California Almonds: Irrigation Management for Hull Rot Control

Hull rot can be a heartbreaker, appearing suddenly at harvest in orchards where growers and PCAs are trying to “do everything right”. Hitting young, vigorous orchards especially hard, it reduces harvestable crop, increases NOW management costs (sanitation), and kills spurs that were future nut bearing sites.

The most susceptible varieties include the most valuable and widely planted –Nonpareil and Monterey – as well as Sonora, Fritz, and Winters. Growers looking to maximize net return from their orchards planted with these varieties should consider a full hull rot management program in June and July, especially at hull split.

A complete hull rot management program includes three approaches: irrigation management in early hull split, adequate nitrogen levels in the orchard, and timely fungicide applications. In this article, we focus on irrigation management with emphasis on Nonpareil.

The goal of irrigation management for hull rot control is to use low/moderate levels of crop water stress in the Nonpareil trees between early onset and 90% hull split – a window of time lasting usually 2-3 weeks beginning about late June and ending mid-July.

The hull split phase is targeted because the fungi (Rhizopus) invade the hulls as the sutures split and produces a toxin that kills spurs and eventually the shoots attached to the fruit. Reducing irrigation and allowing low/mild water stress during hull split reduces standing water in orchards and in turn decreasing, temperature and relative humidity.

Decreasing wet humid conditions in your orchard modifies the growing conditions so hull rot is not as invasive. (See here).

The best tool to guide this practice is a pressure bomb, which directly measures tree water status. The target pressure bomb readings are within a range of -14 to -18 bars. Soil moisture sensors can provide valuable data, too, but the pressure bomb is the critical tool.

To reach these moderate stress levels, the best approach is to “ease up on the throttle” but not “step on the brake” so growers should reduce the hours of each irrigation, not the number of times they irrigate. How much less water to use should be based on the situation in each orchard.

The goal is moderate, but not excessive, water stress in the orchard, so pressure bomb readings taken twice weekly to avoid dropping under -18 bars is ideal.

Once 90% hull split has been reached, full irrigations should resume along with less frequent pressure bomb measurements until irrigation cut off for harvest.

Soil type and variability influence the timing of irrigation changes for hull rot management. The target moisture levels (pressure bomb readings) at hull split can be reached quickly in sandier soils, which hold less water, and more slowly with loam to clay loam textured soils that are “bigger water banks”.

Orchards with more soil variability can benefit more from this hull rot water management strategy if the irrigation system has some capability to irrigate the different soils separately. A reduction in hours of irrigation in heavier soils with higher water holding capacity will induce moderate water stress (-18 bars) and speed up hull split.

Meanwhile, pressure bomb readings in sandier or gravelly soils with lower water holding capacity may signal no further reduction in hours of irrigation and help prevent too much tree stress and hull shrivel. The end result is more uniform hull split throughout the orchard.

Irrigation system can also influence the hull rot management strategy. Orchards with drip irrigation systems, which wet less soil than sprinklers or flood irrigation, can see change in water status with reduced water starting at first hull split on sound nuts, while microjet sprinkler irrigated orchards may need to reduce water beginning at blank nut split to reach the target by the time that sound nuts split.

Orchards with irrigation systems that wet the entire root zone — full coverage sprinklers or flood irrigation – may need to cut back on irrigation one or more weeks earlier than hull split in the blank nuts to achieve the pressure bomb numbers wanted for hull rot control by early and mid hull split.

The practices outlined above are not easy to hit perfectly when starting out. However, the benefits can be significant. We suggest growers first try using the pressure bomb to manage water at hull split in a limited area of their operation. Contact your local farm advisor for information on irrigating with a pressure bomb. More and more PCAs are also providing this valuable service.

Added benefits reported for applying moderate water stress (-14 to -18 bars) during hull split are more uniform hull split, earlier harvest and water and energy savings. These benefits can be especially valuable in managing navel orangeworm through earlier harvest.

June Hull Rot Reminder

Hull rot infections can cost growers a lot of money; money in lost crop this year, money in more sanitation costs this winter, and money in lost spurs and shoots for future crops.

There are two disease organisms responsible for hull rot damage. The first is Monilinia (brown rot), which infects the hull just prior to hull split in early to mid-June and doesn’t show the black mold characteristic of the second hull rot causing organism, Rhizopus, which infects the nut after hull split.

Monilinia hull rot damage is often hard to diagnose as hull rot. The black mold is missing and the brown rot fungus doesn’t show on dried nuts. One or both organisms can infect hulls in the same orchard. Darkening of the xylem tissue below infected nuts is a consequence of hull rot.

The following are practices that, when combined into an integrated hull management program, deliver the best possible control of this costly problem:

  • Manage nitrogen (N) fertilizer inputs to keep summer leaf N levels across the orchard under 2.6% N. Leaf levels of 2.2% N or less = deficient, so 2.6% is more than adequate to maintain orchard health and yield potential. This can be done by carefully matching N inputs to cropload and/or tree growth needs. Spring leaf sampling can help. Don’t apply fertilizer N after May 15 in blocks with a hull rot history. This will not impact nut growth if adequate N applied before this cut off date.
  • When weather conditions (rain or dew) or orchard history of Monilinia hull rot exist, consider an early June fungicide application. Rainfall on June 8 last year, ranging from 0.08” in Arbuckle to 0.60” in Durham, could have produced a Monilinia hull rot infection event. See for effective fungicide materials. In general, FRAC 3, 11, and 19 fungicides (and mixtures containing those chemistries) are effective in reducing, but not eliminating, hull rot strikes. Best pest control results require good coverage, so a properly calibrated sprayer is crucial to efficacy of treatment.
  • As hull split approaches, manage irrigation to deliver moderate water stress (-14 to -18 bars readings on a pressure bomb) during hull split. See article in latest newsletter on this practice. This practice can help tighten up hull split window and so help with early harvest for NOW control.
  • Where conditions and/or orchard history show a need, apply a fungicide (same material options as for Monilinia infections) at early hull split to reduce Rhizopus infections.

The best hull rot control is delivered by a combination of these treatments/approaches

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