Fumigation Factors For The Future

Soil fumigation is typically considered a necessary component of production for many crops grown in Florida such as tomatoes, peppers, eggplant, and strawberries. Fumigation is the pre-plant treatment of soil with a broad spectrum pesticide for the control of soil-borne diseases, weeds, and nematodes. The objective of fumigation is to establish an adequate concentration of the fumigant in the pest zone and maintain that concentration for sufficient time to kill the target pest. Soil fumigants are typically liquid materials packaged under high pressure that volatilize into a gaseous form when released.

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The most widely adopted fumigant was undoubtedly methyl bromide. Its use was considered common practice in raised bed plastic mulch systems throughout the Southern U..S until its mandated phase-out in 1993 by the Clean Air Act and Montreal Protocol. The final phase-out date for new production and importation was January 1, 2005. At that point, growers were still able to use available, existing stocks and new stocks approved through the Critical Use Exemption (CUE) process. Through the CUE process, states submit applications for limited amounts of methyl bromide usage where necessary for successful crop production. In Florida, CUE petitions are created and submitted by the Florida Fruit and Vegetable Association (FFVA) with help from growers, commodity representatives, researchers and extension agents. From 2005-2013 CUEs have been awarded for Florida tomato, strawberry, pepper and eggplant production. CUE allocations are measured as a percentage of the amount of methyl bromide produced in 1991, referred to as the 1991 baseline. Allocations have steadily decreased and have nearly diminished to zero in 2013 with only 2.2% of the 1991 baseline approved. There will be no CUE allocation available in 2014. In August 2013, a petition was submitted requesting allocations for 2015 (supplemental) and 2016. This includes a “rescue treatment” for methyl bromide for use in critical situations. The amounts requested represent almost a 90% reduction from the pre phase-out period.

Since the phase-out began, growers have slowly adjusted to the use of methyl bromide alternatives, with the majority of the industry completely relying on their use in 2013. The transition has not been seamless, and issues continue to arise with their long term use. In the early transition period, 2005-2008, many growers used existing stocks of methyl bromide. In 2008, it was evident that there were shortages in supply and the need for successful alternatives was apparent. Growers were surveyed in 2011-2012 to determine: 1) what alternatives they were using, 2) what pest problems are arising due to their use 3) what additional Integrated Pest Management (IPM) practices are being implemented and 4) production losses associated with alternative use. Grower responses were used to develop a case for the “rescue treatment” and CUE petitions.

Growers have reported increased pest pressure following repeated use of the alternative fumigants. For example, weed populations have increased over time and growers now regularly incorporate additional weed management strategies such as hand weeding, fallow management and herbicide applications. Purple and yellow nutsedges have been identified as the most problematic pests in all four crops. Soilborne diseases and pathogens that have increased over time include fusarium crown rot and Southern blight in tomatoes, peppers and strawberries. Increased charcoal rot incidence has been observed in strawberry. Nematode populations have also increased in all crops. End of season root knot nematode populations have increased in tomato, pepper and eggplant and sting nematode populations have increased and are impacting strawberry production throughout the state. Strawberry growers must make multiple fumigant applications before, during and after the season to try and alleviate pest pressure.

In addition to increased pest pressure additional cultural practices may be required for the successful use of alternatives. Most products require the use of virtually impermeable films (VIF) or totally impermeable films (TIF) to effectively retain fumigant gasses. Many other factors affect the performance of fumigant alternatives such as soil temperature, moisture, soil compaction and the requirement for specialized equipment. With all of these problems and considerations a grower has to be “on the ball” so to speak. Even with the implementation of additional practices and proper fumigant selection for his or her operation, there is still potential for alternatives to fail. A proper alternatives program should be sustainable and sustainability is questionable at best.

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A wide variety of alternative fumigants have been evaluated for use in Florida. However, few have been adopted due to limitations such as cost, odor issues, buffer zone restrictions, limited lateral movement in beds, limited availability, plant back and phytotoxicity issues, and the requirement for additional inputs. In 2010, the U.S. government decided to further reduce allocated amounts of methyl bromide. This decision was primarily based on the idea that the “Georgia 3 Way” system (using Telone II, chloropicrin, and metam sodium) and iodomethane would be acceptable alternatives for methyl bromide. The “Georgia Three Way” was shown to be ineffective under Florida soils and iodomethane was voluntarily cancelled by the registrant and recalled from the market effective December 31, 2012. Thus, Florida growers are facing very limited fumigant options.

