Growing practices that aim for more efficient almond production also may increase pressure from the fungal disease Alternaria leaf spot.
The disease is spreading into new territory in California's almond-growing regions, as well as showing more severity in longtime trouble areas, such as Kern County.
"It's our No. 1 disease problem," says Doug Blair, a pest-control adviser for Paramount Farms Inc.'s eastside operations, where he treats 9,000 acres of almonds for the Los Angeles-based company.
Alternaria damages leaves and can defoliate stricken trees, reducing crop yields the following year. The worst cases Blair says he has experienced cut yields about 40 percent.
In addition, the stress on trees leaves them vulnerable to other problems, particularly mites, he says.
Alternaria outbreaks are getting worse, although Blair says he can't pinpoint exactly why.
The three species that make up the Alternaria spp.complex that cause the disease to thrive wherever daily humidity is high and dew wets leaves every day, says Jim Adaskaveg, a professor of plant pathology at the University of California, Riverside.
"It's a ubiquitous complex of fungi," he says. "If you create the right conditions, it could show up anywhere in the state."
High-density plantings and microsprinkler irrigation, two practices adopted to increase yields and use resources more efficiently, also seem to foster the wet, humid conditions that can push orchards into the Alternaria danger zone, Adaskaveg says.
Rotate fungicides to fight resistance
Cultural practices can help reduce risk and severity, but the primary control is an April-through-June spray program, as temperatures rise and renewed irrigation adds moisture to orchards. Resistance has developed to the strobilurins--Abound from Syngenta Crop Protection and Gem from Bayer CropScience--that were in the first set of tools used. More recently, the organism has developed resistance to the boscalid component in Pristine from BASF Corp. But new products are taking their place, he says.
Inspire (difenoconazole) from Syngenta will be available under a Section 18 registration this year, as it was last year. In the queue for 2010 are Quash (metconazole) from Valent U.S.A. Corp. that is expected to receive full California registration then, and polyoxin-D, a biofungicide from Arysta LifeScience Corp., that also may be registered for that season.
Adaskaveg would prefer to see all three available simultaneously rather than sequentially, to allow greater rotations for resistance management.
"But if they're within a year of each other, I'd be happy," he says.
He's also testing more new materials that could further reduce resistance risks.
A growing problem up north
Above: Alternaria damages almond leaves, leaving them vulnerable to attack from other pests. Scout orchards regularly during the peak spring and summer months.
Joe Connell, a UC Cooperative Extension farm adviser in Butte County, blames increased humidity levels for much of the problem.
Irrigated agriculture, particularly the microsprinklers coming into greater use, modify the environment to create that summer humidity, he says. And as the Central Valley urbanizes, the number of homes with landscape sprinklers also increases, adding more pressure.
Summer fires last year only worsened conditions, as heavy blankets of smoke raised humidity levels, Connell says.
Wherever they are, growers should monitor their orchards for symptoms, keep track of weather conditions that favor the disease and spray where indicated, he says.
Some Alternaria symptoms can mimic those from other causes.
"What looks like a mite problem could be Alternaria," Blair says. "If you have it, make sure that's what it really is."
Steps to reduce disease prevalence
Growers planting new blocks should consider less-susceptible varieties, including Nonpareil, Rosetta and Padre. No variety is completely resistant, Adaskaveg says.
More susceptible varieties include Carmel, Sonora and Winters--which also produce some of the more valuable nuts, Connell says.
The problem, Blair says, comes when mixing in pollinators.
"You usually have at least one variety that's susceptible."
If possible, pick a planting site that doesn't require more frequent irrigation and that features lower humidity, Adaskaveg says.
Connell says that in his area, rivers and rice fields--areas with naturally high humidity--surround the orchards.
Orient tree rows north to south to take advantage of prevailing winds that can help dry up moisture, Adaskaveg says.
For existing orchards, consider adopting irrigation schedules that allow leaves and soil to dry more fully before the next run, he says. Microsprinklers with larger-volume sets that provide quick penetration also can help.
But, Adaskaveg says, "If you're in an area that's prone to the disease, these things won't keep you from getting it."
Tests in Kern County showed that switching to drip irrigation or burying drip lines had little effect, Connell says.
Blair says more hedging work down tree rows in Paramount's orchards seems to help boost air movement to gain drier conditions.
"We've tried all sorts of cultural controls," he says. But the bottom line is that outbreaks are tied mainly to weather, outside growers' control.
"The spores are always there and humidity (in July and August) makes the problem worse," Blair says. "Every year we know it's coming -- we just don't know how bad it's going to be."
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