Fungicide-resistant alternaria creates headaches for nut growers

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

Almond and pistachio growers in California face similar problems with fungicide-resistant Alternaria alternata.

"We're just holding our own now," says Hans Gabski, owner of Gabski Crop Consulting in Chico.

A switch to more-susceptible nut varieties and changes in cultural and irrigation practices have raised Alternaria on the list of concerns over the past 10 to 15 years, Gabski says.

Higher-yielding varieties also tend to be more susceptible to the fungus, making the more-resistant options a harder sell, he says.

At the same time, growers have adopted higher-density plantings that reduce air movement within the block.

"We wore out" strobilurins and Pristine (pyraclostrobin and boscalid) through overuse, Gabski says.

Strobilurin resistance has held steady despite resistance-management strategies, says Jim Adaskaveg, professor of plant pathology at University of California, Riverside.

Several SDHI products, including Fontelis (penthiopyrad), are now hitting the market to help fill the gap.

Other new DMI fungicides that so far haven't shown signs of resistance also are available, he says.

Steward new products to keep them viable

But growers must remember to use these new materials strategically to prevent resistance. "We can't live on any one" of these fungicides, Adaskaveg says.

Alternaria develops later in the season in pistachio orchards, says Themis Michailides, plant pathologist at the University of California's Kearney Agricultural Research and Education Center in Parlier.

Humidity that favors the fungus' growth increases between July and September.

And Alternaria's multiple reproductive cycles within the growing season boost the chances for resistance to appear, Michailides says.

He's tested the SDHI fungicide Luna Sensation (fluopyram and trifloxystrobin) for four seasons in pistachios without signs of resistance, leaving him hopeful that commercial use won't create resistance too soon.

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