Replanting almonds or stone fruit in ground where they once stood can be fraught with problems, some of them coming from unseen peril below – nematodes, most notably.
Greg Browne, a research plant pathologist for the U.S. Department of Agriculture Agricultural Research Service at the University of California at Davis, talked about those challenges at a recent field day at California State University-Fresno.
Residual organisms and residues from previous plantings are the culprits, Browne said. He said also at play are soil physical and chemical properties, rootstock choice, preplant soil treatments and soil water management.
Browne has been researching replant disease for some time and said much is still to be learned. After speaking in a conference room, he would lead participants into an orchard on campus, one of multiple sites where he is studying the effects of anerobic soil disinfestation (ASD) and whole orchard recycling (WOR) as preplant soil management options.
One of the keys to replanting is effective preplant fumigation, but that has become more of a challenge as regulation of fumigation has been stepped up. He said the best approach is to combine chloropicrin with 1,3-dicochloropropene (1,3-D). That’s because the first is good for management of nematodes, though week on prunus replant disease; the second is good for management of prunus replant disease, though week on nematodes.
Browne said new GPS-controlled technology allows for precise fumigant placement. Instead of 100 percent coverage, placement can be in strips, with 50 percent coverage, or in grids, with 20 percent coverage pegged at tree plantings.
He said information on fumigation studies outlining methyl bromide alternatives are available online from California Agriculture publications at http:Californiaagriculture.ucanr.edu.
Browne said replant disease is widespread but not a problem in all soils. He said it is important to take into account whether a particular type of soil is prone to PRD and check on previous experience replanting in similar soils.
Replant disease can be evidenced by sparse feeder roots and less fibrous roots.
“We need to better understand what sets the disease complex up,” Browne said. “Our goal is to identify soil properties and organisms that are well linked to orchard replant performance.”
He said some fungi in the Cylindrocarpon and Pythium species are also linked to replant disease.
Browne said growers and pest control advisers may be able to help in development of predictive diagnostics, soil bioassays and orchard trials “within limits.” He asked that participants let him know of replant sites that could be studied.
Browne continues to research anerobic soil disinfestation (ASD) as an alternative to fumigation “to reset the soil community.”
ASD approaches involved adding rice bran incorporated by a rototiller, adding an auxiliary irrigation system and tarping the soil. Doing that raised the soil temperature and made it anerobic under the tarp.
Browne said fumigation or ASD nearly doubled crop yields compared to controls in which neither was used for the first two years of yield.
Nine alternative carbon substrates were used in ASD trials, including mustard seed meal, rice bran, almond hulls, tomato pomace, grape pomace, pistachio hulls, olive pomace and almond shells, Costs of the material varied widely, with almond shells the lowest at $360 per acre, and mustard seed meal the highest at $2,550.
“We’re trying to get at cost savings,” Browne said. That has meant mechanization of steps that include applying the tarp and irrigation system, as well as looking at variable costs for materials.
His research has raised the question “do we really need that tarp?” considering results that call that into question.
Whole orchard recycling
Researchers worked with Wonderful Orchards to look at the effects of whole orchard recycling. Browne said chips left in that process had an “inhibitory effect” on the growth of potted trees.
“We think that’s related to nitrogen, but it’s still being sorted out,” he said.
At Fresno State, soil treatment with ASD was done in 2017. Trees were planted in 2018. Various treatments included use of almond hulls and shells, rice bran, WOR chips, tomato pomace, irrigation and tarping. All trees are on Nemaguard root stock.
Almond orchard chips were brought in from Kern County and applied at 60 tons per treated acre.
Among research findings is that WOR chips can induce tree stunting at high rates, but that seems to manageable with nitrogen applications at higher levels.
Also, the strongest expression of replant disease occurs in sandy loam and sandy soils compared to clay loam.
In the field, nematologist Andreas Westphal, with the UC's Kearney Agricultural Research and Extension Center, talked of soil organic amendments for mitigation of nematode infestations.
He explained that nematodes can have beneficial impacts, including nutrient recycling, Plant parasitic nematodes include ring, lesion and root knot nematodes found on over a third of almond fresh stone fruit acreage.
Westphal said it is important to sample soil for nematodes.
In Fresno State’s experiment plots he found low populations of ring and lesion nematodes.