Progress Towards Alternatives to MeBr Fumigation
in Bareroot Forest Nurseries in the United States

Susan J. Frankel, USDA Forest Service, Pacific Southwest Region, Vallejo, CA; Jeffrey K. Stone, Oregon State University, Corvallis, OR; Michelle M. Cram, USDA Forest Service, Southern Region, Athens, Georgia; Stephen W. Fraedrich, USDA Forest Service, Southern Research Station, Athens, Georgia; Jennifer Juzwik, USDA Forest Service, North Central Forest Experiment Station, St. Paul, MN; Diane M. Hildebrand, USDA Forest Service, Pacific Northwest Region, Portland, OR; and Robert L. James, USDA Forest Service, Northern Region, Coeur d’Alene, ID

Forest nurseries produce 1.5 billion seedlings each year for planting on National Forests, state and private lands, for reforestation after fire, harvest, or tree mortality. Some seedlings are grown in containers but most are planted in slightly raised seedbeds in the ground, then harvested bareroot. The vast majority of bareroot nurseries fumigate soil with methyl bromide to control soilborne pathogens, weeds, nematodes and insects. Forest nurseries account for less that 3% of the total use of methyl bromide, not huge contributors when looked at from a worldwide agricultural perspective, but nonetheless relied upon by hundreds of nurserymen and foresters.

This article reviews the results of USDA–Forest Service research and field trials evaluating alternatives to methyl bromide in forest nurseries. Since 1993, the USDA–Forest Service, Forest Health Protection, Special Technology Development Program has contributed $1.2 million to the search for alternatives to methyl bromide in forest nurseries. The nurseries (both state and federal) have contributed over $800,000 for a total investment of $2 million over 6 years. In addition, the Forest Service Forest Experiment Stations (Southern and North Central ) appropriated over $2 million from 1995–99.

Eighteen nurseries throughout the United States have participated including: three in California, two in Oregon and Idaho, and one each in: Washington, Nebraska, Minnesota, Michigan, Wisconsin, Alabama, South Carolina, North Carolina, Georgia, Mississippi and Florida. This project represents the work of more than 42 cooperators. Progress from the South, North Central and Western States follows.

Alternatives to fumigation in the Southern United States

Nurseries in the South grow over 1.2 billion seedlings annually, 79% of the seedlings cultivated in the United States. Michelle Cram, USDA–Forest Service, Forest Health Protection, Stephen Fraedrich and Dave Dwinell, USDA Forest Service Research scientists stationed in Athens, Georgia have been working with universities, states, and industry in testing alternatives to methyl bromide for forest nurseries. They tested organic amendments, dazomet, metam sodium, no fumigation, chloropicrin, eptam herbicide and plant growth promoting rhizobacteria at various nurseries.

In the South, slash, loblolly, and longleaf pine are grown in a one year crop cycle. For methyl bromide treatments, methyl bromide (67%) + chloropicrin (33%) is applied at 350 lb/ac and tarped. This is the rate and formulation used by most forest nurseries throughout the United States.

There has been no significant disease outbreak in any of the studies carried out over the past six years but weeds (particularly nutsedge) have been a persistent problem. In all the trials, seedling quality and quantity in the control (non-fumigated) plots were acceptable.

The results of these trials indicate that fumigation is applied more frequently than necessary for some nurseries. However, information is lacking on the rate at which populations of disease-causing organisms and nematodes rebound in Southern forest nurseries following methyl bromide fumigation. There are few predictive indicators available to nursery managers to determine when fumigation is necessary to avoid seedling losses in any particular field. The long-range goal is to work on predictive indicators and pest models for better pest management at Southern forest nurseries.

The North Central States

In Northern forest nurseries, Jennifer Juzwik, USDA–FS Research, and Raymond Allmaras, USDA–ARS, both in St. Paul, MN, along with 10 cooperators are focusing on improving soil quality as an alternative to fumigation for black walnut and white pine seedlings. From 1994 to 1996, investigations were conducted in five Northern nurseries to examine soil and pathogen status in the nurseries and to identify operational practices that could be changed to prevent root disease and minimize fumigation. Alternative soil management plans were devised, focusing on reducing soil compaction and ensuring that plant residues remain near the soil surface. These plans contain guidelines such as limiting all nursery traffic to areas not planned to be in nursery beds. They also tested and modified equipment for cultural activities such as sub-soiling.

