Last year was a bad year for blight in many walnut orchards around California, and this year could be even worse for those growers unless they take action, a University of California expert told growers at a recent seminar.
"Walnut Blight is a familiar foe for many of you and last year was a really bad year if you had it,” said Luke Milliron, a farm advisor for the UC Cooperative Extension for Butte, Glenn, and Tehama Counties. "We had a protracted long wet spring, and we had a lot of infection."
Milliron said early on in the 2017 season he was called to an orchard in Tehama County where almost the entire crop was on the ground.
If left unchecked, Milliron said, areas with immense pressure can lead to more than half the crop lost in a tough year like last year.
He told growers to try combining copper with fungicide application to control blight.
“We've had fairly good control when you combine copper with Mancozeb, because the Mancozeb allows copper to get into the bacteria and have its lethality,” Milliron said.
Another material growers may have at their disposal is Kasugamycin, which Milliron said looks like it will be registered in time for use this spring.
“Timing (of fungicide spray) really comes down to the orchard history,” Milliron said. “Timing the first spray and the second spray are the most important, by far.”
Milliron described a “short and early season for controlling this disease.”
There’s some good news potentially in the works: Milliron said that UC researchers are making promising strides on walnut genetics, and future generations of walnut orchards may have some level of resistance to blight.
For now though, growers must rotate materials and stay on guard.
“With timing it's really frustrating because once you see nuts dropping to the ground and you've got your hands in the air, it's probably too late to do anything about it,” Milliron said.
An explosion of growth in trees in April and May can be susceptible to blight, Milliron said, and if there are conditions for blight to grow, it’s important to go out and protect new foliage.
Last year, he said, blight plateaued by the end of May.
"Heading into 2018, if you had it bad in 2017, you have a lot of innoculum out there,” Milliron told growers. “If 2017 was a tough year, you will have to be perfect in 2018. For the first time we can say there is a need to rotate. Rotate those chemistries."
The Walnut Blight basics from Milliron:
- Early leafing varieties of walnut are far more susceptible than later leafing varieties
- If no orchard history for disease pressure, do bud testing, measuring inoculum in 50-100 buds collected for each block
- Large sprayers are useful for taller foliage--if it’s not covered, it’s not protected
- Growers should avoid half sprays, particularly on the first two sprays
- Watch weather and treat accordingly--full label rates of copper plus an EBDC