pose a threat to the rest of the nation’s citrus groves as they carry the citrus greening bacterial disease. Citrus greening, also known as huanglongbing (HLB), has been identified in every citrus-growing county in Florida. HLB is used interchangeably with citrus greening. Citrus greening attacks a number of citrus varieties, starting with leaf mottling and bright yellowing of the veins leading to a decline, that may kill the trees. The yellowing gives the disease the name huanglongbing or yellow shoot disease.

Growers are working in tandem with USDA, University of Florida (UF) and other entities to try to minimize the effects of HLB while long-term solutions are sought. These solutions include the development of disease-resistant citrus trees and more effective methods to control the insect, but in the meantime, a three-tiered management approach has been recommended that includes removal of infected trees, control of the psyllid and the use of disease-free trees to replant.

While thousands of trees have been removed, the approach has generated controversy, and some growers are working to maintain their trees. Still others have abandoned their groves.

Part of the three-tiered management approach includes removing diseased trees and replanting with only certified disease-free stock. To avoid exposure to the insects and HLB, several nurseries have moved to isolated areas of the state away from commercial citrus areas.

Nursery relocation has resulted in a number of nurseries closing that could not afford to replace all existing stock and move to new locations. With fewer nurseries operating, and less citrus stock available, replacement costs for citrus have increased dramatically.

Photo courtesy of Ryan Atwood, UFL Extension Service.
Citrus trees damaged by citrus greening drop fruit.
Photo courtesy of Mongi Zekri, UFL Extension Service.
Close up shot shows psyllid that transfers citrus greening disease.
Misshapen fruit can result from citrus greening disease.

Science seeks answers

Entomologist Dr. David Hall leads the Subtropical Insects Research Unit at the Horticultural Research Lab of USDA Agricultural Research Service in Fort Pierce, Fla. Hall emphasized that citrus greening is a worldwide problem and that researchers from USDA, UF and other organizations worked with researchers from around the world to develop a plan to address the issue. Hall noted that while Brazil mandated that diseased trees be destroyed, neither the U.S. nor the state of Florida issued a mandate. While many growers have removed trees, removal is not uniform throughout the groves where disease has surfaced as some growers try to save their trees.

“Some growers are just walking away from their groves, because they can’t afford to remove their trees,” Hall noted. “That leaves the diseased trees still there.”

A breakthrough was made this year when USDA Agricultural Research Service scientists sequenced the genome of the bacterium, Candidatus Liberibacter asiaticus, which causes the disease. This sequencing will allow scientists to decipher the genetic code of the bacterium and study its biological features to help identify genes that will assist in developing control methods.

Controlling the insects

The entire state of Florida is under a USDA quarantine that prevents moving citrus plants from the area into other citrus-growing regions.

Dr. Phil Stansly, entomologist at the University of Florida Southwest Florida Research and Education Center emphasized that dormant sprayings are most effective in controlling the insects. “An advantage of winter spraying is that it limits collateral damage on beneficial insects such as lady beetles and lacewings,” Stansly said. “Through the growing season, we encourage scouting for psyllid and adjusting control spraying based on psyllid populations.”

Both foliar spraying and soil drenching are employed in the groves. The chemical imidacloprid is one of the primary products used for both, although it is especially recommended as a drench for young trees.

“Another chemical, aldicarb, is a granular product that can be applied to the soil,” Stansly said. “It is costly but depends on moisture. Even with irrigation running during the winter months, some of the soil profile can remain dry without rain and the chemical does not activate properly.”

Grower response

The Florida Citrus Production Research Advisory Council (FCPRAC) is funding about $7.5 million in research this year. About $20 million has been invested in research.

Doug Bournique, executive director of the Indian River Citrus League, said, “We have 900 growers that range from 40 to 40,000 acres of citrus groves.” While Bournique cited a delay in research funding, he said that “science is showing us pieces of the puzzle that look promising.”

Three-tiered approach

Mike Irey, plant pathologist and research director at Southern Gardens Citrus, strongly encourages growers to implement an HLB control program as opposed to doing nothing. Southern Gardens in Clewiston, Fla., a subsidiary of U.S. Sugar, is aggressively following the three-tiered approach.