Reducing soil compaction improved soil-water relations, preventing environmental conditions conducive to root disease. Maintaining plant debris at the surface prevented deeply buried plant debris (more than 8 inches below the surface) from becoming a reservoir for pathogens. In the past, the upper 8 inches of soil was sterilized by fumigation or solarization, only to be recontaminated from fungal propagules residing 8 to 12 inches below the surface.

Results from their trials show that soil management procedures can be the basis for reduced dependence on fumigation. Nurseries can use tillage to control depth of placement of cover crop residues, and to prevent hardpan creation thereby managing root disease in white pine and black walnut crops.

In 1998, operational-level trials were initiated at nurseries in Minnesota and Wisconsin to test this customized soil management approach in black walnut and white pine without methyl bromide fumigation. Weed and insect management plans are also being tested.

The Western States

Diane Hildebrand, Robert James, Susan Frankel, and Jeri Lyn Harris, USDA Forest Service, Forest Health Protection, are working with Jeff Stone at the Dept. of Plant Pathology, Oregon State University to identify and implement feasible alternatives to methyl bromide in bareroot forest nurseries in the West. The strategy is to decrease pathogen populations by: (1) reducing the food source for pathogens by replacing cover cropping with bare fallowing; (2) promoting growth of competitive or antagonistic microbes resident in nursery soils through soil amendments with high carbon to nitrogen ratio or biological agents; and (3) reducing seedling exposure to infection through alternative sowing and mulching techniques.

Barefallow, with or without periodic tilling, and the elimination of cover crops was as effective as methyl bromide for controlling Fusarium and weeds at a nursery in central Oregon.

At nurseries in Oregon and Idaho growing Douglas-fir and ponderosa pine, barefallow with sawdust soil amendment has resulted in seedling quality and quantity comparable to fumigation. Results have been similar for repeated trials and are now being tested in a larger scale operational demonstration.

In Idaho nurseries, supplementing barefallowing with additives of potential biocontrol organisms is being evaluated.

At a California nursery, high quality seedlings were initially produced without chemical soil fumigants by forming seedbeds in the fall with a winter mulch covering, followed by early sowing in the spring. However, after a few years, losses became excessive on several conifer species due to infection with Macrophomina phaseolina, a fungus previously successfully controlled by methyl bromide. Current work involves evaluating an alternative chemical (dazomet), fallowing with periodic tilling, and water shade treatment to reduce heat stress. As yet, an acceptable alternative to methyl bromide fumigation is not available for this nursery.

Research priorities for bareroot forest nurseries

Despite progress towards developing alternatives to methyl bromide fumigation, much work remains. Research priorities in forest nurseries include:

1. Strategies for management of specific, difficult-to-control weeds such as nutsedge;
2. Long-term maintenance of adequate soil organic matter levels without cover crops;
3. Fertilization regimes without methyl bromide;
4. Development of pathogen-suppressive soils, for example, with organic amendments which have high carbon to nitrogen ratios and/or biological agents;
5. Improvement of soil pathogen assay techniques and disease prediction models; and
6. Evaluation of soil and environmental factors and how they interact with alternative growing regimes.

Summary

In summary, barefallow, improved soil management, and other cultural treatments can be effective alternatives to fumigation in some nurseries. Biocontrol methods such as rhizosphere bacteria or actinomycetes need further development before they are suitable.

Chemical alternatives to methyl bromide such as chloropicrin and dazomet have been effective at some nurseries. With the use of alternative fumigants, fertilization, irrigation and other nursery operations may need adjustment. Herbicides may also be needed for adequate weed control. However, chemical controls are less desirable because of environmental and human health concerns. Conversion to container production is another alternative for some areas.

Acknowledgment

Editor Retires

Doris Stanley Lowe, who served as editor of the USDA newsletter Methyl Bromide Alternatives since its inception, retired from the Agricultural Research Service on June 4, 1999. Her service to the newsletter was very much appreciated.

Ken Vick
ARS, National Program Leader

Last Updated: July 1, 1999