Methyl Iodide: Recent Field Results

James J. Sims & Michael E. Stangehellini, Department of Plant Pathology; J. Ole Becker, Department of Nematology; M.E. McGiffen, Jr., Department of Botany and Plant Sciences, University of California, Riverside, CA92521, and Cynthia G. Eayre, USDA–ARS, Postharvest Quality and Genetics Research Unit, Fresno, CA 93727.

So far, methyl iodide is the only single chemical alternative to methyl bromide that has been shown to be effective. To quickly review, methyl iodide is chemically analogous to methyl bromide. Thus, it is expected to have very similar biological effects. Methyl iodide is not a threat to the ozone layer since it decomposes rapidly in sunlight. As it is a low boiling liquid, it is safer to handle and measure. For laboratory purposes, it is almost exclusively used over the more-difficult-to-handle methyl bromide which is a gas at normal temperature and pressure.

In recent published work we have shown that methyl iodide was more effective than methyl bromide in halting weed seed germination and that it was effective at lower temperatures.

The purpose of this technical report is to announce some preliminary results with methyl iodide in field tests.

Peach Replant

Peach replant syndrome is an ill-defined disease complex which causes stunting of newly planted peach seedlings on old orchard land. Currently, in such a situation the soil is fumigated with methyl bromide at a rate of 300–400 lbs./acre before the seedlings are planted. This treatment ensures that the trees will reach maturity faster and be heathier. We have begun field trials in a peach replant situation comparing methyl iodide and methyl bromide fumigations on USDA/ARS land in Parlier, CA.

We started our first trial in 1997. We fumigated prepared ground with methyl iodide and methyl bromide at 450 lbs./acre using tarp coverage for seven days after treatment. Untreated ground served as a control. Four repetitions of each treatment were randomized with four controls. After one year, the trees planted in the treated plots could easily be picked out from those in the untreated plots. The height and canopies were markedly different. The trees planted in fumigated ground were taller and had larger canopies than the trees planted in the untreated control ground. Trunk diameters of the trees measured in 1998 provided quantitative measure of the visual observations. The average diameter of trees in the respective plots were: control, 20.2 cm, methyl iodide, 32.6 and methyl bromide, 30.4. There was no significant difference between the treated trees and both were significantly different from the untreated controls.

A second trial begun in 1998 is showing similar visual and trunk diameter differences after the first growing season. Therefore, based on this data, methyl iodide and methyl bromide appear equally effective in avoiding the peach replant syndrome.

Vine Decline of Muskmelon

First year results from the comparison of methyl iodide and methyl bromide fumigation of field sites naturally infested with Monosporascus cannonballus, the causal agent of vine decline, have been encouraging. The Desert Research and Education Center of the University of California in the Imperial Valley near Holtville was the site of the studies. We studied two cropping seasons with preplant fumigation only before the first (spring) season; the second season (fall) crop was replanted with no further fumigation. Because there were several different materials being tested in this study, the entire area was subjected to preplant treatment with Telone for nematode control prior to the spring seeding.

In the spring, we applied methyl bromide and methyl iodide at 400 lbs./acre, preplant—through irrigation drip tape under plastic mulch—as hot gasses, using an application system of our design. The treatments were replicated and we included untreated controls in the experiments.

At weekly intervals after planting, we extracted plants from the soil and rated them for evidence of infection. The condition of both the canopy and roots was rated on a scale of 0–4, where 0=heathy, 1=25 percent, 2=50 percent, 3=75 percent and 4=100 percent, collapsed or infected, respectively. For the Spring crop, the root ratings at harvest were: control=2.9, methyl bromide=2.0 and methyl iodide=1.1. The canopy ratings were: control=2.9, methyl bromide=1.1 and methyl iodide=0.9. Both treatments gave excellent and statistically significant control of the disease.

In the fall, we replanted treated plots to evaluate any carryover effect of the chemicals. Root ratings at harvest were: control=3.7, methyl bromide=2.0 and methyl iodide=1.3. The canopy ratings were: control=2.2, methyl bromide, 1.1 and methyl iodide=0.3. More importantly, there was a late season collapse of the vines in all but the methyl iodide plots. In practical terms, this meant that the only crop that could have been harvested in the fall was in the methyl iodide plots. When the canopy collapses, the melons are ruined by sunburn and cannot be sold.

This work will be repeated in 1999. As far as efficacy is concerned, methyl iodide continues to be as effective, or even more effective, than methyl bromide. The next hurdle for methyl iodide will be registration.

Last Updated: January 12, 1999