The so-called “unpruned” method for almond trees continues to show commercial potential for some varieties in multi-year trials at the Nickels Soil Laboratory in Arbuckle.

John Edstrom, a Colusa County farm advisor, explained his progress with the method during the recent 33rd Almond Industry Conference held by the Almond Board of California in Modesto.

Edstrom’s trials, planted in 1997 at the 200-acre University of California research center, are on plots of Nonpareil alternating with pollinators of Monterey, Carmel, and Aldrich. Four training systems, standard, unpruned, mechanically topped, and temporary scaffolds, have been made.

The standard method consists of three primary limbs selected in the first dormant season and secondaries selected in the second season. Centers were kept open, with limb tying and staking as necessary, along with yearly, traditional light pruning.

Unpruned trees had three primaries selected, tipped, and left long at the first dormant pruning. No additional pruning was done, other than to facilitate orchard operations or to remove broken limbs.

Term confusion

Edstrom pointed out that the term “unpruned” causes some confusion in the industry. The trees were indeed pruned to establish the three primaries, which he noted are critical, but they have not pruned been during the past nine years.

Mechanically topped trees were established the same as the unpruned, but machine flat-topped to remove the prior season’s top shoot growth during the second dormant season and again in the spring of the fourth leaf. No additional pruning was done.

Temporary scaffold trees had limbs trained at the first dormant season to favor three permanent, upright primary scaffolds. Lower, less-dominant branches were temporarily retained, with removal of only those that compete strongly with the permanent scaffolds. The temporary limbs were removed gradually during years five to eight, or sooner if they interfered with primaries.

Yields from all four pruning treatments have been about the same, and “now on average we’ve not lost a lick of production from not pruning,” Edstrom said.

Significant savings

“The trees appear well-balanced with acceptable tree architecture, which should be capable of long-term high production. The accumulated reduction in pruning costs ($700 per acre) represents a significant savings to the grower.”

He said some adjustments may be needed according to variety, but in general the unpruned method “continues to perform remarkably well, both in terms of production and tree framework.”

The unpruned trees are somewhat shorter, which helps promote light penetration and facilitates most orchard operations. Nevertheless, he said, in the future the denser fruitwood might cut light penetration and production.

Thus far, the temporary system, he said, “looks questionable. Yields don’t appear to justify the extra pruning efforts.”

On the other hand, he added, “Long-term yields could reveal mature tree yield gains to this idea when compared to the possible declining yields of the unpruned mature trees.”

Mechanically topped trees, particularly Monterey and Nonpareil, appeared to be most negatively affected, and he said “Apparently, heavy topping should be avoided or done very carefully during canopy development.”

Another presenter, Roger Duncan, Stanislaus County farm advisor, has been studying the interaction between rootstocks, tree spacing, and pruning strategies.

Spacing trials

In a trial in the eastern part of his county, he has collected five years of data on combinations of Nonpareil and Carmel with various pruning regimes and spacings ranging from 10 by 22 feet to 22 by 22 feet.

“The early conclusions are that we probably increase early yield with high-density plantings, unless the trees are extremely vigorous. Then we don’t see any advantage to high-density plantings,” he said.

He also found that closer planted trees may require less pruning and are, thus far, more easily managed. “We will have to see what happens 10 years down the road. We have not seen any yield advantage in pruning at this point, although when we prune, it’s for five or 10 years in the future. This trial will have to go on for many years.”

Tom Gradziel, a pomologist at the University of California, Davis, has been working with the selection UCD 36-52 in his projects to improve almond varieties. It shows a high kernel-to-shell crack-out and high quality kernel, plus resistance to navel orangeworm (NOW).

He said unsprayed UCD 36-53 plots had about 2 percent infested nuts, while adjacent varietal standards showed NOW infestations of up to 20 percent. “Most UCD 36-52 damaged nuts appear to have resulted from infestations during the early kernel maturation, with nuts at later stages of maturation appearing more resistant.”

The resistance, he said, is determined by multiple structural and biochemical components.

‘Web-trait’

A second promising source of resistance is the “Web-trait,” shown as a very thin but tightly sealed shell. Selections having the trait withstood heavy NOW pressure during the 2005 season.

Gradziel said the trait also gives resistance to peach twigborer and ants, and it resisted development of aflatoxin following application of Aspergillus flavus. It did not, however, show resistance to Salmonella bacteria when they were applied during controlled conditions.

In a project investigating the spur dynamics and almond productivity, Bruce Lampinen, another pomologist at UC, Davis, has been tracing the links between spur longevity and reproduction and management of nitrogen fertility and irrigation.

The object of the study, being done in 146 acres in Kern County, is to learn how excessive vegetative growth influences spurs. “If you can slow down that process, you may get more productive trees,” he said.

“We hypothesize that management variables such as nitrogen fertilization and irrigation rates, which are know to impact yield, influence the dynamics of spur renewal and fruitfulness.”

Noting that the variables may cause both direct and indirect effects, Lampinen said, “Growth may be a direct effect of adequate nitrogen availability and tree water status. Shading of lower or interior branches, which reduces spur survival, may be an indirect result of excessive vegetative growth.”

Variety trials

Lampinen is also conducting long-term regional field evaluations of almond varieties in Butte, San Joaquin, and Kern counties. In each, he is recording data on growth, pest and disease susceptibility, and noninfectious bud failure symptoms.

UC, Davis graduate researcher Kristi Barckley reported on a project led by Abhaya M. Dandekar of the Department of Pomology to identify the genes that prevent self-fertilization in almonds.

An understanding of the genetics, she said, could lead to technologies to either reduce or eliminate self-incompatibility. Self-compatible varieties would not need pollinator varieties or honeybees and create a more economical and efficient process.