(May 8, 2020) – Prunus Replant Disease (PRD) can occur after a mature almond orchard is pulled and new trees are planted in that same soil. Though relatively common in the Central Valley, the causes of PRD are less understood than growers may assume.
Greg Browne, a research plant pathologist for the USDA Agricultural Research Service and a member of the University of California (UC), Davis Department of Plant Pathology, describes PRD as “growth suppression, mediated in part by soil microbes, in successive plantings of almonds or other stone fruits.”
Research collaborators Brent Holtz and Mohammad Yaghmour are shown calibrating a spreader ahead of a substrate application to ensure the substrate is evenly distributed across the orchard.
“It varies among sites in the Central Valley,” said Browne, who has been studying PRD since 2004, thanks to funding provided by the Almond Board of California (ABC). “PRD’s impact is most pronounced in the first year or two of an orchard’s growth. It can cause severe stunting and reduces early yield – and you never get that back.”
Pre-plant soil fumigation has for many years been a preferred prevention method for PRD. While in 2005 methyl bromide was phased out of most uses in the United States, alternatives such as chloropicrin (highly effective for PRD) and Telone (highly effective for nematodes, beneficial for PRD) have filled most of the void. However, those compounds – like methyl bromide – are coming under increased regulatory scrutiny. Ultimately, soil fumigation alternatives are needed, not just for almonds but for many other tree nuts, fruits and vegetables.
Enter anaerobic soil disinfestation (ASD), also called biosolarization. This practice originated in Japan and the Netherlands, and in the United States one of the first assessments of its utility took place in the strawberry industry.
“I was at a conference nine years ago when I first heard about ASD. It was the coolest thing I learned at that event,” said Gabriele Ludwig, director of Sustainability and Environmental Affairs at ABC.
Substrates, saturation and a tarp
Here’s how ASD works: After an old orchard and its roots are removed, and the ground is ripped, tilled, leveled, etc., a source of readily available carbon (via a substrate) is tilled into the soil. Immediately afterwards, a clear tarp cover is applied over the orchard and the entire soil profile is saturated with water. Typically, drip irrigation is used, and Browne recommends up to six irrigation lines per row, with emitters spaced about one to two feet apart. High soil moisture is then maintained by delivering additional water to the soil several times per week for several weeks. The carbon and associated nutrients in the soil provide a food base for microorganisms, while the wet soil and the heat-and-vapor-retaining tarp accelerate microbial activity, consuming oxygen and creating anaerobic conditions.
Under these conditions, microbes generate organic acids, volatiles and additional conditions toxic to plant pathogens.
“You’re essentially removing the oxygen and changing the soil ecosystem, temporarily,” Ludwig explained.
In addition to testing ASD with rice bran substrate, researchers incorporated ground up almond hull and shell substrate into the soil at their trial sites.
Browne’s lab has tested several ASD carbon substrate sources, with particular focus on rice bran and ground almond hulls and shells, using a typical substrate rate of nine tons per acre. Among Browne’s ASD trials at multiple sites in the San Joaquin Valley, the best and most consistent results were obtained by tilling rice bran substrate into the soil and applying water under a tarp for four to six weeks. That said, in some trials the ground almond hull and shell mixture worked just as well as rice bran. In fact, research led by Dr. Chris Simmons’ lab at UC Davis demonstrated, both in the lab and in the orchard, that ASD with almond hull and shell substrate can generate organic acids that contribute to control of replant pests. Also, in certain trials the rice bran substrate incorporated in the soil stimulated orchard growth equally both with and without tarp and water applied.
In addition, each ASD substrate varies in nitrogen, potassium, phosphorus and other nutrient contents. This reality, along with other factors such as treatment timing, native soil nutrient levels and the presence of Whole Orchard Recycling residues, likely impact ASD results at any given site.
As a next step to this research, Browne is working with colleagues Brent Holtz, Mohammad Yaghmour, Phoebe Gordon, Amisha Poret-Peterson and others to learn more about how soil fertility impacts PRD and its remediation with ASD. Team members hope to improve the reliability of ground almond hull and shell coproducts as ASD substrates due to their wide availability and relatively low cost.
Summer is prime time for ASD
Timing is another key consideration with ASD. Most old orchards are pulled out immediately following the latest harvest. By the time growers implement Whole Orchard Recycling – or at least have their woody material trucked away to a co-generation plant – it’s often late September or early October. For growers who want to get new trees in the ground before the next spring that doesn’t typically leave enough time for ASD to be completed while temperatures still are warm.
“I would caution against a grower trying to perform ASD within such a tight time frame,” Browne said.
Under an ideal scenario, ASD would take place during the summer, after the soil has been well prepared. This would also give growers time to finalize logistics, such as irrigation and water sources and tarping.
“Our general impression based on experiments over the years is that we’ve achieved the best results when we’ve treated in July, August or early September,” he said. “The main reason for that is soil temperature, which helps the microbial community in the soil rapidly use the substrate and generate anaerobic conditions and molecules.
“That means, under most circumstances, taking a year off and not immediately replanting will provide the best results. This can also reduce replant disease, help with land preparation and irrigation installation and, on some larger operations, could provide significant water savings.”
The orchard floor will look similar to this when ASD is complete, at which point it’s time to remove the tarp.
Next steps in research
So far, ASD in almonds has occurred only under trials in grower orchards in part because Browne and his colleagues are still trying to answer all their questions and, in part, because of cost. ASD can run as high as $2,000 per treated acre – about twice as much as fumigation. Thus, the team is continuing to see how to refine or minimize inputs to help lower the cost, such as using almond hulls & shells as the substrate. Another focus of Browne’s research is to better understand what groups of microorganisms – fungi, bacteria – contribute to PRD, which is essential for his team to understand when testing soils to determine if they have PRD.
In addition to ASD's potential to minimize replant disease, current research is also looking at its potential to be effective in reducing plant parasitic nematodes such as ring, lesion, and rootknot nematodes. Andreas Westphal, a specialist in nematology at the UC Kearney Agricultural Research & Extension Center in Parlier, is heading up this area of research in conjunction with Browne’s team.
Who may benefit now?
In the short term, Browne and Ludwig both envision a scenario where ASD could work for growers who need to treat their soil near homes, schools or other spaces where fumigation is not an option.
“I think a grower should start small, in an orchard where it might be more complicated to do fumigation,” Browne said. “They may be leaving that ground untreated, but if so, that’s an area where they may want to consider ASD.”
Early yields in effectively ASD-treated soil are on par with those seen after fumigation and, in Browne’s research, about twice has high as the untreated soil in control groups. That leads him to believe there may be a future for ASD. He recommends interested growers check with their local UC Cooperative Extension farm advisor, who can work with Browne and colleagues to design a system that works for them.
“We’re trying to figure out the ASD process well enough to be in a reasonable position to work with growers,” Browne said. “We’re just now getting there with enough data. We’re on the leading edge of this, but it’s not optimized yet - so stay tuned.”
