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Key Issues at Hullsplit: Hull Rot, NOW Sprays and Sprayer Calibration

Now is the time to plan ahead for pest management activities leading up to and during hullsplit. The three key areas to consider at this time are hull rot, hullsplit sprays for navel orangeworm, and sprayer calibration and coverage. As a result of research supported by the Almond Board of California (ABC), some of the tools and techniques for addressing these issues have been refined.

Past and current ABC-funded research illustrates that effective hull rot management takes an integrated approach that includes regulated deficit irrigation at hullsplit initiation, fungicides (if appropriate), and a balanced fertility program that does not include applying nitrogen in excess of tree and crop demands.

Deficit Irrigation. Hull rot can be reduced 60% to 90% by inducing mild stress at hullsplit initiation, and maintaining that stress for two weeks afterward. This is best executed by tracking tree water status with a pressure chamber; the goal is to reach mild stress with readings of –14 to –18 bars at the onset of hullsplit, and maintaining that level for two weeks.

Tracking tree water status to initiate deficit irrigation to reduce
hull rot is best done with a pressure chamber. The goal is to
reach mild stress with readings of –14 to –18 bars at the onset
of hullsplit, and maintain that level for two weeks.
Best practice is to use a pressure chamber throughout the season, especially before and after irrigations, in order to determine how long it will take to reach this reading. Managing and achieving mild stress without a pressure chamber is more difficult, but growers have been successful with a 50% reduction in applied water during the two irrigations prior to the onset of hullsplit.

Once the stress period is completed, additional irrigation may be needed to recharge the soil profile and ensure excessive stress does not occur during harvest. Accordingly, pressure chamber readings should be done in concert with soil moisture monitoring to ensure deep moisture is not depleted. Watch for mites and their natural enemies during the imposed stress.

A new tool to help predict the onset of hullsplit is the UC Almond Hullsplit Prediction Model. This can be found online at the UC Fruit and Nut Research and Information Center.

Another important consideration is to prevent yield impacts by delaying deficit irrigation until nut fill is completed, which is usually in June, depending on the location and year.

Nitrogen Rates. Research shows that excessive rates of nitrogen lead to a higher incidence of hull rot. Newly revised nitrogen management guidelines are a means to a balanced season-long program. These guidelines are available on the ABC website.

Effective Fungicides. Under situations of high disease pressure, recent ABC-funded research by Dr. Jim Adaskaveg (Plant Pathology, UC Riverside) shows that a single application of a number of registered fungicides applied at the same time as a navel orangeworm insecticide treatment can reduce hull rot caused by the bread mold Rhizopus stolonifer as much as 60% to 70%. These fungicides are in the DMI (FRAC 3) group and the QoL (FRAC 11), and are listed at UC IPM Online. These applications are made when sound fruit in the tops of trees begins to split, or, in other words, at about 1% hullsplit, and therefore can be tank-mixed with navel orangeworm insecticides.

To manage resistance, avoid overuse of any one class of fungicide during the season, and carefully consider these applications. For hull rot caused by Monilinia fructicola, effective fungicide applications will need to be made earlier, in late spring, but this practice needs additional research.

Update on NOW Management. Ongoing ABC-funded field research by both Joel Siegel (USDA, Parlier) and David Haviland (UCCE, Kern County) continues to show there are a number of effective insecticide options for navel orangeworm. These include newer ovicides/larvicides and pyrethroids. Trends across trials conducted by Haviland show the diamides (e.g., Altacor, Belt), other ovicides/larvicides (e.g., Delegate, Intrepid and Proclaim) and pyrethroids all significantly reduced NOW damage.

A navel orangeworm synthetic pheromone blend is now
available in combination with a trap. Almond Board–funded
researchers will be looking at how this commercially available
pheromone trap can be used in making NOW spray-timing
decisions, both in spring and during the hullsplit period.
In each trial, the best treatments provided about 60% control, and the most effective treatments were usually tank mixes of an ovicide/larvicide with a pyrethroid. Both Siegel and Haviland note this improved control results from pyrethroid adult activity; however, they also point out that pyrethroids should be used prudently. Haviland advises that if pyrethroids are used, application should be made only once per season, at hullsplit. This class is harder on beneficial insects and tends to flare mites. In addition, pyrethroid overuse has led to peach twig borer resistance in Northern California almonds, and in other crops, resistance has emerged with other insect species.

Studies by Siegel reaffirm the importance of timing at hullsplit initiation. As stated previously, applications are made when sound fruit in the tops of trees begin to split, or, in other words, at about 1% hullsplit. Being a little early is better than being late. At The Almond Conference last year, Siegel remarked, “Nothing is improved by time.”

A new component to timing hullsplit sprays is the NOW synthetic pheromone blend, which is now available in combination with a trap. Almond Board–funded researchers will be looking at how a commercially available pheromone trap will be a best-fit tool for NOW spray-timing decisions, both in spring and during the hullsplit period.

Sprayer Calibration and Operation Are Critical. This is particularly true for NOW control, in which leaf, shoot and nut growth can reduce spray coverage to nuts in the upper canopy compared to early in the season.

Uniform spray delivery throughout the trees is a challenge as
the season progresses and canopies fill. Air delivery, water
volume, speed, number of nozzles and using an engine-driven
PTO all contribute to successful late-season applications.
A 2012 Almond Conference workshop titled “Spray Coverage: The Missing Link in IPM,” led by Franz Niederholzer (UC farm advisor), Brad Higbee (Paramount Farming Company) and Ken Giles (UC Davis), provides a very good review of sprayer calibration. Another valuable resource is the UC IPM manual, “The Safe and Effective Use of Pesticides.”

In addition to basic calibration, both Franz Niederholzer and Joel Siegel offer key points and research findings to consider for late-season applications and sprayer operation, particularly for nuts in the upper canopy:

  • Air delivery needs to be balanced, and it changes during the season. That is, air delivery is lighter early in the season, but full air is needed when the canopy has closed. Niederholzer notes, however, that minimizing offsite movement is important. While sufficient air volume is needed to move the spray thoughout the target canopy, it should go no farther.
  • There should be sufficient water volume to give good coverage, once adequate carrier air is delivered. Later in the season, higher gallonages are a benefit; for instance, coverage at 150 gpa is better than 100 gpa.
  • Be careful about speed: Just increasing from 2 mph to 2.2 mph has reduced coverage in field trials about 30%.
  • Coverage can be improved by using a double bank of nozzles on each vane.
  • Under comparable conditions, coverage by an engine-driven sprayer is better than a PTO.

A parting thought: Niederholzer says that proper sprayer calibration and operation is a balancing act. Two important goals must be met: One is optimizing pest control, and the other is reducing drift and pesticide runoff from orchards. Sprayer effectiveness, responsible environmental stewardship and more stringent regulations require proper sprayer calibration and operation.

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