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Raspberry fumigation research shows less might be more

Posted by | June 7, 2012

Less fumigant may actually improve raspberry crops, as an ongoing study by Washington State University small fruits horticulturalist Thomas Walters indicates.

Bed Fumigation Apparatus
This bed fumigation apparatus, used in WSU research trials, reduced fumigant use and buffer zones, and resulted in fewer nematodes and larger raspberry plants. Click on image to download high resolution version.

Raspberry plants are finicky about soil and climate and optimal land for commercial cultivation is in short supply. While many farmers rotate crops to stay ahead of pests, raspberry growers don’t have enough suitable land to move their crop. When pests and pathogens find a permanent home for their favorite snack, they can reproduce until they become unmanageable. A soil-borne pest, root lesion nematodes, and a fungal pathogen, raspberry root rot, are particularly troublesome to raspberry growers. Every five to eight years, the culprits get so troublesome that the plants are removed, the field is broadcast fumigated, and new raspberries are planted.

Increasing restrictions have growers seeking alternatives to broadcast fumigation. New Environmental Protection Agency regulations require a buffer zone for two days after fumigation. The size of the buffer zone depends on the field size, the fumigant and rate used and other factors, but it can easily be 600 feet or more. With many raspberry fields in close proximity to houses, this approach is no longer practical.

To more selectively apply the fumigant, Walters borrowed a concept from California strawberry growers, and adapted a bed fumigation apparatus. This multi-stage, tractor-drawn tool uses a shank to cut the soil. The fumigant is injected into the soil through a hose behind the shank. The apparatus then shapes the three-foot wide bed, and pulls a fumigant-resistant plastic tarp over the bed. Raspberry rows are spaced ten feet apart, and the alleyways are not fumigated.

With bed fumigation, the statutory buffer zone drops to 25 feet, making the process more neighbor-friendly. The process uses one third as much fumigant as broadcast fumigation. Due to the expense of the tarp, the cost is about the same as with broadcast fumigation. Bed-fumigation also requires more planning to be sure the beds are properly placed for irrigation.

Since the study began in fall 2010, results have been surprising. Twice a year, Walters and his colleagues from the United States Department of Agriculture measure plant growth and yield and take soil and root samples to evaluate pests. When compared to broadcast fumigation, the raspberries grown with bed fumigation are doing at least as well, if not much better. Soil tests from some sites show ten times as many nematodes in broadcast-fumigated plots, as opposed to bed-fumigated plots. Also, all raspberry plants in bed-fumigated plots are the same size or larger than plants in the broadcast-fumigated plots. Walters will soon compare raspberry yield from the test plots, as well.

Walters notes that this project was possible thanks to cooperation from the USDA, Trident Agricultural Products, Inc., and support from the grower community.