E. N. Rosskopf, Microbiologist/Weed Scientist
USDA-ARS, U.S. Horticultural Research Laboratory
2001 South Rock Road, Ft. Pierce, FL, USA 34945
ERosskopf@ushrl.ars.usda.gov
D.J. Mitchell, M.E. Kannwisher-Mitchell, and S.J. Locascio, University of Florida, Gainesville.
J.P. Gilreath, University of Florida, Bradenton.
S.M. Olson, University of Florida, Quincy.

Abstract

The loss of methyl bromide is a substantial threat to the raised-bed, plastic mulch strawberry production system used in Florida. Field trials were conducted to determine the suitability of alternative fumigants and soil solarization for strawberry production. 'Chandler' strawberries were obtained as bare root and plug transplants. Root material from each type of transplant was plated onto selective media to determine the extent of fungal colonization prior to field planting. Plug transplants carried higher numbers of root colonizing fungi than did bare root transplants. Methyl bromide:chloropicrin and metam sodium treatments resulted in higher yields than those found in the untreated check and plots treated with soil solarization alone. Solarization combined with 1,3-dichloropropene (1,3-D) or metam sodium and 1,3-D were statistically similar to those achieved in methyl bromide plots. Yields from bare roots showed similar trends.

Introduction

Methyl bromide is a low weight, volatile, organic compound that is highly toxic to most living organisms. In agriculture, one of it's principal uses is as a fumigant to disinfest soil prior to the planting of high value crops. Methyl bromide was listed as a Class 1 ozone depleting substance in 1992, resulting in the need to phase out use in agricultural applications. The final methyl bromide phase-out date in the U.S. has been designated as 2005. Methyl bromide production and importation will be reduced from levels as follows: 25% reduction in 1999, 50% reduction in 2001, 70% reduction in 2003, and 100% reduction in 2005. In the U.S., producers of fresh market vegetables and fruits including tomato, pepper, and strawberry utilize production systems that are highly dependent upon methyl bromide for soil disinfestation. Florida is the second largest producer of fresh market strawberries in the United states. During the 1997-98 season the crop was produced on 2,511 ha valued at $161.2 million with an average value of $64,000 per hectare (Witzig and Pugh, 1999). This high value per hectare is dependent on an intensive culture system of soil fumigation, use of polyethylene mulch, fertigation, foliar pest control, superior strawberry varieties, and excellent management. In the production of polyethylene-mulched strawberry, the use of preplant complete soil fumigant provides control of soil pathogens, weeds, and nematodes. Without the control of soil pest and weeds, strawberry yields would be expected to be seriously reduced. Studies reported here were conducted to evaluate several soil fumigants and soil solarization as alternative to methyl bromide for polyethylene-mulched strawberry.

Materials and Methods

Field studies were begun in the fall of 1998 at Gainesville on a Millhopper fine sand and at Bradenton on an EauGallie fine sand with 'Chandler' strawberry. At Gainesville, beds 1.2-m apart were fertilized with 56-25-45 kg N-P-K/ha applied broadcast, fumigants were applied, and solarization treatments were initiated approximately 10 weeks prior to planting. On selected plots, napropamide was applied at 4.5 kg/ha for weed control, metam-Na at 589 L/ha as sprays on the bed, and dazomet (granules) at 448 kg/ha was surface applied on the bed. All were rototilled into the bed. Soil fumigants were injected with two chisels per bed spaced 0.3-m apart to a depth of 20 to 25 cm at 392 kg/ha 67 % MBr + 33 % Pic, 336 kg/ha Pic, 327 L/ha C-35, and 112 L/ha 1,3-D. Drip tubing was placed on the beds before application of 1.5 mil black polyethylene mulch or 3 mil clear thermal infrared-absorbing film (AT Plastics, Inc., Edmonton, Canada). The clear mulches were painted with black latex paint a week before transplanting. Plots were 1.2m x 11m with 3-m of bed planted with two rows per bed 31 cm apart and 31-cm within the rows to 'Chandler' transplants. Double-wall drip tubing (Chapin Twinwall, Watertown, N. Y.) was placed between the two plant rows for irrigation. Additional N-P-K fertilizer was applied with drip irrigation at 5.6-0-5 kg/ha for four applications per month for 5 months. At Bradenton, 81-cm-wide beds spaced 1.5-m apart were fertilized with 56-25-47 kg N-P-K/ha before application of fumigants and herbicides as listed above except dazomet, which was not applied. MBR-Pic, Pic, and 1,3-D were applied through 3 chisels in the bed. Metam-Na and napropamide were applied as above.

Transplants were sampled for fungal colonization of roots prior to planting. Plants were washed, and roots and crowns were removed. Two crown pieces and eight roots from each of 25 root systems were surface sterilized by immersion for 2 min. in a 3 % bleach (Clorox) solution followed by two rinses in sterile deionized water. The tissue was blotted dry and placed on potato dextrose agar (PDA) supplemented with 0.1 ml of tergitol NPX, 10 mg of rifampicin, and 250 mg of ampicillin per liter of medium (PDA+TRA). Root samples were also plated on a medium selective for species of Phytophthora and Pythium fungal colonies were identified to the genus level using taxonomic keys.

Plants were sampled for root disease and incidence of colonization of roots by fungi at the end of the harvest season. At Gainesville, five plants collected from each of the five repetitions of the control and treatments with MBr-Pic and C-35 were washed in tap water, weighed for fresh root weight, and rated for root disease. Samples consisting of two pieces of crown tissue, three lateral roots, and five terminal roots from each plant were surface disinfested, plated on PDA+TRA. Isolates of fungi were identified based on various taxonomic keys.

At Bradenton, three plants were sampled from each replicate of each treatment. Samples were washed, rated for root condition, and root fresh weights were recorded. Nine root samples from each plant were plated onto three selective media in a manner similar to that used for the transplants. Incidence of fungi was then recorded on a per plant basis.

Results and Discussion

Incidence of fungi isolated from containerized plug transplants was generally higher than in the bare-rooted transplants. Although some species of Alternaria, Colletotrichum, Fusarium, Phoma, and Pythium may cause root diseases of strawberry, it was not possible to determine if fungi detected in transplants contributed to the source of pathogens isolated from roots during the growing season and at harvest.

The low incidences of potentially important pathogens, such as species of Colletotrichum and Pythium, indicated that the bare-root transplants were not significantly infested with root pathogens. Conversely, containerized transplants had a high incidence of Colletotrichum spp., Fusarium oxysporum, and Phytophthora cactorum. Although these fungi could contribute to disease symptoms observed during the production season, no pathogenicity tests were performed. Therefore, no correlation can be made. The high incidence of Trichoderma spp. may have been beneficial to plants by providing competition to the pathogenic fungi after planting.

In the Bradenton trial, strawberry yields from all treatments were statistically similar when plug plants were used (Table 4). In bare-root plantings, yields from all treatments, except solarization without fumigant, were similar to those from methyl bromide-treated plots. Only the treatments composed of Chloropicrin/Devrinol and Vapam/Telone II/Devrinol resulted in yields that were significantly higher than the untreated check.

References

Witzig, J.D. and N.L. Pugh. 1999. Florida Agricultural Statistics. Vegetable Summary. FL Dept. of Agric. and Consumer Services, Tallahassee, FL.