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Too Early to Tell. Excitement Over New Plum Rootstock Premature.

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Recent news media reports of the discovery of a plum rootstock for almonds that could be the "silver bullet" for both root lesion and root knot nematodes were too much too soon, say researchers on the project.

"The limited tests that have been conducted on Deep Purple plum rootstock with almonds are not enough to make a prediction of field performance," says Craig Ledbetter, plant breeder at USDA's Horticultural Crops Research Laboratory in Fresno. "We need at least another five years of tests to determine if it could be a viable candidate for commercial use."

Ledbetter found Deep Purple when screening "exotic" germ plasm for resistance to root lesion nematodes in a project funded by the California Almond Board, Prune Advisory Board and California Tree Fruit Agreement.

"In our screening of 205 materials we found 10 that are resistant," he explains. "Deep Purple was one of them."

First Tests on Fruit

To determine just how resistant Deep Purple and the other candidate rootstocks are, Ledbetter planted a block of trees at the Fresno facility in an area known to be a "nematode pit." The block included Improved French prune and apricot on Deep Purple, Marianna and several other rootstock candidates. (Because land was scarce, almonds were not in that first test.) As it turned out, the ground was also laced with bacterial canker.

"We soon noticed that we had a better than 50 percent kill of prunes and apricots on Marianna, but only eight percent on Deep Purple," Ledbetter recalls. "Now, after four years, the only big trees are the ones on Deep Purple."

Two years ago, two Nonpareil and two Padre almond trees were grafted to Deep Purple rootstock in the laboratory orchard. Ledbetter says that they are doing okay, but are showing signs of yellowing.

Many Questions Remain

What Ledbetter has shown so far is that Deep Purple, a fresh market plum variety developed by the University of Minnesota in 1965, is resistant to root lesion and root knot nematodes.

"But the question is, will it make an almond rootstock?" he asks.

That question involves determining whether or not Deep Purple is resistant to oak root fungus, if it is compatible with leading almond cultivars, and if it performs well in the nursery and in commercial orchards.

"We don't even know how easy it is to root this material," he says. "We had 60- to 80-percent success in the laboratory, but when we sent 300 cuttings to a commercial nursery for rooting, only nine percent survived, compared to 88 percent for Marianna and 38 percent for an apricot rootstock that we had been unable to root in the laboratory."

That rooting failure was a setback for Deep Purple and for Ledbetter, who still believes a higher success rate can be achieved. It is one more hurdle to clear before declaring that this rootstock has commercial potential.

John Edstrom, Colusa County farm adviser and manager of the Nickels Estate research facility near Arbuckle, agrees that the answers to all of these questions about Deep Purple are at least five years away.

Compatibility Issue

"It takes at least five to six years to decide the compatibility issue," he says, a lesson that he has learned in 20 years of testing almond cultivars on Marianna 2624 at the Nickels Estate. "For example, we have three-year-old Nonpareil trees on Marianna 2624 that are doing just fine, and everyone knows that this is an incompatible combination. We know that those trees will begin to fail in their fourth, fifth or sixth year."

Work on finding the answers to Deep Purple's viability begins next spring. This summer, Burchell Nursery grafted 32 Nonpareil scions to Deep Purple rootstock supplied by Ledbetter. Successfully grafted material will be shipped to Edstrom at the Nickels Estate for test plantings in commercial orchard conditions. "Five years from next spring we may have an idea of how compatible Nonpareil and Deep Purple are," Edstrom says.

Oak Root Fungus Concern

In addition to compatibility, Edstrom says tests are needed on the new material for oak root fungus, "which is one of two primary reasons that people choose plum root to begin with," he says. Heavy soil is the other. Several nurseries will also be testing the Deep Purple/Nonpareil combination for oak root fungus resistance as well as other considerations. Edstrom notes that the trees may not be virus free and may need to be "cleaned up." "There's a long process that you have to go through before you have a clean, reliable rootstock," he cautions, "and we are in the early stages of it."

And once the material is declared viable for commercial use, nurseries need a year or two to build up a supply to sell to growers. As for Deep Purple, if it is all that it is hoped it will be, it is still five or more years away from being available to growers.