Many California almond growers are battling three diseases — almond scab, alternaria leaf spot, and hull rot — like never before.
However, they have no one to blame but themselves, says a leading plant pathologist.
An overall increase in all three can be laid, at least partially, at aggressive growing practices like closer orchard tree spacing. High nitrogen and water use to push trees to maximum production is also a contributing factor to increases in the diseases.
For scab, overuse of a fungicide class has resulted in high levels of resistance to strobilurin materials in many orchards in Butte, Tehama, and northern Glenn County, according to University of California, Riverside Plant Pathologist Jim Adaskaveg.
While growers are at fault for the upturn in the diseases, they can't really be blamed. The price of almonds over the past six or seven years has made the little brown nuts seem like gold nuggets, and growers want to gather as many as they can while prices are on an unprecedented record run.
The increase in the diseases doesn't mean growers will chop down trees producing the golden almonds the way creeks dried up during the California gold rush.
But, it does mean they must change disease control strategies in the case of scab resistant to a fungicide class. For the more closely-spaced trees, growers can expect to live with high disease pressure problems for the two to three-decade life of the packed orchards.
Scab fungicide resistance
Adaskaveg says a number of orchards in Northern California were identified with strobilurin-resistant scab last season and it is still there this year.
“For the last eight years, we haven't seen much scab. This was due to the introduction in 1998 of Abound and its new class of fungicide,” he told more than 200 growers at the recent Nickels Almond Field Day.
Abound was the first strobilurin, the most effective fungicide on scab that plant pathologists had ever seen. Several other strobilurins came to market after Abound, including Flint/Gem, Sovran, Cabrio, and Pristine, a dual active ingredient containing both a strobilurin and boscalid. Unfortunately, says Adaskaveg, bascalid is ineffective on scab, which in essence makes Pristine a single-site fungicide in the scab resistance scenario, rendering it ineffective in scab-resistant orchards.
The resistance is in orchards where growers used strobilurins more than twice per season; made alternate-row applications of the fungicide; or applied the single site mode of action fungicide by air to trees with full canopies.
The resistance isolates overwintered into 2007, rendering the strobilurin fungicides totally ineffective.
Fortunately, there is a “plethora of good material” to replace the scab-resistant strobilurins. They are all old standbys.
Adaskaveg and Northern California UC Cooperative Extension farm advisors say a dormant or delayed dormant spray of 12 to 15 gallons of liquid lime sulfur per acre has proven very effective.
He told growers strobilurin-resistant or untreated scab was sporulating in March, while the scab treated with liquid lime sulfur didn't sporulate until late April or early May, giving growers four to six weeks of control from liquid lime sulfur alone.
Adaskaveg recommends a follow up in-season treatment of chlorothiolonil at petal fall, followed five weeks later with a treatment of captan, maneb, or ziram.
“Wettable sulfur is also pretty active on scab,” he says.
This scab treatment scenario should prevent summer defoliation from the disease and resulting crop loss. “There should be no defoliation until October or November from scab, making scab a non-economic issue.”
Alternaria leaf spot
Alternaria leaf spot is increasing in two areas, the Highway 99 corridor north of Bakersfield, Calif. and along I-5 in Northern California, according to Adaskaveg. This is where many high density orchards have been planted.
They are irrigated with sprinklers or drip, with 60 to 70 hour irrigation sets, producing high humidity. When high temperatures accompany that high humidity, it is an ideal environment for alternaria leaf spot.
Most of the same materials used for almond scab, including strobilurins, also are registered for alternaria leaf spot; thus the issue of resistance surfaces here as well.
However, like in the scab situation, there is good news. According to Adaskaveg, Rovral, a multi-site fungicide, will soon be given a Section 18 for use against alternaria.
Rovral is now registered on almonds, but at a half pound active ingredient rate, with a pre-harvest interval of five weeks after petal. The Section 18, which has been granted by the California Department of Pesticide Regulation, increases the a.i. on almonds to one pound and the phi to 60 days.
This should help to control alternaria leaf spot with Rovral rotated with strobilurins. He expects federal approval by the end of May.
High density plantings are contributing to the alternaria and scab increases, and aggressive, high nitrogen use and no-stress irrigation are contributing to a rise in hull rot, a problem that has no easy chemical solution.
Hull rot is caused by a soil-borne organism that comes from within the orchard or is blown in. It thrives on moisture in inside the split hull.
“It's difficult to slow an orchard under high density, high fertilization and heavy irrigation enough to let those split hulls dry out,” Adaskaveg says.
Nevertheless, he urges growers to throttle back aggressively-growing orchards in the summer, allowing them to dry out and go through hull split quickly. He recommends reducing leaf petiole nitrogen levels to 2.5 percent. This will not starve the tree, but may make it less vigorous.
He also recommends deficit irrigation at hull split. “I know it's hard to cut back water to slow tree growth when it is 105 and 110 degrees, but stressing trees a little in late June and July could get them through hull split without a lot of hull rot.”
There are fungicides that control hull split, but it is almost impossible to get them inside the split hulls. “Plus, I don't think you will get a fungicide registered for application just before harvest,” Adaskaveg says. “Your best control of hull split is cultural practices.”
All the increasing disease issues are partly due to closer spacing of trees and more intense management to maximize yields. High density plantings preclude air movement and are idea for humidity and heat buildup, two important elements in the spread of diseases.
“Maybe lower density plantings would help,” Adaskaveg says. “I know that's a bad word, but it may be a compromise worth considering. Otherwise, you're going to live with some of these disease problems for the life of the orchard.”