The effects of a humid monsoonal weather pattern which stalled over much of California in July are beginning to show up as increased hull rot in many almond orchards.
The actual disease severity appears to depend on a grower’s cultural practices and an orchard’s stage of hull split during the high humidity.
“It’s sporadic, but it's definitely increased and it's concentrated because of the monsoonal moisture,” said Mel Machado, director of member relations for Blue Diamond Growers, Salida, Calif.
“Any time you see increased humidity you will have more pressure. Some growers treated, but they're saying it was difficult to stop."
David Doll, University of California Cooperative Extension (UCCE) pomology farm adviser in Merced County, says hull rot is caused by the fungi Rhizopus stolonifer or Monolinia fructicola and are naturally found in orchards.
Monolinia, also known as brown rot fungus, is more common in the Sacramento Valley. It produces tannish to buff-colored spores on the inside and outside of the hull.
Rhizopus, also called black bread mold, is primarily found in the southern San Joaquin Valley. Gray to black Rhozipus spores can be found between the hull and the shell.
Both organisms require an injury to invade healthy plant tissue. In this case, hull split initiation when the tissue just begins to rupture which provides the gateway.
In addition to invading hulls, the fungi produce toxin that can kill the shoot attached to the fruit. Other green fruit along the dead shoot will not mature.
Since the fungus causes dieback of shoots and fruiting wood, Doll says growers can see reduced yields in future years.
Unlike many other pests, hull rot does not affect the quality of almond kernels, he says. It increases the number of stick tights which are not harvested which reduces yields.
Doll says stuck nuts pose a challenge for winter orchard sanitation crews removing the mummies.
Over the years, UC has developed an integrated approach to minimize almond hull rot which includes matching nitrogen rates to anticipated yields and inducing moderate irrigation stress during hull split.
The goal is to temporarily modify the microclimate around each nut to make it less conducive for the fungi.
Machado said, “The University of California says nitrogen and irrigation management are key pieces to the puzzle in minimizing hull rot. It all boils down to humidity and the microclimate in the split hull. When you have humidity levels like we did this summer, it will be tough to mitigate."
Research led by University of California, Riverside plant pathologist Jim Adaskaveg also has shown that a single application of a sterol inhibitor or strobilurin fungicide timed with the Navel orangeworm hull-split insecticide spray can reduce Rhizopus-caused hull rot from 60-70 percent.
Doll adds that fungicide is not promoted as a stand-alone hull rot treatment and instead should be part of an integrated approach.
Even many growers who followed a multi-faceted course this season had high incidents of hull rot due to high ambient humidity, which Doll says was 10-15 percent above average for several weeks.
“I've seen farmers who have never had hull rot have problems who have problems this year," Doll said.
Machado says Blue Diamond field staff has reported hull rot more in the Nonpareil variety than in other types, which he attributed to that variety's stage of hull split during the monsoonal period.
“The Nonpareil variety split the most during the high humidity period,” he said. “I was in an orchard with Carmel (Aug. 11) and they weren't even ‘smiling.’ It just depends on the weather conditions in the orchard when splitting occurs.”
Machado says typically if the hull seal is tight then you there is not a problem. If the hull is split and dried out, there is usually not a problem.