Methyl Bromide Alternatives By A.R. Chase

As the deadline approaches for complete Methyl Bromide phase-out, more research is proving the efficacy of replacements.

Methyl bromide is an important part of ornamentalproduction. The combination of methyl bromide and chloropicrin has long beenused to control weeds, nematodes and plant pathogens like Pythium. The majorityof this fumigant is used for strawberries, fruit trees and vegetables inFlorida and California, but there is a substantial amount of the product usedin floriculture production. Some industries have found acceptable alternativesover the past five years and no longer use methyl bromide. Floriculture hasbeen struggling to find an acceptable alternative.

The production of field-grown cut flowers, some in-groundshade house flowers and caladiums rely on availability of methyl bromide (MBr)for economically acceptable crops. I have even met a few greenhouse growers whostill use soil as a part of their potting medium and fumigate the resultingblend with MBr before use.

The California Cut Flower Commission (CCFC) took the lead infunding research on MBr alternatives in ornamental production in the early1990s. Research has involved everything from alternative fumigants;solarization; treatment of soil with steam, microwaves or UV rays; soilfertility; and amendment with green manures and biological agents. Currentalternative fumigants are 1, 3-D (Telone), chloropicrin and metam sodium(Vapam), which can be applied alone and in combination. In some cases,application through drip irrigation systems has been developed with excellentresults. In addition, the use of granular Basamid has been researchedextensively, often in conjunction with Telone or chloropicrin.

Much of the new research sponsored by the California CutFlower industry has concentrated on weed control. Research on Fusarium wiltfungi (on mini-carnations and bulbs such as Dutch iris) and nematodes is alsoongoing. Some of the key crops in these trials have been ranunculus, gladiolus,callas lilies, delphiniums and stock. Although MBr is used in saran houses inboth California and Florida, the bulk of the product is used in fieldproduction, and therefore, much of the research has been done in the field.

Possible Replacements

In the early 1990s, a group of scientists at the Universityof California, led by Dr. Jim Sims (UC-Riverside) started extensive researchinto the use of methyl iodide (MI) as a replacement for MBr. Their results werevery encouraging, but it was not until 1999 that Arvesta (formerly Tomen-Agro)began to develop MI as a new product. Arvesta has continued research into MI(trade name will be Midas), and a label was submitted to the EPA a little overa year ago. The initial label will include bulbs and ornamentals, as well astomatoes, peppers and strawberries.

Midas is a liquid at room temperature, making it a littlesafer to handle than MBr (gaseous at room temperature). It also has a muchshorter half-life than MBr and is unlikely to damage the ozone layer since itfalls apart before it can reach the ozone. Midas can be applied through a dripirrigation system, making it more flexible in use patterns than MBr. It hasmuch the same spectrum of activity — works on weeds, nematodes and fungalpathogens and appears to remain in the soil longer than MBr, again because itis a liquid at room temperature. When fields are planted too quickly afterpulling the plastic used in Midas application, some toxicity has been reported.

Many other alternatives are being researched at this time,including sodium azide. This poison as been around for at least 50 years buthas no agricultural uses at this time. In fact, development of sodium azide wasprobably curtailed when MBr became widely available in the 1970s. Under someconditions, sodium azide explodes, which can obviously be a problem with itsusage. Presently, there are two companies developing a liquid sodium azide. Thefirst is American Pacific, which is working with Auburn University. The secondis Cal-Agri Products. Trials with these newer experimental formulations havehad mixed results in both California and Florida, but research continues.

An annual MBr alternatives meeting provides a forum for the manyUSDA, university and private researchers to exchange ideas, report results andlearn the latest in the political arena. For 2003, the meeting will be held inSan Diego in November and will rotate to Orlando the following year.

Research Results

Over the past two years, Chase Research Gardens has beenhelping out with some MBr alternative trials for cut flower production. Many ofthe trials we visited in 2001 and 2002 were conducted by Dr. Clyde Elmore, arecently retired weed scientist from the University of California-Davis. Clydehas been conducting trials at The Flower Fields (Carlsbad, Calif.) incooperation with Mellano and Company. One of the trials was designed toevaluate Iodomethane (Midas) in a 50:50 combination with chloropicrin.Treatments were shank applied and tarped. Two rates were compared to aMBr/chloropicrin standard at 350 lbs per acre and an untreated control. We havealso rated the trials periodically, and some of the results from this year’strials are presented in Figure 1, left.

Compared to the tarped-only control, plant vigor (rated on ascale from 1 [dead] to 5 [excellent]) was better for all fumigated plots onboth the March 18 and the April 4 evaluations. The degree of flowering was alsoevaluated and looked identical to vigor for the April 4 rating. All fumiganttreatments were very effective in killing last year’s white flower seed,compared to the tarped-only control. Those that did grow were on bed ends wherethe fumigant concentration was apparently too low for 100-percent kill.

