Evaluating Methyl Bromide Alternative Fumigants on Tomato Under Polyethylene Mulch in Florida

D. W. Dickson¹, S. J. Locascio², and, D. J. Mitchell ³. ¹Departments of Entomology and Nematology, ² Horticultural Sciences, and ³Plant Pathology, University of Florida, Gainesville, FL 32611–0620.

Our objective over the past 5 years has been to evaluate the most promising alternative multi-purpose soil fumigants. Comparisons are made against two formulations of methyl bromide (MeBr 98–2 and 67–33). The trials are conducted in fields infested with root-knot nematodes, soilborne diseases, and weeds. The test systems include tomatoes grown in drip irrigated polyethylene mulch culture. The most promising alternative soil fumigants identified to date include 1,3 dichloro-propene (Telone^®) combined with either chloropricin (pic) at 17 percent (C17) or 35 percent (C35), or metam sodium (Vapam^®) combined with pic or 1,3–dicloropropene (1,3–D). However, to date, there has been little work on the latter combination. When either 1,3–D or metam sodium are applied singly or in combination with pic, a herbicide must be used. The only herbicide available for nutgrass suppression is pebulate (Tillam^®).

In past work, metam sodium, applied as a single treatment, has not provided acceptable nematode control, whereas C17 at 35 gal./acre + 4 lb./acre of pebulate has in most cases provided good control of plant-parasitic nematodes, some soil fungal diseases, and suppressed nutsedge in polyethylene mulched tomato. Consequently, the latter treatment is currently considered the best alternative for methyl bromide. However, our biggest data gaps regarding the use of these fumigants as substitutes for MeBr are the lack of performance data in fields infested with diseases such as fusarium wilt, fusarium crown rot, bacterial wilt, and southern blight, and seedling diseases caused by Pythium and Rhizoctonia. The latter diseases will affect plant stands.

The following is a summary of some MeBr alternative trials conducted at the University of Florida, Gainesville, during the fall of 1997 and spring, 1998. We tested the alternative fumigants at two sites: the Horticultural Unit (Millhopper fine sand) and the Agronomy Unit (Arredondo fine sand). The treatments were applied with standard industry equipment. We injected the fumigants 10 inches deep with 3 chisels per bed into 3-ft-wide beds (30-ft long). MeBr 98–2 was tested at 400 lbs./acre, 67–33 at 350 lbs./acre, C17 at 35 gal./acre, C35 at 18, 24, 30, and 36 gal./acre, and metam sodium (42 percent formulation) at 75 gal./acre. Pic applied in combination with metam sodium was tested at 75, 100, and 150 lbs./acre.

We sprayed metam sodium and pebulate over the bed surface, incorporating 4 to 6 inches deep into the soil. The treatments were immediately covered with polyethylene mulch. Irrigation was applied via drip tubing before transplanting on some metam sodium plots in an effort to enhance fumigant activity. All treatments were tested in an approved statistical field design and replicated 5 to 6 times. Solar-Set and Agriset 761 tomato seedlings were transplanted for the fall and spring trials, respectively. We counted purple and yellow nutsedge seedlings that grew through the mulch on the side of the bed where the drip irrigation tubing was placed. After harvest, six tomato plants were dug, and roots were rated for the presence of root-knot nematode galling. We also made a count of plants infected by soilborne diseases, southern blight or bacterial wilt.

In the fall study (1997), marketable yields with MeBr, C35 (24 gal./acre), and metam sodium + pic were significantly higher than with the check. Yield with metam sodium alone was similar to that with the check. Additional water applied before transplanting with metam sodium did not increase yield over treatment with metam sodium that had no additional irrigation water. Nutsedge was controlled with MeBr and all treatments with pebulate. Metam sodium alone or with pic but without pebulate did not reduce nutgrass. Nematode root-gall ratings were high on tomato grown without fumigants, low with MeBr, and intermediate with all other treatments except with the low rate of C35 (18 gal./acre).

In the 1998 spring trials, yields at the Hort Unit were highest with MeBr, C35 (30 gal./acre), and C17 (35 gal./acre) and were significantly lower in the untreated control. Yields with other treatments were not different from the untreated control. Nematode root-gall ratings were lowest in plots treated with MeBr, C35 and C17, and significantly higher with the untreated control. Root-gall ratings were significantly higher with all metam sodium treatments and were not different from the untreated control. Nutsedge control was excellent with MeBr, metam sodium + pebulate, and C17 and C35 + pebulate treatments. The addition of pic with the metam sodium + pebulate resulted in nutsedge counts that were similar to that with the check treatment.

At the Agronomy Unit, 42 percent of the plants in the untreated control plots were diseased (bacterial wilt or southern blight). In the treated plots, the number of diseased plants ranged from 0=metam sodium + pic (75 gal. + 150 lbs./acre), and C35 (36 gal./acre) to 13 percent=metam sodium alone. Other plots were intermediate between these percentages. None of the metam sodium treatments reduced root-knot nematode galling compared with the untreated control. This is consistent with previous data. Also, the lower rate of C35 (24 gal.) did not result in a significant reduction in root galling. The remaining treatments resulted in 85 percent to 90 percent reduction in root galling. The MeBr treatments, C17 and C35 treatments (except C35 at low rate), and metam sodium + pic + pebulate increased yields compared to the untreated control.

In summary, the combinations of various rates of pic with metam sodium have given inconsistent results. In the fall study, this treatment provided fruit yields similar to that with MeBr, but in the two spring studies, the treatment combinations looked good in only one treatment. It is important to point out that neither pic, metam sodium nor combinations of the two reduced root-knot nematode galling on plants at harvest. As in past studies, C17 and C35 + pebulate have given the most consistent results when compared with MeBr. C35 was most effective at 30 to 36 gal./acre. These higher rates generally provide control of root-knot nematodes similar to that provided by MeBr.

Last Updated: January 12, 1999