Fungicide-resistant alternaria creates headaches for nut growers

05/31/2012 04:09:00 PM
Renee Stern

Also promising in his pistachio tests are Luna Experience (fluopyram and tebuconazole), and, for Alternaria late blight, Fontelis, Quash (metaconazole) and Quilt Xcel (azoxystrobin and propiconazole).

Adaskaveg recommends uniform concentrations of all fungicides. "I'm not an advocate of alternate-row spraying," he says.

Correct timing is also crucial.

"Protective treatments are better than corrective," he says.

If you must spray during high disease pressure, compounds with multi-site modes of action are less risky for resistance than fungicides with a single active ingredient.

Adaskaveg is working to change regulations on chlorothalonil fungicides to shorten the preharvest interval, currently at 150 days.

"It's a multi-site material, and if we could get a 60-day PHI that would help," he says.

Cultural controls

Avoid cover crops, which increase orchard humidity, Michailides says. Where cover crops are necessary, remove them by the end of May to avoid aggravating Alternaria problems.

Orient new plantings with prevailing winds to improve air movement within the block, and hedge canopies in tight plantings to open up the trees to sun and air, he says.

Irrigation also can raise orchard humidity. Adding gypsum may aid water infiltration if surface pooling is an issue, he says.

Northern California growers rely on solid-set systems for irrigation and frost protection, Gabski says. But that often increases orchard humidity.

One option is to install a secondary double-line drip system for regular irrigation needs and keep the solid-set for frost protection, he says.

"We don't really control Alternaria," Gabski says. "We can only manage it."

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