Application of MeBr Alternative Fumigants by
Drip Irrigation Systems for Strawberry Production in California

Husein Ajwa and Tom Trout, USDA–ARS, Water Management Research Laboratory, Fresno, CA

The most likely alternatives to methyl bromide in the short term are alternative fumigants [e.g., 1,3–dichloropropene (1,3–D), chloropicrin (CP), and methyl isothiocyanate (MITC)]. Although these fumigants are not ozone depleters, they are potentially hazardous to the environment and humans if not properly applied. Water-soluble formulations of these alternatives can be applied with irrigation water through the same irrigation systems that are later used to irrigate the crops. The amount of water used to apply the fumigants, application rates, and soil conditions determine the success of these fumigants in controlling soil pathogens and weeds in the strawberry beds. The objectives of our research are to develop application methods for alternative fumigants based on drip irrigation systems and to maximize efficacy and minimize human and environmental risks.

This research has been conducted at two strawberry field sites located in Salinas (USDA–ARS Spence Farm) and Watsonville (Monterey Bay Academy), CA, for the last three years (1996–98). Preplant treatments with the soil fumigants [Telone C35 EC (58% 1,3–D plus 32% CP), chloropicrin EC (93% chloropicrin), and Vapam (42% sodium methyldithiocarb-amate)] were drip-applied in early October of each year approximately four weeks before planting. Variables that are being evaluated include amount of water used to apply the fumigants (15, 25, or 35 mm); application rates (manufacturer recommended rate and 60% of that rate); application of combinations of fumigants; and number of drip lines. The fumigants were applied through two drip lines in 76 cm wide beds (132 cm center-to-center) covered with green polyethylene film. In addition to the drip-applied fumigants, MeBr/CP (67% MeBr, 33% CP) at 475 kg/ha and Telone C35 (61% 1,3 D, 35% CP) at 355 l/ha were shank injected with two chisels into the bed at a 30 cm depth. The soil gas concentration of 1,3–D, CP, and MITC in the middle and at the edge of the raised beds were monitored for two weeks beginning 24 hours following application.

Strawberry variety Selva was planted at 30-cm spacing in early November. Yields were collected at least once a week during the production season (April to September). Disease rating and yield from the drip-fumigated beds were compared to yields from non-fumigated beds and from beds fumigated with MeBr/CP and Telone C35 by shank injection.

Monitoring the fumigant concentrations in the soil gas showed that a minimum of 25 mm of irrigation water is needed to deliver the chemicals to the edge of the raised bed using two drip lines. The optimum distribution uniformity of fumigants across the bed was obtained with 25 mm of irrigation water using four drip lines per bed and with 35 mm of irrigation water using two drip lines. The concentrations of 1,3–D and CP in the soil gas were greatest 24 to 36 hours following application. The decline in concentrations of these fumigants in the drip-applied treatments was slower than that in the shank-injected treatments. However, no detectable amounts of fumigants were found in any of the treated soils 14 days following application.

At both sites, bed shank injection with Telone C35 at 355 l/ha (38 gal/a) produced strawberry yields equivalent to those produced by shank fumigation with MeBr/CP. The highest yield (110% relative to MeBr/CP) was in the drip-applied Telone C35 EC treatment at 390 L/ha in 35 mm of irrigation water. Yields from beds treated with the same application rate of Telone C35 EC applied in less amounts of irrigation water (25 or 15 mm water) were, in general, lower than yields from the MeBr/CP treatment.

For all of the alternative fumigants tested, strawberry yields significantly increased relative to the nonfumigated treatment. At the Watsonville site, Verticillium dahliae was a major problem that resulted in a substantial yield reduction in the untreated plots (28 to 50% relative to MeBr/CP). Reduced rates of drip-applied Telone C35 EC (235 l/ha) or Chloropicrin EC (135 l/ha) alone or in combination with Vapam did not control V. dahliae or Pythium spp in this soil. Yields from the Vapam treated beds were lower than yields from the Telone C35 EC treatments and ranged between 67 to 79% relative to MeBr/CP treatment. At the Salinas site, the highest yields were also in the drip-applied Telone C35 EC. However, this site has low populations of V. dahliae, and yields from the nonfumigated beds were 91% relative to yields from the MeBr/PC beds.

Weed control by the drip applied fumigants was not evaluated during the first two years. Results from the 1998 trials showed multiple applications of Telone C35 EC through drip irrigation tremendously reduced weed germination, and weed counts in all of the Telone C35 EC treatments were much lower than those in the MeBr/CP or Telone C35 shank injection treatments.

Application of Telone C35 EC through drip irrigation systems to strawberry beds shows promise in controlling soil pathogens and weeds and producing strawberry yields nearly comparable to present production with MeBr/CP. This application method can reduce costs since separate application equipment is not required. It is expected to be safer than present methods of shank injection since laborers are not required to be in the field during application. It will require good irrigation systems and dependable injection equipment. We expect that emissions from drip application will be lower than with shank application.

Our research will continue to determine optimum drip application rates (minimum rate for consistent efficacy) and conditions (soil water content and amount of water carrier) for all three fumigants alone and in combination to control pathogens and weeds for strawberry production in various areas in California. Because the loss of MeBr will come before this research is completed, we have, in cooperation with the California Strawberry Commission, begun field tests with drip-applied fumigants on growers’ fields. Using our best estimates from the research plots, large trials with drip applied fumigants were started on 5 growers’ fields in 1998/99. With these trials, we have an opportunity to verify efficacy under a wide range of soil, climatic, and cultural conditions. We also work with the manufacturers to develop safe, practical field-scale application equipment. Growers have an opportunity to test the methods under their operating conditions, determine how they might have to change their practices with the new fumigants, and determine what problems require further work.

Last Updated: July 1, 1999