The Impact of Methyl Bromide Alternatives in Tomato on
Double-Cropped Cucumber

J. P. Gilreath and J. P. Jones, University of Florida, Gulf Coast Research & Education Center, Bradenton, FL 34203 and J. W. Noling, University of Florida, Citrus Research & Education Center, Lake Alfred, FL 33850.

Most of the methyl bromide alternatives research in Florida has focused on identifying viable alternatives in tomato or some other primary crop, but often that is only half the picture for many tomato growers. The economics of tomato production are such that many growers have adopted a production strategy which spreads the cost of soil fumigation and fertilization over two crops for the farm to remain profitable.

This is achieved by killing the first crop at the end of the season and leaving the raised, polyethylene film mulched beds in place, undisturbed and then growing a second crop on them. Thus, the residual fertilizer is utilized and the cost of soil fumigation, mulch and, in some cases, microirrigation tubing is spread over two crops. This procedure of growing a second crop using the inputs for the previous fumigated crop, is referred to as double-cropping. The first crop is usually tomato, but may include pepper, and the second crop is typically a cucurbit, such as cucumber or watermelon. Cucurbits are favored because they are cheaper to grow and are not in the same botanical family as tomato, thus avoiding some potential disease problems.

Previous work by the authors and other members of the Florida Methyl Bromide Alternatives Task Force has determined that the most likely chemical replacement for methyl bromide in tomato will be a combination of Telone C–17 or C–35 soil fumigant and Tillam herbicide. The effectiveness of this alternative has been demonstrated in both small plot research and large plot trials on commercial farms. Although sometimes not as effective as methyl bromide for soilborne pest control, including nutsedge control, this combination generally has performed quite well. Cooperating tomato growers indicate that the mechanics of application would be acceptable.

One concern that has plagued this and all alternatives research is the fact that most research is being done on sites which received methyl bromide in previous years. Another concern expressed by growers has been what impact the combination might have on double cropped cucurbits. Their concern involves herbicide residues in the bed and resurgence of soilborne pests, such as root knot nematode. Cucurbits are well known for their sensitivity to herbicides and there are few herbicides labeled for their use. Tillam is not one of those. In some experiments, Telone C–17 or C–35 has controlled root knot nematodes as well as methyl bromide, statistically speaking, but the absolute number of nematodes present or the extent of galling might be slightly higher. Growers fear that these slightly higher numbers might translate into significantly higher incidence and severity in the double crop.

To address the issue of residual activity of Tillam, we conducted experiments over the past several years at the Gulf Coast Research and Education Center to determine the effect of Tillam on cucumber growth when grown in the same beds following tomato and soil fumigation with methyl bromide, chloropicrin, Vapam, or Telone C–17 or C–35. Some of these studies also were designed to assess the impact of the previous season's soil fumigation program on overall growth of cucumber and root knot nematode populations that develop on the cucumber roots.

The time interval between Tillam application for tomato and cucumber planting is the shortest in the spring. Conditions during the fall season and over the brief winter period prior to cucumber planting are less conducive to herbicide degradation than following a spring tomato crop. For these reasons, we conducted the Tillam studies in the spring following a fall tomato crop. Results of these and related studies indicate that the level of Tillam remaining in the bed at the time of cucumber planting is not sufficient to cause any damage to cucumber.

The resurgence of root knot nematodes was determined in both fall and spring double cropped cucumbers. Late season nematode resurgence was a problem even with methyl bromide, but it was a bigger problem with the alternative fumigants. Root galling due to root knot nematodes was worse on the fall cucumber crop. Root galling at the end of the crop season was greater with methyl bromide in the spring than with some of the other fumigants, but the plants were more vigorous. A more vigorous plant has a larger root system which can sustain higher nematode populations. Fumigation with Vapam, chloropicrin or Telone C–17/C–35 resulted in more vigorous plants, higher yields, and less root galling than no fumigation, but in general they were not as effective as methyl bromide.

Thus, it appears that none of the fumigant alternatives will be as effective as methyl bromide when viewed in a double crop scenario. Research is continuing on this and related topics.

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Last Updated: October 6, 1998