Ongoing research by Dr. Mark Mazzola, plant pathologist at the U.S. Department of Agriculture, suggests incorporating oil-seed meals into the soil before replanting an orchard might help combat replant disease.
Mazzola, based in Wenatchee, Washington, has been exploring biological alternatives to the standard treatment of fumigation with Vapam (metam sodium) or Telone C-17 (dichloropropene and chloropicrin). Methyl bromide is rarely used to fumigate Northwest orchard soils because it is being phased out of use and is comparatively expensive.
Replant disease in Washington appears to be caused by a complex of organisms in the soil, including the fungi Cylindrocarpon, Phytophthora, Pythium, and Rhizoctonia as well as Pratylenchus, the lesion nematode in some sites.
Mazzola has tested various seed meals as soil amendments, including canola and mustards, which have helped suppress some of the replant disease organisms. His recent research has focused on understanding the mode of action of the seed meals and how they affect the soil biology.
He believes that resistant rootstocks will be part of the nonchemical solution. Some rootstocks developed at Cornell University, Geneva, New York, are tolerant of replant disease. These include Geneva 11 and G.41, which are commercially available.
David Granatstein, sustainable agriculture specialist at Washington State University, said Mazzola’s research looks promising, but the economic repercussions of poor tree growth in a new orchard are so serious that most growers—including organic growers—daren’t risk foregoing fumigation until the results are proven. It’s been estimated that growers can lose 10 to 20 percent of their potential yields for the life of the orchard because of replant disease.
In some other parts of the United States, growers leave the ground fallow for a couple of years and grow cover crops before planting fruit trees, but Granatstein said it’s not clear that it controls replant disease as well as fumigation does.
“Nothing is at the level of consistency at this point that would make people comfortable to not fumigate when they’re replanting,” he said.
One of the problems with finding alternatives to fumigation is that replant disease is difficult to diagnose, and the organisms involved vary from site to site. To be viable, an alternative treatment would need to control all the organisms causing the problem, not just some of them. Also, there is no good way for growers to assess the risk of replant disease developing in their orchards.
Organic growers replanting orchards tend to fumigate the land and not pick a commercial crop until the third year, as it takes three years from the application to recertify the orchard as organic. However, most of the orchards being converted to organic are already in existence, so growers have sidestepped the issue of how to treat for replant disease, Granatstein said.
To avoid needing to fumigate, organic growers who are upgrading orchards can graft over the trees, rather than replace them—as long as the rootstock and spacing are still appropriate, he said.