Sac Valley Almonds: Hull Rot Bad In 2018, What Now?

Early hull split, when the hull begins to open at the suture, is a don’t-want-to-miss timing in almond production. The crack in the hull does two things:

First, it releases nut volatiles so the navel orangeworm (NOW) female can find the nut (and lay eggs).

Second, splitting gives wound pathogens like Rhizopus and Aspergillus an opening to infect the hull.

Spraying at the right time (b2 stage, see photo below) when hull split first begins (in the upper, southwest side of the tree) is critical to effective hull rot and NOW management throughout the orchard.

Because it takes time to get across orchards, consider starting spraying just before nuts get to this stage. Spraying the suture is the key. That’s where the NOW females prefer to lay eggs and that’s where the hull rot spores can infect.

Delivering a good cover spray to tree crops is a lot like painting a large house. For good weather protection of house siding, even paint coverage (no gaps), is needed.

In almonds at hull split, the whole canopy, leaves and all, must be evenly sprayed to protect the nuts —to leave no gaps you have to “paint the whole house”. There are 4-8 acres of leaf surface area in an acre of mature, vigorous almond trees.

Multiple studies in almonds from Colusa to Fresno have shown that 150-200 gallons per acre (GPA) spray volume delivers better NOW control than 100 GPA. Additional steps help deliver the best coverage possible. Slow tractor speed — 2 MPH – gives the sprayer fan time to move the spray material throughout the canopy. Spraying in dry, warm air (relative humidity below 40% and temperatures above 80o F) can reduce spray coverage 50% in the tree tops as spray evaporates.

Bring Enough Water

For the best possible spray coverage and NOW and hull rot control in mature almonds at hull split, use 150-200 GPA, drive 2 MPH, and spray when relative humidity is above 50% and temps below 80oF. (If this approach sounds too expensive and time consuming, try it out where you have the most NOW and hull rot pressure.)

From the authors: This is the target (2b hull split stage) for both reaching target water stress (-14 to -18 bars) and fungicide application for Rhizopus hull rot. No opening to the shell is visble.

Hull split is a critical timing for pest control in almonds. NOW and hull rot pressure have increased the last few years. Proper spray timing and delivery will help make reject sheet reading less painful and almond growing more profitable.

Hull rot can be an extremely damaging disease hitting almond orchards in late spring through hull split (HS). It can cost growers hundreds of dollars per acre in lost crop and clean up (winter sanitation), and is a growing problem for almond growers in the Sacramento Valley.

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.

The Players

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. Identification of the specific causal pathogen may help with management for the following growing season.

Hull rot is traditionally caused by Monilinia and Rhizopus (Figures 1-2). Over the last 2-3 years, the pathogens Aspergillus niger and a species of Phomopsis have also been isolated from stick-tight nuts (Figure 3-4).

Bad Year In 2019? ID The Cause

In the case of a bad hull rot infection in 2019, it may be wise to send sticktights to a pathology lab to confirm the species involved.

A summary of the symptoms and known control measures for each species are listed in the table below.

Orchards with high leaf nitrogen (N) levels and good irrigation practices often show symptoms at harvest compared to orchards with lower N status and less than perfect irrigation.

Multiple research studies have documented reductions in hull rot strikes when:

  • Moderate water stress is imposed for a limited time just prior to start of hull
  • And the orchard nitrogen levels are in the moderate range (2.4-2.6% N in summer samples).

In 2018, we encountered a “banner year” for hull rot. Multiple PCAs and growers who have been involved in the almond industry for many years tell us this is the highest density of sticktights they have seen.

Hoping Not To Repeat 2018

Why was last year so bad for hull rot?

Higher humidity from fire smoke is one possibility. If irrigation sets weren’t adjusted down (less water applied) in response to reduced orchard water use during the smoky period, that could have increased orchard soil moisture and orchard vulnerability to hull rot.

Also, in orchards hit by frost last February, if fertilizer N rates weren’t reduced to match the crop loss, they could have had higher orchard N levels and higher hull rot risk.

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. T

his 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. Precise fungicide applications.

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 UC Fungicide Efficacy and Timing publication listed in the accompanying table.

For Monilinia hull rot, early June is the best timing.

Hull rot is a major disease of almonds and seems to be getting worse. Careful attention to nitrogen fertilization and irrigation programs, with timely fungicide sprays (in a combined approach/package deal) are the best way to control this damaging disease.

Editor’s Note: Click here to download related image gallery.

This article appeared in the Sacramento Valley Almond Newsletter, a PDF file.

Click on table to enlarge


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