Mar 15, 2018
Brassica seed meal shows promise in curbing apple replant disease

Early experiments show brassica seed meal and anaerobic soil disinfestation (ASD) may greatly help in preventing apple replant disease.

USDA Agricultural Research Service Plant Pathologist Mark Mazzola spoke at the recent Great Lakes Fruit, Vegetable & Farm Market EXPO in Grand Rapids. Soil amendments of brassica seed meal seem to be making the soil of old orchards hospitable to new trees.

Apple replant disease has been observed in orchards around the world: New plantings will grow poorly and/or inconsistently where the previous orchard flourished.

Scientists don’t all agree which pests cause the phenomenon. When starting the study years ago, “there was a lot of disagreement about what the causal mechanisms were,” Mazzola said. His group eventually concluded that although the disease-causing pests may vary depending on the site and region, that at a certain level the “same players” are active: Fungi, plant-parasitic nematodes, and oomycetes – microorganisms that are also known as “water molds” and include phytophthora.

Soil fumigation is a common pre-planting technique for wiping out pests in the soil before planting, while ASD and brassica are alternatives. ASD refers to the use of soil microorganisms to control soil-borne pests. Brassica seed meal treats or amends the soil in several different ways, Mazzola said. Although the seed meal is thought to chemically react with the soil for a fumigation-like effect, “it’s far beyond simple chemistry,” he said – it also has anti-fungal and anti-nematode action.

Tests were conducted in 18 different orchards, and the seed meal performed well.

Mark Mazzola

“We obviously have developed a system that’s limiting re-invasion of the site by some of the pathogens of interest to us that are causes of agents of replant disease,” Mazzola said. “And we wanted to look at, really, what is soil microbial ecology of the system and how might that be contributing to this type of response. And so we’ve done some high-frequency sequencing of the rhizosphere microbial community at the end of the second growing season.”

Two years after the trial, fumigated soil was back to its original condition, while the soil treated with the brassica seed meal amendments was distinctively different from both the fumigated soil and a control group. In the soil treated with seed meal, researchers found favorable microbes that worked by trapping or parasitizing nematodes and oomycetes. It seems that the treated soil was weaponized against re-invasion by pests that cause apple replant disease.

“I think we’ve demonstrated that ASD and the use of brassica seed meal amendment can provide significant levels of anaerobic soil disease control and yields, at least on the sites we’ve studied so far,” Mazzola said. “Efficacy of either system relies in part on the soil microbiome and microbiome activity and I think we’ve demonstrated that this resource can yield a system that’s more resistant than one that is derived from soil fumigation.”

Additional trials of the study are planned for the future, Mazzola said – larger field trials that will involve the growers themselves.

“All of these treatments at this point in time are experimental,” Mazzola added in an interview after the conference. “We want to make certain they work.”

MSU eyes apple replant disease

While USDA researchers tackle the problem from a national level, Michigan State University (MSU) is looking at apple replant disease in its own backyard.
The project is analyzing Michigan soils and will experiment with cover crops that may help prevent apple replant disease.

Julianna Wilson, a tree fruit integrator in MSU’s Department of Entomology spoke at the Great Lakes EXPO about a newly-formed study that will examine apple replant disease in Michigan.

Wilson, who is heading up the MSU multidisciplinary team project, said that so far, a soil survey was conducted in May 2017 in orchards that will eventually be replanted.

Twelve growers who are collaborating with the project have allowed soil samples to be taken from 20 different blocks of 1990s-era apple orchards. The apple growers, she said, represent a variety of different locales in the state from the Ridge area to Belding and west-central Michigan. The soil samples also cover 45 different rootstock/scion combinations of apple trees.

Rootstock susceptibility to disease, and local soil traits, are thought to be among several factors affecting how disease takes hold in replanted apple orchards.”It’s a really tricky situation,” Wilson said.

Researchers will follow up with the orchards, which will be replanted in spring 2019.

The team will also replant an orchard at MSU’s Clarksville Research station. They will experiment with letting the ground lay fallow, growing a cash crop on the land or using cover crops, such as pearl millet or sun hemp.

“That was the suggestion of our nematologist (Marisol Quintanilla),” Wilson said. “She came from Hawaii most recently and was working in vegetable systems using some cover crops to suppress nematodes.”

Grasses such as pearl millet don’t play host to root lesion nematode, while sun hemp may act as a biofumigant when incorporated back into the soil, Wilson said. The list of cover crops used in the study could change.

– Stephen Kloosterman, Assistant Editor

Above (top photo): Apple replant disease treatments, from left, across rows of fumigation/seed meal/no treatment control. Photo: Mark Mazzola/USDA Agricultural Research Service




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