The search for an effective alternative to methyl bromide

June 2006
For more than 30 years, New Zealands strawberry industry has relied on methyl bromide for preplanting sterilisation of soil. This minimises pest and disease problems and helps growers maintain high productivity year after year.

However, methyl bromide is a serious depleter of the ozone layer and is being phased out. In 1997 it was decided there would be a total ban on the use of methyl bromide in 2005, but that deadline has been extended with a critical use exemption (CUE).

Dr Ian Horner from HortResearch Applied Plant Pathology group and others have been working with the strawberry industry on alternative soil fumigants. With trials in strawberry gardens, runner beds and at HortResearch facilities, various fumigants have been tested. Results have been encouraging, with yields equal to or close to the level of control of pathogens under methyl bromide.

However alternatives have less mobility in the soil, less weed control and longer plant-back times.

In the 2006 season, most growers who fumigated used methyl bromide, under an allocation from the CUE, although the volume available under the 2006 CUE is lower than last year. An application has been made for next year (likely to be the final year of the exemption), but the type of questions posed by the international panel in response to the NZ application indicates that they are taking a more hard-line approach. Supplies next year, if granted at all, are likely to be substantially reduced.

One factor that will count against NZ is grower reluctance to trial alternative products in part of their gardens. The panel cite this as a major consideration in granting continued supply of methyl bromide. This year a number of growers trialled alternative products in an effort to counter this argument.

It appears that the 2005/06 season was a relatively good one in terms of fumigation success, and general root or plant health. A few losses from Phytophthora were reported, but the incidence was low compared to some other years. While this is good for the industry, it has made research into soil and fumigation problems more difficult. Its hard to troubleshoot when theres no trouble to shoot. Ironically, researchers can make more progress towards solving long term problems in a poor year than in a good year.

Growers fumigate soils for two weeks under plastic during the late summer/autumn before planting for spring strawberry production. Gas is injected under the plastic and left to disperse. Growers currently preparing ground have been urged to put in a few rows of alternative products (e.g. Telone C35 or chloropicrin) for comparison. Where possible, such strips should be applied in the middle of blocks and clearly marked, so direct comparisons can be made with adjacent rows of methyl bromide. They could also try turning off the gas for a couple of 4-5m strips to help gauge the background disease problems. A number of growers have implemented such trials this year.

The main alternative fumigant/practices trial in Roselea garden in Hawkes Bay in 2005 tested three varieties of strawberries (Camarosa, Gaviota and Pajaro) under various alternative fumigant and non-fumigant soil treatments. Comparisons were made in soil that was either good or poor (too wet) during soil preparation and fumigation. A lot of data is yet to be analysed, but overall trends are becoming clear.

Key findings to date, averaged across varieties are:

Methyl bromide, iodomethane and chloropicrin (in good soil only) gave similar fruit yields.

Telone C35 and chloropicrin (in poor soil conditions) averaged 10 to 15% lower yields.

Fumasol yielded 24% & 59% less than MeBr in good and poor soil conditions.

Untreated, urea, compost and mustard oil treatments yielded 40-60% less than MeBr.

Root health and overall plant health was generally best in methyl bromide, chloropicrin, iodomethane, and Telone C35 treatments. Fumasol lagged slightly behind the other fumigants.

Responses differed for different varieties, complicating interpretation.

If soil conditions were poor at fumigation, fruit yields in some treatments were lower compared to fumigation in good soil. This was particularly noticeable in Fumasol and chloropicrin treatments, but was not a factor with methyl bromide or iodomethane. Telone C35 gave mixed results for the different varieties.

None of the potential biological controls (various Trichodermas and compost tea) tested in sub-plots gave any improvement over the untreated control.

Initial pathogen kill and destruction of weed seeds showed that all the tested fumigants gave good control of both pathogens and weeds, but other treatments had minimal effect. Counts of weeds in early spring showed a similar response.

A separate Roselea trial tested phosphorous acid treatments to control Phytophthora on three varieties (Pajaro, Camarosa and Gaviota). Both pre-plant dip and post-planting sprays were tested. Results differed for the different varieties.