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Protective structures fight HLB

California and Arizona citrus nurserymen are building or planning to construct protective structures to keep the destructive Asian citrus psyllid insect and the disease it vectors, citrus greening, out of nursery operations.

The expensive undertaking is designed to protect the investments of nurserymen and their customers; commercial growers and retailers.

“We have to move from an outdoor system to an indoor system,” said Georgios Vidalakis, University of California (UC), Riverside plant pathologist and director of the Citrus Clonal Protection Program. “Nurserymen need to grow clean (disease-free) plants to stay in business.”

Vidalakis and other citrus leaders from the U.S. and overseas discussed how Western citrus nursery operators can best meet the challenge of the Asian citrus psyllid (ACP) and citrus greening (Huanglongbing disease or HLB) during a two-day workshop in Riverside, Calif., in June. About 100 Western citrus nurserymen, researchers, government officials, and industry leaders attended.

ACP and HLB could be the largest threat ever to the Western citrus industry.

HLB causes tree death in one to two years flowing infection. Disease symptoms in leaves include blotchy mottle, yellow veins, vein corking, and green islands. Die back occurs in twigs. While about 1,300 psyllids have been trapped in California’s San Diego and Imperial counties since last summer, HLB has not been detected in California, the California Department of Food and Agriculture reports.

Arizona is ACP and HLB free so far, says Glenn Wright, University of Arizona Cooperative Extension citrus specialist.

The Western citrus-growing belt includes California’s central San Joaquin Valley with additional production in the state’s southern low desert region. Yuma County is Arizona’s largest citrus production area with additional groves mostly in Maricopa County. Western citrus is primarily grown for the fresh market.

Western citrus leaders have closely followed the ACP and HLB’s destructive path across Florida’s citrus-for-juice industry which now includes all 32 citrus-producing counties. The insect was first found in the Sunshine State in 1998 followed by the initial HLB detection in 2005. HLB is also found in Louisiana and South Carolina groves, plus in Spain and Brazil.

HLB in Florida likely occurred from multiple introductions in budwood or illegally imported plants, Tim Gast told the crowd. The agronomist with Southern Gardens Citrus, Clewiston, Fla., said the company has lost about 16 percent of its trees (about 400,000) to HLB. The infected trees were removed.

Florida mandates protected structures in all citrus nurseries, a policy likely to be copied in California. All Florida nursery stock propagated after Jan. 1, 2007, must originate from a greenhouse structure and site approved by the Florida Department of Agriculture and Consumer Services.

California nurseries

California has about 40 citrus nurseries ranging from several acres in size to more than 100 acres. Production is located from the Sacramento area to San Diego County. Most nurseries are located in inland areas with a few near the coast.

Tom Delfino, executive director, California Citrus Nursery Society, says the most common process for growing citrus nursery stock includes removing buds from highly-valued budwood source (mother) trees. The buds are placed on root stock to grow into increase trees. Buds harvested from increase trees are then budded onto rootstock and grow into nursery stock. The nursery process averages about one to three years.

Protective structures are essential to help keep the nursery stock process safe from pests and diseases. The two types of protective structures include: the greenhouse which features plastic or glass non-permeable that prevents the wind from blowing through; and the screenhouse with permeable walls allowing wind movement, Delfino says.

Most California citrus nurseries have greenhouses designed to retain heat for accelerated growth, but offer inadequate protection from pests and diseases. Several nurseries have protective structures to protect budwood source trees. Delfino says several large nurseries are pursuing large protective structures to protect entire nurseries.

The cost to place an entire nursery under a protective structure is expensive. A basic screenhouse cost $5 to $15 per square foot, Delfino says; about $500,000 for a one-acre facility (about 43,000 square feet).

Exclusion screening

James Bethke, University of California Cooperative Extension nursery farm advisor, San Diego County, supports protective structures with exclusion screening. The benefits include pest exclusion, reduced damage from the wind, and disease-free nursery stock.

Other bonuses include more effective integrated pest management, easier release of beneficial insects, plus reduced pesticide use and resistance.

Exclusion screening drawbacks, Bethke says, include light reduction, retained heat from thermal screens, required screen washing, retrofitting costs, and reduced airflow.

“There will be air restriction and airflow changes,” Bethke said. “For nurseries located in California’s interior, nurserymen will most likely have to move and cool the air. Those along the coast may not have to do that.”

Exclusion screens are made from: polyethylene; polyester; brass, stainless steel, and nickel; and unwoven filters.

“The best products are polyethylene woven screens which are extremely strong, very durable, and last for many years,” Bethke said. A polyethylene woven screen structure at UC Riverside remains in excellent condition after 20 years.

Shade cloth is not a viable option for citrus nurseries due to its large holes.

Bethke encourages nurserymen to determine the optimal screen-hole size based on the size of the nurseryman’s target pest.

“For many growers that’s the citrus leafminer which is a large insect,” Bethke said. “If thrips are the target pest, then a very small-holed screen is needed. I’ve seen a videotape of western flower thrips moving through a small hole. It’s incredible.”

Adult ACPs are 1/8 inch to 1/16 inch in size.

Bethke warns nurserymen to steer clear from screen hole-size decisions based simply on a name or an advertised hole size.

“Actual hole size is the most critical issue,” Bethke said. “A 50 mesh size from different companies can have different-sized holes since different strand width and strengths create the weave.”

The four basic roof styles for protective structures include the truss or A-frame (most expensive), sawtooth, arched, and flat shade, says Dan Howard of AgraTech, a greenhouse manufacturer in Pittsburg, Calif. The arched roof is the most common screenhouse in California and the most recommended for budwood protection, he says.

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