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United States Department of Agriculture

Agricultural Research Service

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Research Project: Increase Fumigation Efficacy with Alternatives to Methyl Bromide Using Low Permeability Tarps

Location: Water Management Research

2013 Annual Report

1a.Objectives (from AD-416):
Demonstrate that using low permeability tarps (e.g., TIF) can improve efficacy while controlling emissions and can be potentially used with lower rates.

1b.Approach (from AD-416):
Two field trials will be conducted to collect soil fumigant (1,3-D, chloropicrin and methyl iodide) and pest control (nematode, pathogen, and weed) data under TIF tarps. Fields with naturally occurring nematode and pathogen populations will be selected. Treatments (4 replicates) will include: control (no fumigation), full rate (maximum allowed) in bare soil, under standard tarp, and TIF tarp, and reduced rates (2/3 and 1/2 of full rate) under TIF. Soil-gas concentrations will be monitored at pest sampling locations and depths. Correlation between fumigant dosage and pest control, and treatment effects will be statistically analyzed. Tarp permeability will be measured before and after installation. Fumigant change in the air under tarp will be monitored to estimate the proper time for the safe removal of TIF tarps to minimize exposure risks, a major concern of regulatory agencies. Almond trees will be planted the following spring and tree responses to treatments will be monitored.

3.Progress Report:

This project addresses the in-house project (NP 211) Objective 5: Develop various application methods, soil amendments, and physical barriers to reduce the emissions and enhance efficacy of the chemical alternatives to methyl bromide. This project is in collaboration with University of California Cooperative Extension farm adviser in Merced County, California, and researchers or extension specialists in University of California, Davis. One large and comprehensive field fumigation trial was conducted from 29 Nov. 2012 through 3 Jan. 2013. The specific objective of this trial was to determine fumigant emission, distribution or concentration changes in soil profile, efficacy on pest control (nematodes and pathogens), and tree response to fumigation treatments. The trial was conducted in an almond orchard near Merced, California. The treated area for this study was about 5 acres. Almond trees in this field were pulled out after harvest in fall 2012. The soil was prepared by the grower based on their schedule for replanting and common practices used. Treatments included three surface sealing methods [bare, standard polyethylene (PE), and totally impermeable film (TIF)] and four application rates (full, 2/3, 1/3, and 0 rates) of Telone C35 (35% CP, 63% 1,3-D, and 2% other ingredients). A total of 12 treatments with 6 replicates were applied in a randomized complete block design. The fumigant was applied on 29 Nov. 2012 through shank injection to 18 inch depth with spacing of 20 inches between shanks. Fumigant emissions from PE and TIF tarped plots as well as near the edge of the TIF tarp were determined using passive flux chambers. Changes of fumigant concentrations in the soil profile were monitored and compared under different surface sealing treatment. Air fumigant concentrations under the two different tarps from different application rates were also monitored. Prior to fumigant injection, soil samples were collected for nematode population count and soil water content measurement. The field was identified with high populations of nematodes (chiefly pin and some ring nematodes) prior to fumigation. Soil samples were collected at the end of the trial and the efficacy on residential nematode survival was determined. In addition, bioassay bags containing soils infested with citrus nematodes and pathogens were buried in soil at different depths and retrieved at the end of the trial for survival count to determine treatment effects.

All field samples have been processed and the data are being compiled. The emission data illustrated again that TIF tarp can significantly reduce emission peak flux compared to the standard PE tarp. Under the low temperature conditions throughout the trial, chloropicrin (CP) emissions were extremely low in comparison with 1,3-D, only detectable from the full rate, and most other monitored plots showed non-detectable emissions. Fumigant concentration in air under the tarp reached a peak in 2 days under PE, but in 6 days under TIF before starting to decline. This was observed for all three application rates. This is likely due to the ability for TIF to retain fumigants that diffuse to reach the soil surface and easily pass through PE film as emission loss.

For fumigant distribution in soil profile and changes over time, 1,3-D and CP followed a similar pattern and also at similar concentration ranges during the first week. Among the treatments, because of large field variability there appeared to be no difference in fumigant concentration between PE tarped and TIF tarped plots at the same application rate. The field varied in topography. Heavy rain occurred at the beginning of the field trial and resulted in very non-uniform soil water distribution at different locations in the soil profile. This was believed to contribute to the field variations. Prior to fumigation, the field showed high nematode population (chiefly pin and some ring nematode and spiral nematode detected) with the highest population below 3 ft. All full rate treatments and the 2/3 rate under TIF provided 100% kill for residential nematodes in the top 3 ft soil. The 2/3 rate under bare and PE and all 1/3 rates as well as non-fumigated controls showed much higher survival of the residential nematodes in the top 3 ft soil. However, from 3 to 5 ft depth, all treatments including the full rate under TIF showed survival of nematodes. The data indicate that difficulties in controlling nematodes at deeper soil depths in replanting orchard continue to be a challenging task. The efficacy data were supported by the soil gas data that showed low concentrations in soil depth below 3 ft for all treatments, especially with the reduced rates. Almond trees were planted early spring 2013. Tree response to the fumigation treatment is being monitored.

Last Modified: 8/27/2014
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