Peach Replant Disorder

Peach replant disorder is a nebulous problem that occurs when young trees are planted into old orchard soil. Nematodes, Phytophthora spp., and other soil pests may be abundant in the old orchard soil and attack the young root system of newly planted trees. Resulting symptoms, such as poor growth and nutritional deficiencies, are unevenly distributed across the field. Because methyl bromide is such an effective biocide, it easily controlled replant disorder even though the specific causes in a given orchard were not identified.

Replant disorder plagues other crops as well, including grapevines and other fruit trees. Nationwide there are 4.4 million acres of orchards and vineyards, with California having a little less than half of this. In the past, the soil would be fumigated with 300–400 lb/acre of methyl bromide before the young trees were planted. That is no longer feasible due to the rising cost of methyl bromide. Import and manufacture of methyl bromide in 2001 is limited to 50 percent of the amount used in 1991. The result has been an increase in the cost from $1.00/lb a couple of years ago to approximately $3.50/lb now. "Methyl bromide is basically no longer an option for perennial growers," said Thomas Trout, an ARS agricultural engineer in Fresno, California. "Most growers can't afford the $1,800 per acre it costs to apply methyl bromide."

Due to the phaseout of the fumigant, researchers are now focusing on what causes replant disorder and what the best alternatives for controlling the disorder might be.

Just What Is Peach Replant Disorder?

Peach replant disorder is an ill-defined disease complex that causes stunting of newly planted peach trees in nonfumigated orchard soil previously planted to peaches. It also affects other crops that use peach rootstock, including almond. "Mature trees seem to develop resistance, but when a young tree is planted in the same soil, it doesn't perform well," explains Trout. According to Greg Browne, ARS research plant pathologist at Davis, California, there is much to learn about the problem. The soil harbors a complex mix of organisms that may contribute to replant disorder. He says, "some replant disorder culprits are well documented, including several parasitic nematode species, oak root fungus, and Phytophthora, while much less is known about possible roles of other suspects. Even in the absence of the well-documented causes, replant disorder can occur." Browne's lab is investigating possible roles of many microbes collected from peach and almond replant disorder sites.

"In the past," relates ARS research plant pathologist Sally Schneider, "because methyl bromide has such a wide spectrum of activity, scientists didn't need to know the specific cause of replant disorder in a given orchard." That is no longer the case. "Potential alternatives sometimes are effective against only one pathogen group, such as fungi or nematodes, rather than the broad spectrum of pests and pathogens controlled by methyl bromide." Schneider also says, "better diagnoses of what is causing replant disorder in a particular orchard are needed in order to select the management alternatives that are best able to control the pests and pathogens actually present in a given orchard."

Methyl bromide is capable of killing most life stages of soil-dwelling organisms as well as the roots of old trees and vines. Without fumigation, some old roots are likely to remain alive for at least 3 years after tree removal. These woody roots continue to provide a food source for a variety of soil-borne nematodes, insects, and plant pathogens. These pests and pathogens can then attack and damage the root systems of the newly planted trees.

What To Do?

Trout and Husein Ajwa, an ARS soil scientist, investigated fumigation and fallowing effects on replant disorder on peaches at the USDA-ARS research facility in Parlier, California. The researchers tracked tree growth and the level of nematode populations in the replanted orchard following treatment with various fumigants and fallow periods of up to 3 years. Pin nematodes predominated in the plots, with very few other plant parasitic nematode species found. Although pin nematodes are not commonly considered important plant parasites, their population levels are used as indicators of treatment effectiveness.

Fallowing resulted in reduced pin nematode populations and improved tree growth, dramatically reducing the populations of pin nematodes, especially after 2 and 3 years. All nematodes found after 3 years of fallow were more than 3 feet deep in the soil. Each additional fallow year in the study resulted in improved tree growth, but even a 3-year fallow was not as effective as methyl bromide.

Methyl bromide and 1,3-D (Telone) were used as soil fumigants to control nematodes, and no nematodes were detected in plots treated with either chemical. Plant growth in plots treated with drip-applied Telone C-35 (InLine) was better than growth in the no-fallow check plots and as good as plant growth in plots treated with methyl bromide. An interesting result in one of the studies was that the best first-year growth was in plots drip-fumigated with chloropicrin. Chloropicrin is not considered a good stand-alone nematicide, but is a good fungicide. This result suggests that there are important aspects of the peach replant problem that are fungal in nature. Trout's study will continue through at least 4 years to quantify the effects on tree productivity by measuring fruit yield and quality.

For Future Consideration

Methyl iodide, or iodomethane, is a promising chemical fumigant that is currently in the registration process. In field trials conducted at the Parlier facility by Cynthia Eayre, an ARS research plant pathologist, iodomethane was as effective as methyl bromide in its ability to control peach replant disorder. "Iodomethane is applied using the same equipment as methyl bromide, provides the same control, but is more flexible because it is a liquid at ambient temperature instead of a gas, like methyl bromide. This means we can apply it through drip lines in addition to shank-injected applications," said Schneider. The registrant is planning to have iodo-methane approved for use by 2004.

For now, Telone will probably be the most widely used option. Though fallow periods provided some relief from replant disorder, it is not a realistic option for most growers. The cost of leaving an orchard fallow for 2 to 3 years is too high to be economically acceptable. "A switch to Telone by perennial growers will begin this fall due to the high cost of methyl bromide," asserts Trout.

Last Updated: September 5, 2001