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Almond disease control season long challenge

California almond growers hardly have time to clean and put away their fungicide spray rigs for the season before they start servicing harvesting equipment due to a growing array of season-long disease challenges.

Brent Holtz, University of California Cooperative Extension Farm Advisor for Madera County, says disease pressure starts with the first almond blossom in February and continues well into July. No fewer than six major diseases can threaten almonds. California’s almond harvest begins in August.

Holtz detailed diseases old and new for growers and Pest Control Advisers recently in an almond orchard near Chowchilla, Calif., where the farm advisor conducted a wide array of fungicides trials to control an equally long list of diseases.

The turnout was big because growers and PCAs have had a full array of disease control challenges for two straight prolonged, wet springs and summers.

The disease season begins with brown rot and ends with possible anthracnose control measures. In between can be major threats from shot hole and scab as well as a newcomer to the family of Central Valley rogues gallery of diseases, apple powdery mildew. And then there is what Holtz calls the “lower limb dieback complex,” where diseases not normally found in almonds are showing up in dead, lower limbs.

The newest disease, powdery mildew, is really not new, but it is showing up in Central Valley almond orchards on a more regular basis. Holtz said when it was first identified, he thought it was an “anomaly,” but it apparently is here to stay.

The apple form of powdery mildew is a different fungus than grape powdery mildew. In Madera County, Holtz said it is associated with land where apples once grew. The disease can also ravage peach orchards.

“It seems to be more of a problem in southwestern Madera County where S&J Ranches reported 10 percent to 15 percent more blanks due to apple powdery mildew,” said Holtz.

Fortunately, there are fungicides labeled for almonds that can control powdery mildew. Treatment timing is like the scab treatment measure, two to five weeks after pedal fall.

“Unfortunately, Nonpareil is one of the more powdery mildew susceptible varieties,” Holtz said.

Holtz adds the word “complex” to one of the most puzzling disease issues facing almond growers, because he believes there are many factors causing lower limbs to die back, particularly on the more susceptible varieties, Butte, Padre and Monterey. Nonpareil, Carmel, Aldrich and a few other varieties are affected to a lesser degree, according to Roger Duncan, UCCE farm advisor in Stanislaus County and one of the lead researchers in trying to identify the factors involved in the loss of lower limbs.

Pathogens like Phomopsis, Botryosphaeria and Phytophthora have been identified in the dead and dying lower limbs, according to Holtz, where shading apparently also plays a role in the loss of limbs.

“Sometimes these infections are found in old hull rot, shot hole or anthracnose infections,” Holtz said, adding that he is uncertain if these newly identified pathogens are coming into limbs weakened by another disease or are infecting leaves as the first pathogens.

Because the problem involves many factors, there is no recommendation now for controlling it. Holtz believes the best way now to control it is to prune and burn the infected limbs. He recommends this during the season when the problem is evident. Waiting until the trees are dormant makes it more difficult to identify the limbs that have died back due to disease.

Pruning and burning may be an early control mechanism for another growing disease problem, anthracnose. “The early stages of anthracnose in an orchard produces spores in dead wood that can be devastating to an orchard. If you start to see anthracnose, I would suggest pruning it out and taking it to the burn pile before the disease gets to the stage you cannot get rid of it and are forced to treat for it.”

Anthracnose has not become a major problem in Madera County, but is starting to show up more frequently from Madera north to Chowchilla.

It is a major problem in Northern California where Holtz said Butte County, Calif., growers may spray as many as seven times to control anthracnose.

Monterey seems to be the most anthracnose-susceptible variety. The disease appears from five weeks after pedal fall until the first of July. It is more common in wet years like the past two seasons.

The traditional treatment period is from bloom to five weeks after pedal fall. This year wet weather extended that susceptibility period until 10 weeks after pedal fall and that is why Holtz believes more anthracnose has become evident in Madera County.

Establishment of anthracnose, which causes profuse gumming from nuts, should change the way growers manage diseases, according to the farm advisor.

Strobilurn fungicides like Abound, Cabrio, Flint and Sovran are the only materials effective on anthracnose. These materials also work well on early season diseases like brown rot and shot hole. “I would recommend growers not use strobilurns early for the early spring diseases if they have anthracnose and save the strobilurins for later use against anthracnose.

“I like to see Rovral used for brown rot and shot hole because there has been no reported resistance to the active ingredient in Rovral, and it is a good multi-site fungicide,” he said.

Fortunately, there is a good selection of fungicides for disease control in almonds, but growers should track their use not just by product names, but by fungicide classes used to make sure different modes of action are rotated to prevent resistance buildup.

“Scala was recently registered and it is a good fungicide, but it is important to understand it is in the same chemical class as Vangard and you do not want to rotate with those two materials. The same is true for products like Flint and Abound,” he said.

Another new product, Pristine, is a mixture of two fungicide classes, noted Holtz, and using it makes it more difficult for pathogens to become resistant.

To aid growers and PCAs in resistance management, the Fungicide Resistance Action Committee is establishing a number system to identify fungicides of similar classes. For example, Scala and Vangard are identified by the number 9; Abound and Flint are in the same class together with the number 11. These numbers are on product containers to make it easier to rotate products.

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