Evaluation of Reduced Methyl Bromide Rates in Field Grown Perennial Crop Nurseries

Authors: Hanson, Bradley; Gerik, James; Schneider, Sally

Submitted to: Western Society of Weed Science Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/1/2009
Publication Date: 3/12/2009
Citation: Hanson, B.D. J.S. Gerik, S.M. Schneider. Proceedings Western Society of Weed Science. 62:17

Interpretive Summary: This report presents research data on fumigation research in a garden rose nursery and a stonefruit tree nursery in 2005-07 and 2006-09. Reduced rates of methyl bromide applied with standard or low permeability plastic tarps were compared to the standard rate and to two alternative fumigants. In these two trials, nematode, pathogen, and weed control was similar among treatments suggesting that reduced rates and low permeability tarps should be considered for use in perennial crop open-field nurseries in California.

Technical Abstract: Preplant soil fumigation with methyl bromide (MB) has commonly been used in field grown nursery crops to provide broad-spectrum control of plant parasitic nematodes, disease pathogens, and weed propagules. Although MB use was officially phased out in 2005 due to negative effects on stratospheric ozone, the fumigant is still allowed in certain crops under the provisions of annually requested Critical Use Exemptions. In California nursery stock nematode-free certification requires a minimum of 300 lb/A MB in sandy soils or 400 lb/A in clay loam soils (33.6 or 44.8 g/m2); however the United Nations Methyl Bromide Technical Options Committee (MBTOC) suggests that rates of 26 g/m2 for nutsedge and 20 g/m2 for pathogens and other weeds should be effective where low permeability barrier films (LPBF, i.e., virtually impermeable film [VIF]) are not available. Where LPBF is available, MBTOC suggests that rates of 17.5 and 15 g/m2 should provide effective control of nutsedge and pathogens, respectively. Two field trials were conducted in 2005-07 and 2006-07 to evaluate pest control with reduced-rate MB treatments in a garden rose nursery near Wasco, CA and a fruit tree nursery near Visalia, CA, respectively. Each trial arranged in a randomized complete block, split-plot design with fumigation treatment (untreated, MB standard, 7-8 reduced MB rates, and two 1,3-dichloropropene treatments) as the main plot and two rose or Prunus rootstocks as the split plot factor. Broadcast fumigation treatments were applied in the fall, hardwood cuttings and/or seeded rootstock were planted in the winter, and crop growth and control of nematodes, pathogens, and weeds were monitored for 1-2 growing seasons. Nematode control (citrus nematode bioassay) was very effective to a depth of 36 inches with all treatments. Control of Pythium spp. (good control) and Fusarium spp. (poor control) was similar among treatments. Weed populations, handweeding times, and seed viability reductions were generally similar among treatments; however, 1,3-dichloropropene treatments usually had slightly higher weed populations and handweeding requirements. Nursery stock productivity did not differ among reduced rate MB treatments and the industry standard treatment in either trail. VIF tarps and low MB rates are not currently allowed in California perennial crop nurseries but, if regulations change, these treatments should be considered. Although the results of this trial were favorable, it is important to note that the experiments were conducted in first-year nursery sites with low nematode and weed populations. The issue of nematode control remains critically important to the industry because certification requires “non-detectable” levels of parasitic nematodes. Regardless of fumigation treatment, supplemental weed control strategies will be needed to provide sufficient season-long control and, due to increasing labor costs, herbicides are likely to become a more important tool in perennial crop nurseries. Additionally, it is possible, if not probable, that long-term repeated use of low rates of MB or alternative fumigants could reveal weaknesses in pest control not evident in single-cycle field trials.