With growers shaking the last of the Chandler trees this season in mid-October, the 2013 walnut harvest in Tehama and Glenn counties was quickly winding down.
Once the harvest is completed, it’s a good idea for growers here and elsewhere in the state to check soil moisture levels and replenish them, as needed, before the start of fall rains, notes Rick Buchner, University of California Cooperative Extension orchard advisor in Tehama County.
Typically, growers stop irrigating their orchards far enough in advance of harvest to provide a dry surface for the nuts after they’ve fallen from the trees and to support harvest equipment.
“Usually, by now soils are getting pretty dry,” Buchner says. “I’m a big fan of restoring soil moisture as soon as possible after harvest.”
One way to determine the trees’ need for water is the use of soil augers to visually evaluate moisture conditions at various levels of the soil profile. Another is to base irrigation amounts on crop evapotranspiration ETc – the amount of water lost from the leaves through transpiration and from the soil through evaporation. This figure, usually measured in inches, is an estimate of how much water a tree consumes in a day. It’s based on such factors as type of soil, depth of root zone, air temperature and humidity, wind speeds and orchard management practices. This information is often available in local newspapers and for Tehama County at cetehama.ucanr.edu. Go to On Farm Irrigation scheduling tools and click on weekly soil moisture loss reports.
“Based on soil moisture levels at the start of harvest and how much moisture the trees used weekly in terms of ETc,` you can calculate how much water you need to apply with your post-harvest irrigation,” Buchner says.
Other options for determining soil moisture include such instruments as tensiometers, gypsum blocks and neutron probes. Pressure chambers are useful to measure actual tree water stress.
Regardless of which approach you take in managing your post-harvest irrigation program, the key is not to overdo it, Buchner adds.
“The idea is to apply only enough water to prevent stress on the trees until Mother Nature provides free irrigation in the form of rain, which usually starts later in the fall,” he says.
Another important part of caring for your walnut trees after harvest is removing mummies to eliminate over-wintering sites for the navel orangeworm. That could be especially critical this fall and winter. Buchner notes.
“I haven’t seen many grade sheets, yet, but I suspect this year’s walnut crop suffered unusually heavy navel orangeworm damage,” he says.
He attributes high NOW trap counts this season to several factors.
The first NOW flight in his area began about three weeks earlier than normal. This provided time for an extra generation, a fourth one, to develop. The adults that laid the eggs for this fourth generation began flying in Tehama County about Aug. 19. As a result, worms began emerging as the hulls were splitting in both walnut and almond, providing easier entry for the worms to infest the nuts. Navel orangeworms is a scavenger insect and hull damage from codling moth, walnut blight and/or sunburn favor NOW infestation. Sunburn damage was higher than usual this year due to several mid-season hot spells.
Over the past few years, the incidence of Botrysphaeria (Bot) has been on the rise here and in other areas of the Sacramento Valley, Buchner notes. In an effort to combat this growing threat, more and more growers have been treating trees with fungicide sprays as a way to reduce the disease inoculum and prevent death of limbs and spurs. University of California scientists haven’t determined effective spray timing for Bot. But, some growers speculate that a post-harvest application may be a possibility.
“There’s a lot of interest in it and UC researchers are ramping up studies to better understand what materials are effective in controlling Botrysphaeria and the best time to apply them,” he says” “Right now, however, we do not have enough information to be certain that post-harvest applications are effective.”