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Combat Hull Rot Today to Protect Next Year’s Yields

The threat of hull rot is present in many orchards but can be effectively treated with a combination of regulated deficit irrigation, avoiding over fertilizing with nitrogen and alkaline treatments.

6/11/2020

(June 12, 2020) – As temperatures rise throughout the Central Valley and the industry approaches the brink of hull split, it’s time for growers to seriously consider their plans to combat hull rot in the orchard.

hull rot_monilla
With Monilinia infections, as shown above, Yaghmour said growers will see a brown area on the outside of the hull and often a tan, fungal growth will appear in the brown area on the inside or outside of the hull. (Photo courtesy of UC ANR).

During hull split, the crack in the hull creates an inviting entry point for air- and soil-borne fungi common in most orchards. These fungi are attracted to the nutrients and water in the hull, and once inside the hull these pathogens release toxins that affect the fruiting wood – or spur – on which the hull hangs.

University of California (UC) Agriculture and Natural Resources (ANR) orchard systems advisor Mohammad Yaghmour, Ph.D., has studied hull rot for the past few years in California’s Central Valley. He said using integrated pest management (IPM) principles is a key way growers can prevent hull rot and be proactive to ensure it doesn’t occur in their orchards.

“It is advised to use as many tools as possible from your IPM toolbox to combat this disease and not just to rely on one method,” said Yaghmour, who advises growers in Kern and Kings counties. “This includes cultural and chemical controls such as employing regulated deficit irrigation, when possible, and avoiding applying too much nitrogen or over fertilizing.”

Research has shown that over fertilizing with nitrogen can promote hull rot. Yaghmour advised growers to manage fertilization by keeping mid-summer leaf nitrogen percentages within the critical values of 2.2-2.5%. No nitrogen should be applied between the completion of kernel fill and harvest, he said.

Unlike some pests and diseases, hull rot is not a threat to the current year’s crop – it affects the shoots and twigs that will produce next year’s nuts, ultimately impacting growers’ yield the following crop year.

Three main culprits

There are three main fungi that cause hull rot to occur in the orchard:

  1. Monilinia (airborne spore; more common in the wetter Sacramento Valley)
  2. Rhizopus stolonifera (soil fungus; more common in the San Joaquin Valley)
  3. Aspergillus niger (soil fungus; more common in the San Joaquin Valley)

Symptoms of each pathogen’s presence are shriveled and dry leaves, which indicate that the hull is infected and nearby wood is at risk. “Sometimes, the whole branch dies,” Yaghmour said.

With Monilinia infections, Yaghmour said growers will see a brown area on the outside of the hull and often a tan, fungal growth will appear in the brown area on the inside or outside of the hull. Twigs that were infected the previous year can harbor the fungus, as can infected mummy nuts from the previous season if they were not removed and destroyed during winter sanitation.

“Removing mummy nuts and conducting thorough winter sanitation is not only vital to combat Navel Orangeworm,” Yaghmour said, “it’s also necessary to fight against other diseases – like hull rot.”

Rhizopus is indicated by a black or gray fuzzy fungal growth on the inside of the hull, located between the hull and shell. It produces fumaric acid, which kills spurs and shoots. Though it lives in the soil, wind and dust transport the fungus up into the branches during hull split.

Aspergillus is marked by flat, jet black, powdery spores between the hull and the shell. Like Rhizopus, it comes from the soil and is carried by wind and dust into the tree.

Rhizopus and Aspergillus can both infect the same tree, Yaghmour said.

mohammad yaghmour
“Removing mummy nuts and conducting thorough winter sanitation is not only vital to combat Navel Orangeworm, it’s also necessary to fight against other diseases – like hull rot,” Yaghmour said. (Photo courtesy of UC Davis).

Treatments: deficit irrigation, chemical controls

In the almond lifecycle, the hull split production stage known as “Deep V” – when the hull starts to open and the suture begins to split – is when treatment for hull rot caused by Rhizopus is most valuable.

Treatment can take two forms: cultural controls such as the use of regulated deficit irrigation and chemical controls.

Regulated deficit irrigation involves reducing the amount of water applied to trees beginning at hull split. Growers should apply mild-to-moderate stress (-14 to -18 bars stem water potential) for two weeks to properly execute this type of irrigation. While appropriate decreases in water application are helpful, Yaghmour said it is still important to maintain irrigation frequency even as the volume is lowered.

“If you irrigate two times a week, continue to do that while also realizing that you’ll likely need to reduce the amount of water applied by 10 to 20% to reach the targeted mild stress, depending upon your soil type and irrigation system,” he said. “And remember, what we want is for the tree to consume the water around the root zone and induce mild stress while avoiding high stress that may negatively impact the tree and yields.”

Yaghmour recommended that growers use a pressure chamber when implementing regulated deficit irrigation to check their trees’ stress levels. Once deficit irrigation is complete, growers should resume normal irrigation until harvest dry down. To understand how much to deficit irrigate – or really, water stress – their trees, growers must first understand the various soil profiles within their orchard/s, which is a key factor in the effectiveness of deficit irrigation, and plan accordingly.

Another advantage of regulated deficit irrigation is that it promotes more uniform hull split in most trees. Growers are encouraged to read UC ANR’s Publication 8515 for more information on how to properly execute regulated deficit irrigation.                                                                       

Regarding chemical controls to combat hull rot, timing is everything. Extensive research performed by UC professor and plant pathologist Jim Adaskaveg, Ph.D., and funded by the Almond Board of California shows that at the Deep V stage the hull rot infection rate across the orchard is around 40%, but rises to 70% to 80% if growers wait until the hulls are split more than a centimeter.

Fortunately, growers can look to several fungicide options that have a “good and reliable” rating to control hull rot, Yaghmour said.

For Monolinia, fungicides work best when applied three to four weeks before hull split (in early June). Hull rot caused by Rhizopus stolonifer can be managed by a single application at hull split, often timed with a Navel Orangeworm insecticide spray. Another option against Rhizopus stolonifer, Yaghmour said, is to use alkaline treatments such as dipotassium phosphate, calcium hydroxide and Cinetis. He said the efficacy of these three applications is comparable to that of fungicides and can “significantly reduce” hull rot within an orchard. The theory behind their activity is that they might neutralize the fumaric acid produced by Rhizopus that kills fruiting wood.

In terms of treatments for Aspergillus niger, Yaghmour said research is ongoing, with preliminary results showing promise for FRAC groups 3 and 7+11 fungicides.

Nonpareil, Butte, Winters most at risk

Certain almond varieties are more susceptible to hull rot than others. Data shows that Nonpareil is very susceptible to hull rot, and that Butte and Winters also have high infection rates. On the other hand, Monterey, Carmel and Fritz are considered to have very low vulnerability to hull rot fungi.

Yaghmour said UCCE researchers Brett Holtz, Ph.D., and David Doll, published[1] an article in 2013 on hull rot management, in which they ranked varieties according to their susceptibility to hull rot. Still, Yaghmour said more work is needed to analyze varietal susceptibilty to hull rot in different regions of California.

“It could be that the pollinizers show less hull rot compared to Nonpareil since, by the time hull split takes place in the pollinizer varieties, they might be under moderate stress from harvest,” Yaghmour said. “Regardless, it’s important that growers know which varieties are most susceptible and then manage conditions as best they can.”

[1] http://thealmonddoctor.com/2013/06/18/almond-hull-rot-cultural-and-chemical-management/