For the past eight years or so, the Monterey almond variety has exhibited an increasing number of maladies in scattered Northern California orchards. Although researchers have yet to conduct replicated trials to pinpoint the causes, anecdotal reports and observations point to overwatering as a possible instigator, said Luke Milliron, UCCE orchard advisor for Glenn, Tehama and Butte counties.
“It’s really a head-scratcher. It’s a really difficult thing to study,” he said.
J.R. Gallagher, a PCA for Butte County Rice Growers Association in Richvale, said he’s found symptoms similar to what Milliron described but in the Carmel variety. But he didn’t see them in the Nonpareil or Aldridge trees that were in the same orchards.
Gallagher even had Milliron out several times to look at the orchards. He wondered if part of the issue with Carmel is they’re related to Monterey. Both were chance seedlings from Nonpareil-Mission crosses.
“I don’t have Monterey, but I’m just seeing similarities and I’m concerned,” he said. “What Luke was explaining about the Monterey, I was seeing last year with Carmel on Krymsk. The leaves were yellow.”
The common denominator among the troubled orchards is Krymsk 86 rootstock. Growers in the northern Sacramento Valley have widely adopted it because of its vigorous root system and strong anchorage.
“Many varieties behave poorly on Krymsk, especially if there’s too much water, and Monterey is one of those,” Milliron said.
But he said rootstock doesn’t explain the issues entirely with Monterey, since growers in Australia are experiencing some of the same challenges with the variety and they don’t use Krymsk 86.
For growers and PCAs dealing with Monterey maladies or issues such as Gallagher has seen, Milliron recommended adopting pressure bombs or automated technology like FloraPulse to ensure they’re irrigating to meet the trees’ needs and not under or overapplying water.
Gallagher did just that last year and worked with the grower of a problematic orchard to use a pressure bomb in the worst area of the block. The weekly pressure bomb readings were a tremendous help, he said. The grower was able to use that real-time data to fine tune the irrigation cycles to improve tree vigor.
In orchards that may have water-penetration problems, Milliron also suggested planting a cover crop or applying gypsum to help open up the soil.
List Continues to Grow
The four issues seen so far in Monterey are yellow curled leaf/yellowing, leafing failure, flowering failure and Botryosphaeria/Phomopsis dieback.
Milliron said he hasn’t heard from UC colleagues about Monterey problems outside the northern Sacramento Valley, but he pointed out this region receives significantly more rainfall than the north and south San Joaquin valleys. In addition, many newer Sacramento Valley orchards were planted on marginal or heavy soils with poorer drainage that were former rice fields.
Gallagher also cited the larger number of solid-set sprinklers used in the northern Sacramento Valley. Farther south, most orchards are on drip or microsprinklers, which allow more precise water application.
Drip irrigation also allows growers to irrigate immediately after shaking, reducing potential water stress to the trees. Growers with microsprinklers or solid-set irrigation typically wait to irrigate until the nuts are picked up to avoid wetting the crop. By doing so, they may be stressing the trees.
Leaf Yellowing
Farm advisors began receiving calls in 2018 about unique foliar symptoms (yellowing, rolling leaves with marginal necrosis and poor growth) in Monterey. The symptoms, seen in young trees grafted onto Krymsk 86 or Rootpac-R, were like those seen in union mild etch disease. The latter problem is typically seen in Monterey, Butte and a few other varieties grafted onto Marianna 2624 plum.
Research into union mild etch found overwatering was a contributor, and it appears the same may be true for the most recent leaf yellowing with Monterey, Milliron said.
But unlike union mild etch, there’s no report of etching at the bud union with the most recent cases with Monterey on Krymsk 86. Based on his observations, Milliron said the same treatments apply to both issues: don’t overirrigate, limit fertilizer and let the trees recover naturally. The severely affected trees may die, but he said many others will recover.
Leafing Failure
In 2018 and again in 2020, UCCE farm advisors received calls about leafing failure in several varieties but most notably Monterey. These were not the first reports, with symptoms being noted periodically even before 2000. More recently, farm advisors also reported the issue in 2000, 2006, 2010 and 2017.
The earlier reports came not only from the Sacramento Valley but also the San Joaquin, and they involved several different rootstocks.
While most of the symptoms were on mature trees, some were also seen in young orchards. Symptoms ranged from a few trees on the orchard’s edge to one mature Northern California orchard that had about 50% of Monterey pollinizers affected. Not every orchard with Monterey has experienced the issue either, and Milliron said he’s also seen perfectly healthy orchards with Monterey pollinizers.
Leafing failure differs from non-infectious bud failure, seen predominately in Carmel but also in other varieties, including Nonpareil. With leafing disorder, the vascular tissue is green and alive if you cut into affected branches or shoots. Vegetative buds may be late in pushing or push from some parts of a branch but not another.
With bud failure, on the other hand, flower buds typically bloom normally but may be delayed a few days. It’s the vegetative buds that never emerge and likely died the previous fall.
In an earlier trial, UCCE irrigation specialist Bruce Lampinen was able to induce delayed leafing in Monterey in plots that received more than 100% of evapotranspiration. In other words, they were overirrigated.
Milliron said the leafing malady also has been documented both in California and Australia following wet years. In California, bad leafing disorder years of 2018 and 2020 followed the wet years of 2017 and 2019 spring. In addition, the 2024 season followed an unusually wet 2023.
Flowering Failure
Flowering failure also appears to be tied to excess water the previous year. The problem appears more prominently lower in the canopy where water status is the wettest. Smaller Monterey trees can easily be overirrigated if the grower is managing applications for larger Nonpareil trees.
Flower buds begin forming about nine months prior to bloom, so any type of stress, including overwatering, may have a negative effect on the following year’s bloom.
Although he doesn’t have research data, Milliron said he suspects wide swings in soil moisture during the season also may put undue stress on Monterey, possibly contributing to the maladies.
“These trees go from too wet in the spring from weather or overwatering and simply too much water, and you get root damage,” he said. “Then the trees are experiencing water stress late in the season.”
Spring is typically when trees push roots critical for water uptake. But too much soil moisture can kill fine roots, which reduces the trees’ ability to take up water. This may become apparent in the fall during harvest when trees appear stressed but there’s adequate soil moisture.
Milliron said these situations are when a pressure bomb is invaluable to determine plant water status.
Newest Issue
This spring, Milliron said he answered several calls about sudden branch dieback of Monterey due to Botryosphaeria and Phomopsis cankers. This is the least understood of the maladies.
But he pointed to an observation from a Glenn County producer who said his Montereys on heavier ground with nuts stayed green through hull split. These trees, presumably the wetter ones, now have severe dieback compared to trees that experienced typical hull split and harvest water stress.
To help Milliron and his colleagues learn more about these maladies, he asks growers and PCAs who may have related valuable information to text him at 530-828-9666.