Summary. Several alternatives to methyl bromide (MeBr) have been validated in vegetable production systems in the southeastern United States. All of the alternatives have demonstrated the potential to replace MeBr in the specific cropping systems in which they were evaluated. However, all alternatives lack the same broad spectrum of control achieved with MeBr fumigation and have more stringent application requirements. Thus, additional knowledge of pest biology and application technology is required by the growers to implement these alternatives. Growers must also cope with the management of additional information and a more complex decision making process regarding the selection and implementation of alternatives.
Key words: methyl bromide, chemical alternatives, solarization, cultural prcatices, strawberry, pepper, tomato
Florida is the leading producer of fresh market tomatoes and pepper in the United States. Combined these two commodities comprise 23,760 ha and produce over US $745 million worth of tomato and pepper. MeBr fumigation is conducted on 93 % of the tomatoes and 83 % of the peppers grown in Florida and together they account for 25 % of the total consumption of MeBr in the United States and 8 % of the global methyl bromide consumption.
A closer examination of the crop production systems used in Florida is necessary to understand many of the constraints impacting the development of alternatives to MeBr. Since the 1960's, growers have used a production system in which seedlings are transplanted into pre-formed 76 - 100 cm wide by 20-25 cm high beds that have been fumigated with methyl bromide and covered with polyethylene plastic. MeBr is applied to the raised beds by shank or chisel injection methods. A high level of inputs is required to sustain economic yields. Preharvest production costs can exceed $16,000 per ha while expected yields in tomato can reach as high as 2300 kg / ha. Land suitable for these production systems is limited and associated rental costs are high, forcing growers to practice as sustained monoculture over many years. This in turn escalates the build-up and subsequent damage inflicted by soilborne pests.
Since 1996, the University of Florida has conducted 69 large scale field demonstration trails of chemical alternatives to MeBr. Thirty-six were conducted on tomato, 28 on strawberry and 3 on pepper. Each demonstration trial was conducted on a commercial area by the grower. The minimum size for treated areas was 0.2 ha. Attempts were made to collect crop yield information independently from two sources: 1) small research subplots and 2) grower pack-out from entire treated blocks.
The alternative fumigants evaluated were various mixtures of 1,3-dichloropropene and chloropicrin. In addition, the herbicide pebulate was included in the tomato trials and the herbicide napropamide was included in the pepper trials.
Using data collected from grower pack-out, the average loss in the alternative plots was 1.12 % when compared to the MeBr treated plots (Noling and Gilreath, 2000). In the 12 trials were yield data was collected from the grower, the alternative treatment had higher yields in 3 trials and methyl bromide resulted in higher yields in 9 trials. Disease and nematode pressure was low in all of the trials. Problems were encountered with soil incorporation of the herbicides and phytotoxicity was observed when pebulate was not thoroughly mixed into the soil prior to planting. Recently, trials have been implemented using broadcast applications of 1,3-dichloropropene plus chloropicrin to avoid problems associated with worker exposure and requirements for workers in the field to wear full protective clothing. A deep placement 76 cm coulter system has been adapted to optimize fumigant diffusion patter and retention when compared to tradition shank injection systems.
Since 1995 soil solarization has been validated in 21 large scale demonstration plots in commercial production farms by the U.S. Department of Agriculture, Agricultural Research Service. The minimum size for treated areas was 0.2 ha. Attempts were made to collect crop yield information independently from two sources: 1) small research subplots and 2) grower pack-out from entire treated blocks.
In addition, additional large-scale field plots were established to evaluate various mixtures of 1,3-dichloropropene and chloropicrin at low rates in combination with soil solarization.
Soil solarization practices were modified to be compatible with the standard crop production systems. Strip solarization was performed on raised beds using clear, low density polyethylene film or clear, virtually-impermeable-film. Following a six to eight week solarization period, the film was painted white with latex paint to allow it to function as horticultural mulch.