On April 22, 2003, the Pythium severity of each plot wasrated using the following scale: 1 = none, 2 = few plants with wilting orstunting, 3 = up to 25 percent of plants dead and/or showing wilting andstunting, 4 = 26-50 percent of plants showing wilting and stunting or dead andmissing, and 5 = more than 75 percent of plants in plots missing or showingwilting and stunting. Disease was moderate in the control plot, but bothMidas/chloropicrin (300 lbs.) and MBr/chloropicrin (350 lbs.) had little if anydisease apparent. However, the higher rate of Midas/chloropicrin showed slightdisease or at least what appears to be disease. It is possible that the wiltingand yellowing typical of Pythium are due to some other factor in thistreatment.

Dr. Elmore has been working with Glad-A-Way (one of theworld’s largest producers of gladiolus) in Santa Maria, Calif., as well. Lastfall a trial was run using Midas and chloropicrin (33:67, 50:50 and 67:33)applied at 300 lbs. per acre compared to MBr and chloropicrin in a 50:50 mixapplied at 350 lbs. per acre and an untreated control. Dr. Elmore’s weed datashowed excellent results with Midas when it was used at 50 or 67 percent of thefumigant mix but slightly reduced control when used at only 33 percent. The abilityof these mixtures to kill cormlets from previous crops was also shown. Theimage above shows an untreated area with many cormlets (new gladioluspropagules) growing between the rows. The other image above shows the abilityof Midas and chloropicrin to kill these weed cormlets when used at 50 or 67percent of the fumigant mix.

A second gladiolus trial at the same site, compared Vapamand Basamid combined with Inline (a mixture of 1, 3-D and chloropicrin) to anuntreated control and to an experimental formulation of sodium azide. Sodiumazide was very effective in killing many of the weeds that were present but notas effective in killing the cormlets (40-percent kill). All three of thecommercial alternatives were very effective in weed control, as well as cormleteradication. Other similar trials showed the benefit of adding Inlinetreatments to either Vapam or Basamid to reduce weeds.

One final set of trials has recently been completed by Dr.Elmore working with Ano Nuevo Flowers (located just north of Santa Cruz,Calif.). One of his trials at this site evaluated a 50:50 mixture of Midas andchloropicrin used at 100, 200 or 300 lbs. per acre. These treatments werecompared to an untreated but tarped control. In addition, a virtuallyimpermeable film (VIF) was compared to the normal high density (HD) plastic.Along with weed count data, Dr. Elmore calculated the costs of hand weedingeach treatment. Figure 3, page 45 shows that under the HD plastic 300 lbs. peracre of Midas:chloropicrin was needed to achieve the same level of control as200 lbs. per acre under the VIF. These data are critical to our success inusing Midas once it is legal.

Drip Trials

Dr. Husein Ajwa (chemist, University of California-Davis)has been instrumental in setting up several drip applied trials with Dr.Elmore. This year’s trial at The Flower Fields included the same treatmentsdescribed above and also accounted for occurrence of white flowers and weeds(see Figure 2, page 45). Applications were made late fall 2002. We again ratedvigor on March 18 and April 4. All drip applied products significantly reducedthe severity of Pythium compared to the control. Chloropicrin (300 lbs.) andboth Inline treatments (at 150 and 300 lbs.) showed no signs of Pythium at thisrating. The 150-lb. chloropicrin rate and the Midas/chloropicrin treatments hadvery few diseased plants. Sodium azide had an overall rating of about 2 (slightdisease) that was mainly affected by a single replicate with higher thanaverage disease expression (for that treatment). This may have been due todifferences in application efficiency, soil conditions or even distribution ofthe naturally occurring Pythium inoculum. The Vapam treatment was lesseffective than the others at this rating.

All drip-applied products reduced the number of whiteflowers that grew from last year’s crop. In many treatments, the white flowerswere on bed shoulders, indicating that the products had not reached the entirebed as applied. Lowest counts of white flowers occurred in Metam, Midas/chloropicrinand the 300-lb. rate of chloropicrin.


At this point, it appears that there will be a number ofalternatives for MBr for control of weeds and diseases on cut flowers. Productssuch as Vapam, Basamid, Telone and Inline can each be valuable tools,especially when used in conjunction with each other. Although Midas looks likea very good product, we cannot use it yet, and learning the correct applicationratio (with chloropicrin) and rate per acre will be critical to insure a safe andsuccessful fumigation. I have not covered many of the truly experimentalproducts that are being researched at this time. They are in various stages ofdevelopment, and none look to be as promising yet as Midas. I want to thank theCalifornia Cut Flower Commission and the Society of American Florists (onbehalf of the Florida growers) for the opportunity to work on this criticaltopic with them.

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A.R. Chase

A.R. Chase is plant pathologist at Chase Agricultural Consulting LLC and
can be reached at [email protected]

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