According to grower surveys, tomato growers began switching from Chloropicrin 50:50 to Pic-Chlor 60 in 2010. Subsequent surveys in 2011 and 2012 showed a major increase in Pic-Chlor 60 use, and a major decrease in 50:50 usage. Chloropicrin 50:50 is a formulation of methyl bromide that contains 50% chloropicrin and 50% methyl bromide. It was originally adopted by the grower community due to cost, availability, and formulation containing methyl bromide. However, research has repeatedly shown it to be less effective than fumigant mixes with higher concentration of methyl bromide. Pic-Chlor 60 is a combination product of 1,3-dichloropropene (40%) and chloropicrin (60%). The cost and availability of PicClor60 make it currently the most popular fumigant for Florida tomato growers. However, it does not provide sufficient efficacy against nutsedges or Fusarium species.

Strawberry acreage is split with 30% using K-Pam or Vapam, 28% using either Telone C-35 or InLine, and 33% using Pic-Chlor 60 as the primary fumigant. K-Pam and Vapam are products which require additional compounds to provide sufficient efficacy. In drip applications, growers see problems with lateral movement to the shoulders of the beds. In areas where these products have been used without rotation with other alternatives growers have reported diminished efficacy. Some have speculated that this occurs due to the breakdown of the material by select microbial agents. Telone C-35 and other 1,3-dichloropropene containing products show a lack of herbicidal activity and may cause issues with phytotoxicity and plant back when under high soil moisture and cool temperatures. Pic-Chlor 60 is the primary alternative used among pepper and eggplant growers.

Efficacy research has been ongoing since the decision to phase-out methyl bromide. Researchers consistently observe inconsistency of alternatives and high variability within any given trial. Trials are dependent on pest presence and consistent weather conditions over several planting seasons. These conditions are extremely difficult to capture. Although many of the alternatives have been shown to be efficacious in a given trial, they are not consistently effective and come with high risk when compared to methyl bromide. For further information on available alternatives and their efficacy please refer to EDIS publication SP170-23.

The CUE petition submitted in August 2013 includes a request for a rescue treatment of methyl bromide for fields in which alternatives have been used, and no other method of control has been effective at keeping a given pest below a defined action threshold. The action threshold is the point at which treatment is necessary. The proposed treatment is a formulation of 67% methyl bromide and 33% chloropicrin. The use rate is 235 lbs. per treated acre under VIF or 200 lbs. per treated acre under TIF.

Individual fields would be considered for a rescue treatment if they met defined criteria. To be considered for a rescue treatment, growers would be required to document pest presence at certain time periods during the crop season. Specific criteria have been developed by specialists for the most prevalent pests in each of the four crops.

For tomato, the two key pests that have been identified are the nutsedges (Purple and Yellow, Cyperus rotundus and C. esculentus) and Fusarium wilts (Fusarium oxysporum f.sp. radilycopersici). The criteria triggering the need for the rescue treatment would be 30% of the field with 10 or more weeds present per linear bed foot. For Fusarium, an assessment of the disease would be made at first or second harvest to document the level of inoculum in the field. In strawberry, three of the most destructive pests have been identified including: Sting Nematodes (Belonolaimus longicaudatus), Nutsedges (Purple and Yellow, Cyperus rotundus and C. esculentus), and the hard-seeded winter annual weeds including, Black Medic (Medico spp.), Carolina Geranium, (Geranium carolinianum), and the Clovers (Mellilotis spp.). For the hard-seeded winter annuals, the presence of one weed can indicate the potential for unsustainable production in subsequent seasons. For Sting Nematodes, “green seeker” technology mounted on a tractor can be used in conjunction with GoogleEarth to determine the percentage of a field in which plants are stunted due to Sting Nematode damage. For pepper and eggplant crops, it was determined that Nutsedges (Cyperus rotundus and C. esculentus) would be the key pests triggering the need for a “rescue treatment.” The criteria would be the same as described for tomato and strawberry.

A committee of industry representatives, researchers and Extension agents created the criterion with the realization that these are not the only pests that may cause subsequent crops to be unsustainable. However, according to research observations and grower survey responses, these pests would be the most likely to trigger the need for a “rescue treatment.” A decision for the treatment will most likely not be reached before fall of 2014. In the meantime, it is important that growers document pest issues and bring them to the attention of consultants, researchers, and Extension agents. Growers may continue using products purchased prior to the end of 2013 with 2005 labels. Research toward finding viable alternatives will continue to be conducted.

References

Botts, D. 2013. Application for Methyl bromide Critical Use Exemption for Florida Tomato, Strawberry, Pepper and Eggplant. Submitted for review August, 2013.

Noling, J.W. and A.W. MacRae. 2012. Alternatives to Methyl Bromide Soil Fumigation for Florida Vegetable Production

Snodgrass, C., M. Ozores-Hampton, A.W. MacRae, and J.W. Noling. 2013. Fumigation Practices and Challenges among Florida Tomato Growers: Survey Results. 2013 Florida Tomato Institute Proceedings.

 

 

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