Prune orchards of the future may vary

Soil type, tree spacing, rootstock selection and cropload management are among the factors that could determine how the prune orchard of the future performs, researchers say.
Tim Hearden

Capital Press

Published on April 12, 2018 9:56AM

University of California Cooperative Extension pomology farm adviser Franz Niederholzer says such factors as rootstock choice and cropload management will determine how prune orchards perform in the future.

Tim Hearden/For the Capital Press

University of California Cooperative Extension pomology farm adviser Franz Niederholzer says such factors as rootstock choice and cropload management will determine how prune orchards perform in the future.

RED BLUFF, Calif. — What will the prune orchard of the future be like? It depends on a variety of factors, researchers say.

New orchard layouts could depend on soil types, while new crop management techniques and new varieties other than the now-dominant French prune could resist diseases and reduce the need for thinning, said Franz Niederholzer, a University of California Cooperative Extension pomology farm adviser.

“The way it looks now may not be the way it turns out,” Niederholzer told about 60 growers during a recent workshop in Red Bluff.

Niederholzer gave attendees electronic devices to vote in instant surveys and asked them to envision the “perfect orchard.” Majorities indicated they would want it to produce 6 dry tons per acre with spacing of 18 feet wide by 14 feet between trees.

Asked what they think are the biggest obstacles to planting prunes in 2018, most growers cited prune prices and cost of production, including pruning costs. Most growers said their next plantings would be entirely new orchards rather than interplanting.

In actuality, optimal spacing can depend on soil types, which can influence a tree’s production. Soil can vary even in small blocks, and Niederholzer suggested that growers consider soil mapping and matching spacing and irrigation to the map.

“Consistent production is really important,” he said.

Also, planting and managing trees in such a way that it maximizes sunlight on the trees will increase yields, he said. In a recent newsletter, he explained that replacing an orchard spaced 18 feet-by-16 feet with one with rows 17 feet or 16 feet apart could increase per-acre returns by as much as $760, assuming a $2,000-per-ton average dry fruit price to the grower.

While new planting schemes can boost production, researchers are looking into ways to improve orchard health by developing new rootstocks.

UCCE advisers have been evaluating alternative rootstocks in California Dried Plum Board-supported field trials throughout the Sacramento Valley.

Among the research:

• Scientists in Butte County are testing rootstocks that reach full bloom at different times, with the intent of spreading out bloom timing and reducing the risk of a crop failure because of bad weather, according to a UCCE prune newsletter.

Prune production took a nosedive in 2016 after cold, wet and windy weather created adverse conditions for bees during the height of pollination.

• Farm advisers are watching disparities in tree trunk sizes on test plots in Butte and Yuba counties, according to the newsletter.

The trees with larger trunks in Butte County had a higher dry yield per tree, a higher “dry away” ratio (the ratio of green fruit weight to dry fruit weight) and smaller fruit, while the smaller-trunked trees had lower dry away ratios and larger fruit.

This tree size and yield disparity may be partly caused by soil and water differences between the sites, researchers said.

In the prune orchard of the future, a grower choosing a rootstock will need to consider whether the objective is to plant larger, more vigorous trees or to plant smaller trees at a higher density, the newsletter explained.

Several rootstocks in the Yuba County plot were particularly prone to gumming and tree loss from bacterial and Cytospora canker in 2013 and again in 2017, according to the researchers. The incidence of canker in prunes has added a sense of urgency to the need to develop new varieties.

Commercial production of French prunes in California began in 1859, and an improved variety is used by 95 percent of the industry, said Bob Johnson, a doctoral candidate doing research at UC-Davis. Phellinus, a fungus that causes wood decay, has had more than 100 years to adapt to the variety, he said.

“We should really be thinking about diversifying the prunes that we grow,” he said.

UC-Davis researchers Ted DeJong and Sara Castro have received $244,598 in grants in the past two years to develop new prune varieties that would have traits desirable to the industry.

New varieties under development have a lower dry away ratio, meaning they’d reduce dehydration costs. One variety being evaluated by growers starting this year, called “H135-58,” offers a wide harvest window and partially dries on the tree.


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