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.
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.
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."