Proc. Fla. State Hort. Soc. 105:326-330. 1992. SWEET CORN WEED CONTROL EVALUATIONS ON MINERAL AND ORGANIC SOILS G. S. Crnko with atrazine, was the major herbicide used in sweet corn Seminole County Cooperative Extension Service grown in organic soils. Metolachlor, a related acetanilide IFAS, University of Florida herbicide, contains labeling restrictions for use on organic Sanford, FL 32773 soil (Stall, 1991). Pendimethalin has a label for use on sweet corn applied W. M. Stall preemergence or early postemergence in several states. Horticultural Sciences Department Sweet corn has been damaged when pendimethalin was IFAS, University of Florida applied preemergence in Florida. In initial studies pen Gainesville, FL 32611 dimethalin applied to sweet corn postemergence at the spike growth stage did not injure plants grown on mineral J. M. White soils. Central Florida Research and Education Center With the availability of limited preemergence her IFAS, University of Florida bicides, the use of postemergence herbicides may be in Sanford, FL 32771 creasing. Cultivation is used to control weeds between rows. However, grass and broadleaf weeds can become a Additional index words. Sweet corn, Zea mays var. rugosa, her great problem in the plant row. Sethoxydim and linuron bicides, metolachlor, pendimethalin, atrazine, bromoxynil, are both labeled for use as a directed spray. Tank-mixes pyridate, sethoxydim, linuron. of these materials had not been tested for control of both broadleaf weeds and grasses. Abstract Several herbicides were evaluated on a mineral soil Bromoxynil has a label for control of emerged broad in Gainesville and on an organic soil in Ze 11 wood, FL in 1991 leaf weeds on field corn and popcorn, but not on sweet and 1992 for weed control in sweet corn [Zea mays L. var. corn. The use of two new sulfonylurea herbicides, nicosul- rugosa Bonaf .)• Metolachlor and pendimethalin were applied fuion and primisulfuron is not recommended in sweet at several rates alone and tank-mixed with atrazine for corn due to the differential responses of sweet corn cul- preemergence control of weeds and evaluated for crop safety. tivars to these herbicides. Phytotoxicity with these two her Bromoxynil was applied alone and bromoxynil and pyridate bicides may also be enhanced when organophosphate in were tank-mixed with atrazine applied post over-the-top and secticides are used (Monks et al., 1992; Stall and Bewick, were evaluated for broadleaf weed control and crop safety. 1992). Sethoxydim, linuron and tank mixes of sethoxydim-linuron, The purpose of these studies was to evaluate potential and sethoxydim-atrazine were applied post-directed and herbicides for weed control and phytotoxicity on mineral were evaluated for crop safety and weed control. Metolachor and organic soils in Florida. and pendimethalin treatments did not cause crop toxicity and controlled grass weeds. Several broadleaf weeds were not effectively controlled season long, especially on the organic soils. Bromoxynil and pyridate applications effectively con Materials and Methods trolled the broadleaf weeds found in these fields with no Experiments on mineral soils were conducted at the damage to sweet corn. Sethoxydim, linuron and the tank Horticultural Unit, Gainesville, Florida during the Spring mixes applied post-directed did not provide any crop damage, of 1991 and 1992. Plots for both years were on 4 foot while controlling very large weeds. center beds in a randomized complete block design on a Pomona sand with 1.2 and 2.2 percent organic matter and Florida is the leading state in the nation in fresh market a pH of 6.5 and 6.8, respectively. sweet corn production. In the 1990-91 crop season, 51,700 Sweet corn 'SS-8701' and 'Dazzle' were planted 11 Apr. acres of sweet corn were planted in Florida. This is down 1991 and 6 Apr. 1992, respectively. Plots were fertilized at from 58-59,000 acres planted the previous three years planting and sidedressed once at a rate of 91-29-91 and (Freie and Young, 1991). 