Originally posted July 2018; Updated July 2019.

Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties & Allan Fulton, UCCE Water Resources Advisor, Tehama, Glenn and Colusa Counties

Hull rot can be a heartbreaker, appearing suddenly at harvest in orchards where growers and PCAs are trying to “do everything right”. Hull rot is a general term for hull infection by one of several pathogens. Infected nuts don’t shake off at harvest and must be removed by winter sanitation to eliminate future navel orangeworm (NOW) feeding sites. Often the infection of the hull results in death of the spur and attached shoot, reducing bearing surface of the tree. The list of hull rot pathogens is growing, and now includes Rhizopus, Monilinia, Aspergillus, and Phomopsis. Aspergillus infections can lead to staining of the kernel and reduction in nut quality. The most susceptible varieties commonly planted include: Nonpareil, Monterey, and Wood Colony. UC research is underway, supported by the Almond Board of California, to look at one of the new pathogens on this list — Aspergillus – as well as continuing efforts to manage Monilinia and Rhizopus hull rot infections.

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.

The best tool to guide this practice is a pressure chamber (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.

The best, current approach to hull split management includes three parts:

1. Moderate water stress approaching hull split: The target is -14 to -18 bars stem water potential (SWP) for two weeks beginning just before ANY hull split (late June). The goal is to gradually reach this goal by reducing the hours of each irrigation set, not the number of irrigations. After 2 weeks at that moderate stress level, return irrigation to full ET. To hit that target, growers must start reducing irrigation at different dates, depending on soil water holding capacity (texture). Growers on heavy ground (clay loam) may need to begin to back off on irrigation as early as June 1, up to 30 days before expected HS. Growers on lighter, sandier ground may be able to wait closer to HS before easing up on the water. The key to successful hull rot management with irrigation is getting the orchard to -14 to -18 bars just before the suture starts any separation and keeping it there for 2 weeks, then return to full ET irrigation. This moderate stress in a short period does not reduce yield. Use a pressure chamber (pressure bomb) to make sure water stress reaches the target on time, but doesn’t exceed the target. If you don’t reach the target water stress before hull split, you won’t help control.
2. Careful nitrogen management. Adequate, but not excessive orchard N helps control hull rot. The target is <2.6% summer leaf N. Don’t apply N between May 15 and harvest in orchards with hull rot history.
3. Fungicide application. To control Rhizopus hull rot with fungicides, the best spray timing is 2b hull split stage – the same as for NOW sprays using softer, long lasting insecticides like Intrepid® and Altacor®. FRAC group fungicides 3, 11, and 19 provide “good and reliable” control when carefully applied (see article in this newsletter on spray coverage). Check the UC Fungicide Efficacy and Timing publication. For Monilinia hull rot, early June is the best timing.