In tomato, average marketable yields in the solarization treatments were 5 % less than the adjacent MeBr fumigated plots. In pepper, average marketable yield were 2 % less than adjacent methyl bromide fumigated plots. When solarization was combined with deep disking down to 25 cm depth prior to application of the plastic, yields were 23 % greater than adjacent methyl bromide fumigated plots. In plots were solarization was combined with low rates of alternative fumigants, yields were 13 % than in the adjacent methyl bromide fumigated plots.
Solarization was found to provide adequate control of weeds including yellow and purple nutsedge. Disease pressure was low in all plots. Solarization did not provide acceptable levels of control for root-knot nematodes. Technical problems associated with application of solarization included failure to provide adequate paint coverage to the plastic following termination of the solarization period. This resulted in excessive heating of the soil and some damage to the subsequent crop. Drip irrigation tubing was melted by the solarization treatment when tubing was placed directly on the surface of the soil beneath the plastic film. This problem was corrected by burying the tubing at to a depth of 5 cm.
Variability in results in the solarization treatments can be attributed to the complex mode of action of solarization and the influence of ambient conditions. Solarization works through a combination of physical, chemical, and biological changes in the soil profile. Many of these changes depend on soil type, moisture, and resident microbial populations. Additionally, thermal inactivation is a function of time and temperature and varies depending upon the ambient conditions during the time of solarization. Thus, it is not possible to prescribe a precise treatment period that will provide a broad level of control prior to application. This in turn creates a level of uncertainty among growers and remains one of the biggest factors impeding the widespread adoption of soil solarization. Soil solarization should not be perceived as a stand-alone replacement to pre-plant fumigation with MeBr for soil disinfestation due to problems with consistency of a range of environmental conditions and cropping systems. However, soil solarization is compatible with most nonchemical methods for pest management and deserves serious consideration as a fundamental component of pest management programs that use the biological knowledge of pests to select and integrate tactics promoting safe, profitable, and durable pest management. Its importance and potential contributions to IPM programs have been discussed
The benefits of crop rotation and minimum tillage were incorporated into an alternative production system by designing a low-input production system for tomato using minimum tillage practices in existing Bahia grass pasture. Florida alone has over 2.5 million acres of improved Bahia grass pasture. Through a design that is compatible with pasture crops, the alternative system increases access to those pastures. In addition to reducing input costs, minimum tillage techniques conserved the integrity of the mulches.
The alternative production system was validated in a 3 ha demonstration plot established by a commercial tomato grower. Comparisons in pest pressure, production costs, and marketable yields were made in an adjacent 3 ha plot fumigated with MeBr. Marketable yield in the alternative production systems was 36 tons per ha as compared to a yield of 42.5 tons per ha in the methyl bromide fumigated plot. However, production costs were reduced by $2000 per ha in the alternative production system. Thus, the net return (per ha) was $2888 in the alternative production system and $2320 in the conventional production system. The results demonstrated that profitable yields can be obtained in a vegetable production system designed to meet several environmental and economic goals.
Chellemi, D.O. 1998. Contribution of soil solarization to integrated pest management systems for field production. In: Soil solarization and integrated management of soilborne pests (J.J. Stapleton, J.E. DeVay, and C.L. Elmore (eds). Pp 322-332. FAO Plant Production and Protection Paper 147.
Chellemi, D.O., Olson, S.M., Mitchell, D.J., Secker, I., and McSorley, R.M. 1997. Adaptation of soil solarization to the integrated management of soilborne pests of tomato under humid conditions. Phytopathology 87:250-258.
Chellemi, D.O., Rhoads, F.M., Olson, S.M., Rich, J.R., Murray, D., Murray, G., and Sylvia, D.M. 1999. An alternative, low-input production system for fresh market tomatoes. Amer. J. Alternative Agric. 14:59-68.
Noling, J.W. and Gilreath, J.P. 2000. Methyl Bromide: Progress and Problems Identifying Alternatives. Pp. A3-A15 in `Citrus and Vegetable Magazine' June 2000. Vance Publishing Corp.