How to Manage Pests

UC Pest Management Guidelines

Walnut kernels are mature and of lightest color and highest quality when the packing tissue between the kernel halves turns brown. To maximize kernel quality and minimize insect and mold damage, harvest as close as possible to the time when the most nuts have reached the packing tissue brown stage. The problem often encountered is that hull dehiscence (separation of the hull from the nut) occurs later than kernel maturity, and hot weather can further delay this process. To speed up hull dehiscence, accelerate maturity, and promote fruit abscission, ethephon is often used. The use of ethephon on the earlier-maturing varieties avoids the late season walnut husk fly and navel orangeworm flights.

Walnuts are either harvested with one or two shakes. When two shakes are planned, ethephon advances hullsplit and allows harvest to be conducted closer to the time when most nuts are at the packing tissue brown stage and of highest quality.

For a two-shake harvest, ethephon is applied when 95 to 100% of the nuts have reached packing tissue brown. Harvest can usually begin 7 to 10 days earlier than the normal harvest date, followed by a second shake about two weeks later.

For a one-shake harvest, ethephon is applied 10 to 14 days before the normal harvest date. This timing is when 100% of the nuts have reached packing tissue brown. With this application timing, nut removal is increased and a second shake is not needed.

Notes on the use of ethephon:

• Harvest is normally 14 days after application, but it can be as early as 7 days and as late as 18 days. Hot weather will cause it to be longer. It is recommended that test shakes be made to determine when the trees are ready to harvest.
• Do not spray low-vigor, stressed, or diseased trees, and don't apply when temperatures are below 60°F or above 90°F.
• Apply the spray within 4 hours of mixing the application.
• It is essential that you cover nuts thoroughly. Ethephon is not translocated from leaves to nuts.
• Treat at one time only the acreage that you can harvest in a reasonable amount of time.
• Recommended rates of ethephon may cause slight yellowing of leaves and some leaf drop on healthy trees.

PUBLICATION

UC IPM Pest Management Guidelines: Walnuts
UC ANR Publication 3471
General Information
C. Pickel (Crop Team Leader), UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
J. E. Adaskaveg, Plant Pathology, UC Riverside
J. A. Grant, UC Cooperative Extension, San Joaquin County
J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
R. P. Buchner, UC Cooperative Extension, Tehama County
K. K. Anderson, UC Cooperative Extension, Stanislaus County
W. J. Bentley, UC IPM Program/ Kearney Agricultural Center, Parlier
W. W. Coates, UC Cooperative Extension, San Benito County
R. B. Elkins, UC Cooperative Extension, Lake County
W. H. Krueger, UC Cooperative Extension Glenn County
D. Light, USDA, Albany, CA
M. V. McKenry, Kearney Agricultural Center, Parlier
A. Shrestha, UC IPM Program/Kearney Agricultural Center, Parlier

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