WCN Dec e

WINTER PREP IN WALNUTS
Start Thinking about Control Strategies for
Weeds and Disease
By MITCH LIES | Contributing Writer
Winter annual weeds pop up in the spray strip in this young walnut orchard just as trees
are defoliating. December and January offer ideal times for certain weed management
programs (photo by L. Milliron.)
December and January provide
ideal opportunities for walnut
growers to get a head start on
weed and disease control programs.
In terms of weed control, that may
mean laying down some long-residual
preemergence herbicides to keep orchard
floors clean going into spring. In
terms of disease control, the early winter
is an ideal time to measure walnut
blight inoculum levels and prepare your
season-long control strategy.
Luke Milliron, UCCE farm advisor
for Butte, Glenn and Tehama counties,
said most growers have a good sense
of the walnut blight inoculum in their
orchards from monitoring nut drop the
previous May, June and July. In cases
where they don’t, he advises to scout in
winter months and sample for inoculum
levels in walnut spurs with terminal
buds.
According to the UCCE walnut
blight sample guidelines, buds can be
sampled up to the time they start to
open, or anytime from December into
early April for late-leafing varieties. But
earlier sampling provides more time for
designing disease control strategies.
The sampling guidelines include a
recommendation that growers cut 100
or so three-inch-length dormant spurs
with fat terminal buds from several
trees in an orchard. “Walk the entire
area, collecting a random sample,” the
guidelines state. “One or two buds per
tree should spread the sample adequately…
One sample could easily represent
50 acres if experience suggests reasonable
uniformity.”
Growers or PCAs should place
samples in paper bags, which will
allow samples to breathe and eliminate
condensation, and store them in a cool,
dry place before mailing to a lab. UCCE
advisors can help interpret lab findings
and discuss the relative disease risks.
Getting a bead on walnut blight inoculum
levels and utilizing an aggressive
spray program, if necessary, are
keys to staying ahead of a disease that
ranks as the number one disease threat
to walnuts, according to Milliron.
The disease is caused by the bacterium,
Xanthomonas arboricola pv. juglandis
(Xaj), which overwinters inside
dormant bud scales and causes infection
in spring when it is rain splashed
onto developing shoots and flowers.
Low Blight Pressure
Fortunately, blight pressure has been
low the last two years, Milliron said,
and inoculum levels should be low this
winter. “A lot of people will be going
into this next spring hopefully with
very little blight pressure, because we’ve
had those two back-to-back dry years,”
he said.
In cases where blight pressure is
high, Milliron advises growers to act
early. “If you know you have high blight
pressure, you are going to start earlier
in terms of sprays, really quite early,
just the very start of prayer stage or
catkin emergence,” Milliron said. “And
you are going to be back with a second
34 West Coast Nut December 2021