WCN August 2020

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Clonal
VX211 VLACH
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Successfully using the plant
growth regulator (PGR), ethephon,
to potentially hasten walnut harvest
or improve nut removal and hullability
has been described as part art and part
science.
To obtain the desired results, growers
first need to determine if walnut
kernels are mature and receptive to
the PGR by accurately sampling the
orchard. Then they must factor in other
variables, such as variety, tree stress
levels, weather conditions, harvest
equipment availability and whether
the huller/drier they deliver to is ready
to receive their nuts, said Bob Beede,
University of California Cooperative
Extension farm advisor emeritus for
Kings County.
Ethephon can be used in two ways,
based on a grower’s goals. If it is applied
when at least 95% of the kernels are mature,
it can promote an average of four
to five days earlier harvest, depending
on the season. This may help the grower
harvest more desirable lighter-colored
kernels while potentially minimizing
navel orangeworm exposure, he said.
If done correctly, using ethephon this
way could potentially net a grower an
average of 5 cents per pound more.
A PGR application made five to
seven days after 95% kernel maturity
may aid nut removal and reduce the
chance of having to shake a second
time. Regardless of their choice, Beede
said, growers need to begin by assessing
nut maturity.
“One of the key elements to using
ethephon is to accurately determine
the physiological stage of what’s known
as packing tissue brown, or PTB,” said
Beede, who has conducted numerous
trials with ethephon during his tenure.
“It doesn’t simply mean talking to your
friends or going to the coffee shop and
asking people when they’re putting on
their ethephon because trees in sandy
soils often times will mature sooner
than trees on heavier ground. Water
stress and high nitrogen also affect
PTB.”
A plant growth regulator, ethephon
boosts the release of the plant hormone
ethylene, which promotes ripening and
senescence.
Sampling for PTB
Walnut kernels reach peak maturity
and maximum oil accumulation when
the PTB has changed from a bright
white to an oak-brown color. At this
stage, the kernels are the coveted light
blond color. While growers wait for
hull split and eventual harvest, the PTB
continues to darken as does the kernel.
Kernel maturity typically occurs up
to 21 days before an untreated harvest
but may occur later in well-irrigated
and/or heavily fertilized orchards.
Start sampling about two weeks
before PTB is expected to begin, Beede
Continued on Page 22
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Collect 100 samples, cut them open and dig out the kernel. The packing tissue should be
oak-brown in color. The photo at left is immature and the packing tissue hasn’t reached
that color yet. At right, the packing tissue is an oak-brown color, so the nut has reached its
peak maturity (photos by Bob Beede, UCCE.)
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