West Coast Nut April 2021

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need to use a fungicide.”
Yaghmour cautioned growers,
however, from relying on fungicides for
their control strategy. “Fungicides will
not provide 100% control of this disease.
You will need an IPM approach,
involving good nitrogen and water
management,” he said.
“We know for sure, for example, that
regulated deficit irrigation will significantly
reduce the hull rot in the orchard,”
Yaghmour said. “And we know
that if we over-fertilize with nitrogen,
then, for sure, we are going to get high
incidence of hull rot.”
Modify N Based on Crop Load
Previous UC research has shown
that nitrogen application rates above
250 pounds per acre resulted in trees
most severely affected by hull rot, and
hull rot strikes were higher in low crop
years.
In order to reduce hull rot, nitrogen
rates should be modified based upon
crop load to keep the trees sufficient,
the report noted. Further, the report
recommended growers base nitrogen
rates on analysis of leaf nitrogen content.
“If properly sampled, the critical
value for mid-summer leaf nitrogen
percentage is 2.2% to 2.5%,” a recent
UC report stated, and nitrogen should
not be applied after kernel development
is complete. Applications after
that point will be directed to the hull,
making the hull more susceptible to
infection. Kernel development typically
is completed in late spring, but can
Symptoms of hull rot caused by Rhizopus stolonifer include a fuzzy fungal growth on the
inside of a hull.
extend into early summer in abnormal
years.
Further, the report noted that UC
research has shown that inducing
a slight to moderate water stress at
hullsplit can reduce hull rot. Irrigation
should occur when the stem water
potential is four bars below baseline,
usually -14 and -16 bars, depending on
the weather, the report stated. Deficit
irrigation should be carried out for two
weeks, then full irrigation should resume
until harvest dry-down. Typically,
the report noted, a 10% to 20% reduction
in applied water will be needed,
depending on the soil and irrigation
system. It added that a properly timed
deficit irrigation can reduce hull rot by
80% to 90% without affecting yield or
kernel size.
Phoebe Gordon, UCCE orchard
crops farm advisor in Madera County,
cautioned growers from over-stressing
trees, however. “If someone doesn’t
monitor stem water potential and has
issues with hull rot, I’d start at a 10%
reduction in water during the critical
period,” she said. “Cutting back on water
without monitoring tree stress with
a pressure chamber is risky as it is. Your
trees may already be stressed.”
Another consideration in any hullrot
management program is varietal
susceptibility. Nonpareil, Sonora and
Winters have a very high susceptibility
to hull rot caused by Rhizopus stolonifer,
Yaghmour said, while Monterey
has a low susceptibility to the disease.
According to the UC Statewide IPM
website, hard-shelled varieties such as
Mission, Davey and Drake may exhibit
rotted hulls, but rarely exhibit shoot
dieback.
With proper fungicide use, varietal
selection considerations and good water
and nitrogen management, Yaghmour
said control of hull rot is within reach
of an almond producer. But attention to
all three approaches is needed.
“If we use all three approaches, and
use them appropriately, we can manage
hull rot,” Yaghmour said.
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26 West Coast Nut April 2021