“Citrus greening was identified here in 2005. There’s a very long latent period from the time the tree is infected to when symptoms begin showing up,” Irey said. “Similarly, because of the long latent period, growers have been using various control measures for a couple of years now and are seeing the disease getting worse, not better. What they need to understand is that it takes time for effects of the control efforts to become evident. I tell people that it will get worse before it gets better. They have to stick with the program long enough to see the fruits of their efforts.”

Irey said, “Our production costs have increased 40 to 50 percent over the past three years.” Southern Gardens had about 16,500 acres of citrus trees before citrus greening surfaced as a major concern, down from an earlier 21,500 before trees were removed due to canker. Groves are located in the southern part of the state, where the citrus greening infection rate is among the highest.

“We have removed over 400,000 trees from about 2,800 acres since citrus greening was identified in our groves in 2005,” Irey said. “Southern Gardens’ costs and losses associated with greening have been significant. However, it appears that we have turned the corner, and we are seeing the results of our control efforts with greatly reduced levels of new infections.”

Testing for citrus greening disease is done in the Southern Gardens lab.
Citrus trees damaged by citrus greening drop fruit.

Irey said, “Southern Gardens has been very proactive in its response to greening and has made laboratory facilities available to the industry. Southern Gardens had the capacity and expertise and made the decision to open its laboratory to the outside. The testing process has been made available at no cost to growers, researchers and homeowners. Initially it was also used by nurseries in developing disease-free bud-wood for replanting stock.” Irey noted that FCPRAC and UF are now assisting with testing costs.

Nutritional approach to strengthen trees

When citrus greening showed up in his grove, Immokalee grower Maury Boyd decided not to remove diseased trees. Instead, he modified a nutritional spray he had been using in his groves for several years. “We started seeing improvement in the diseased trees,” he said. Boyd is president of McKinnon Corp. in Oakland, Fla. He demonstrated his results to Dr. Bob Rouse, UF citrus horticulturist. UF field trials, funded by about $300,000 from local growers, have been established, and Rouse has recommended the nutritional spray to growers who decided not to remove trees.

Rouse said, “If growers decide they can’t afford to remove their trees, we don’t take a position on that, but try to help them keep their trees healthy.” In addition to insect control, trees are sprayed with a mixture containing both micro and macronutrients along with Serenade and SAver, both of which are systemic acquired resistance (SAR) elicitors. It is believed that the SARs enhance the plant’s natural immune system. Rouse said, “We target the first spray when new growth appears, and the young tender leaves are most able to absorb the nutrients.” Three sprayings that are mostly applied to the foliage are recommended with timing in spring, beginning of summer and late summer.

Citrus trees exhibit damage from citrus greening.
Maury Boyd, Mongi Zekri and Bob Rouse discuss nutritional spraying to help trees withstand citrus greening.

Dr. Mongi Zekri, a UF Extension Service citrus agent, works in a five-county area of southwest Florida. Zekri said, “We’re working on education with the growers on controlling the vector, the psyllid, and on nutritional spraying.” He is encouraging growers to participate in region-wide insect control measures, particularly two winter aerial sprayings, to more effectively control the insects. UF has also established a testing laboratory at its Southwest Florida Research and Education Center where growers may have samples tested for HLB.

Looking ahead

Citrus greening represents a major threat, and while long-term solutions are sought, controversy on the best ways to address the problem continues. Stansly said, “There’s a tipping point where some growers can’t afford to remove trees or they go out of business.”

The problem is multifaceted and reaches beyond the groves. “The infrastructure here in Florida developed over a long period,” Hall said. “If we lose the citrus, we lose the packinghouses, the equipment, and the suppliers.”

Growers, along with state and federal organizations and institutions, continue to look for ways to survive this most recent challenge, and approaches are ranging from nutritional sprayings that Rouse recommends to complete removal of diseased trees.

Irey said, “What we are doing now is not sustainable. We’re trying to hold on until a solution is developed. Growers in general are pretty resilient and resourceful, and I have no doubt that we’ll come up with a solution.” Irey sees a much higher level of grove management required in the future. “The days of 40 and 50-year old groves are gone. Groves will have a shorter life span that will require a much higher level of management,” he said.

The Web sites www.imok.ufl.edu and www.saveourcitrus.org contain additional information and links related to citrus greening.

Nancy Riggs is a freelance writer and frequent contributor. She resides in Mount Zion, Ill.