Excessive boron in the soil and irrigation water is problematic for almond trees. The problem with boron in almonds is a little different as opposed to other crops.

“In stone fruit, including almonds or anything of the Prunus background, they use a certain kind sugar to transport energy around the tree that other plants don’t use as much of,” according to Katherine Jarvis-Shean, UCCE orchard systems advisor for Sacramento, Solano and Yolo counties.

Boron essentially piggybacks on top of that sugar and keeps recirculating it in the plants. In most plants, boron comes up from the soil with the water, then goes to the leaves and for the most part stays in the leaves, Jarvis-Shean said.

With walnuts for example, the edge of the leaves will die and look as if they burned—more of a classic salt damage appearance.

“That’s what it would look like if almond trees got chloride damage,” Jarvis-Shean explained. But with almonds the boron keeps recirculating and causes a different kind of damage than it does in other trees.

Issues in Almonds

In almonds, boron goes to where the energy is being used: the growing tips and the fruit.

“It [boron] can kill back growing tips like young shoots, and it often causes the fruit to stick onto the trees,” Jarvis-Shean said, adding there are more sticktights when there is high boron, and the sticktights are harder to shake off.

“The biggest problem from a production standpoint is that this sugar/boron brew will often congeal or thicken so that the sugar pipes get sort of clogged and will burst out at the weakest point, which is their branching point,” Jarvis-Shean said.

When the sugar and water are together it creates big, gumball sap wounds where the scaffold branch meets the trunk. These wounds are vulnerable to infections, and if there is a fungal infection, it’s right at the heart of the tree.

“That is just like a fungus’ dream,” she said. “That can lead to pretty quick tree decline, or certainly trees that never really take off.”

The infection that follows those gumballs will weaken the tree to the point it should be removed. If the tree isn’t removed, those gumballs can impact the trees’ productivity from the dead shoots, and it can increase management costs with more sticktights.

Resistant Rootstock Research

Rootstock research for boron-resistant rootstocks was started in 2011 by Carolyn Debuse in Yolo County, and it has since been funded by the Almond Board of California. The research site is in a grower orchard and being maintained by the grower. Only commercially available rootstocks were used in the trial, Jarvis-Shean said.

The research site in Yolo County is in a high boron area. There are also a couple of other hotspots down the westside of the Central Valley that have historically had water coming out of the Coastal Range that is high in boron.

“It’s gotten into the soil, and it’s gotten into the groundwater,” Jarvis-Shean said.

The research site has high boron in the soil and the water, whereas in the southern San Joaquin area growers who deal with boron usually only have it in the soil because their water comes from state or federal water projects—clean water from somewhere else—not well water or local surface water, Jarvis-Shean said.

At the research site, there are nine different rootstocks from different genetic backgrounds that are being screened for how they yield in high boron conditions, Jarvis-Shean said.

The almond boron rootstock tree genetic backgrounds used in the research project are:

• Titan SG1/peach-almond
• Nickels/peach-almond
• Brights 5/peach-almond
• FxA/peach-bitter almond
• Hansen 536/peach-almond
• Viking/peach-almond, myro plum-apricot
• Krymsk 86/myro plum-peach
• Rootpac-R/myro plum-almond
• Lovell/peach

Current Results

Almonds and other stone fruit accumulate boron in the fruit, so leaf boron levels are not a good indicator of toxicity in almonds. To measure boron levels, the hulls were tested at harvest to determine boron levels, Jarvis-Shean said.

Three rootstocks have been found to perform well with high boron. “There’s been a pretty solid trend for the last I’d say, easily four years, that the peach/almond hybrids with the exception of Hansen have done really well,” Jarvis-Shean said, adding Lovell and Krymsk-86 have done poorly in comparison.

Krymsk-86 is very popular in the northern Sacramento Valley because it’s less likely to blow over, and it’s also used in Sacramento/Yolo counties because it does well in heavier soils.

“This trial has actually been really important because a lot of people were starting to grow on Krymsk-86 in this area, even in the high boron area,” Jarvis-Shean said.

Before results from the trial were available, some orchards were planted using Krymsk-86. “They’re just stuck with that for now, and they’re just seeing a lot of these symptoms of high boron. Krymsk-86 would otherwise be a great rootstock to plant on in this area,” Jarvis-Shean said, adding that with the high levels of boron in the area, it’s not recommended.

The peach/almond hybrid rootstocks are pretty vigorous, so it’s important that growers understand the implications of that when determining their spacing and nitrogen management, Jarvis-Shean said.

Titan SG1, Nickels and FxA have consistently performed better year in and year out, Jarvis-Shean said.

“I’d say the only small hurdle is Nickels in particular,” Jarvis-Shean said.

When Jarvis-Shean first started releasing the results of this trial, Nickels was not widely propagated. “It’s kind of tough for nurseries to produce, so availability might be tough. But in terms of once you get the plant put in the ground, it makes for a fine tree.”

The research will continue a few more years monitoring yields to ensure they are sustained in trees on high-performance rootstocks and to gather more information on “middle-of-the-pack” rootstocks like Viking and Rootpac-R, Jarvis-Shean said.

Growers who need to make decisions now, based on current results, should choose Titan, Nickels and FxA the best boron-resistant rootstocks.