30-0-28, (N-P-K) pounds per acre, respectively. Overhead Control of weeds is a major problem in sweet corn pro irrigation was used as needed. duction. An average annual loss of potential production of Experiments on organic soils were carried out in grow 10% has been estimated per year in sweet corn production ers fields in the Zellwood, FL, area. The 1991 trials were in the southeast (Losses due to weeds committee, 1984). on the Crakes Farms and in 1992 on the Lust Farms. The This loss may be higher in Florida due to year round weed soil at both sites was a Lauderhill muck (approx 60% OM) growth and the varying soil types. with a pH of 6.8. 'Showcase' seed was planted 25 Mar. Most of the sweet corn in Florida is grown on the or 1991 and 'Ultimate' 11 Mar. 1992 on 3 foot row centers in ganic soils of the Everglades and the Zellwood areas. Her a randomized complete block design. Maintenance of plots bicides cleared for use for preemergence control of ger was done according to grower practice except no cultiva minating annuals, especially grasses are more restricted for tion was used. Seep irrigation was used in both years. organic soils than mineral soils. Herbicides were applied using a CO2 powered back Alachlor has been restricted from use in Florida due to pack sprayer calibrated to deliver 30 gpa. In the mineral confirmed ground water contamination. Alachlor along soil, a 2 nozzle boom at 20 inches between nozzles were used. In the organic soil, the nozzles were on 18 inches Florida Agricultural Experiment Station Journal Series No. N-00750. centers. Post-directed sprays were applied with a single 326 Proc. Fla. State Hort. Soc. 105: 1992. Table 3. Effects of postemergence herbicides on vigor and weed control in sweet corn in mineral soils, Gainesville, FL 1991-92. Ratings (%)z 6-10-91 Rate Applic 6-15-92 Herbicide lb/A Timing Vigor AMACH^ RAPRA Vigor AMACH CHEAL DIGSP Hoed Check 77 90 92 80 100 100 100 Weedy Check 75 0 0 78 0 0 0 Bromoxynil .375 EPOST? — — — 93 98 100 0 Bromoxynil .375 EPOST — — — 93 98 100 0 +Atrazine 1.0 Pyridate .45 EPOST 55 95 90 95 100 100 0 +Atrazine + COC 1 Pyridate .7 EPOST 77 100 100 95 98 100 0 +Atrazine + COC 1 Pyridate .9 EPOST 72 100 100 88 100 100 0 +Atrazine + COC .2 Sethoxydim .2 POST 62 0 0 85 0 0 80 +COC DIR Sethoxydim .2 POST 82 90 95 80 70 50 60 +Atrazine + COC 1 DIR Sethoxydim .2 POST 90 92 93 98 95 63 63 +Linuron + COC 1 DIR Linuron 1 POST 89 93 94 93 100 59 53 + COC DIR LSD (.05) 25 12 19 18 24 8 19 zRatings are 0 = no vigor (dead) or no control to 100 where no loss of vigor and complete control. See Table 1. y(AMACH) smooth pigweed, (RAPRA) wild radish, (CHEAL) lambsquarter, (DIGSP) crabgrass. "Application - (EPOST) early postemergence, (Post DIR) post directed. plus atrazine treatments. Good control of smooth pigweed Control of purslane was significantly lower with was obtained even though the applications were not made bromoxynil alone than with bromoxynil and pyridate + over the tops of these weeds (Table 4). atrazine at Zellwood in 1992. Sethoxydim did not control, The bromoxynil and all 3 rates of pyridate plus atrazine nor was expected to control, purslane, but atrazine + provided excellent control of both smooth pigweed and sethoxydim and linuron treatments provided excellent lambsquarter at Gainesville in 1992. The post-directed control of this weed. linuron also provided excellent control of smooth pigweed Bromoxynil, bromoxynil + atrazine and all 3 rates of but control of lambsquarter was only fair. Both weed pyridate + atrazine provided no or poor control of crab- species were sprayed along the side of the plants at the grass and barnyardgrass in 1992. post directed stage. Smooth pigweed was killed, but Crabgrass was tall and extremely thick at Gainesville in lambsquarter was only partially controlled by this applica 1992. Control of crabgrass with sethoxydim and the tion. sethoxydim tank mix was better than with linuron alone. Table 4. Effects of postemergence herbicides on vigor and weed control in sweet corn in organic soils, Gainesville, FL 1991-92. Ratings (%)z 5-30-91 5-14-92 Herbicide lb/A Timing Vigor AMACHy PANDI Vigor POROL ECHCG Weedy Check 70 0 0 100 0 0 Bromoxynil .375 EPOSTX — — — 88 73 20 Bromoxynil .375 EPOST — — — 90 95 43 +Atrazine 1.0 Pyridate .45 EPOST 75 73 78 98 98 50 +Atrazine + COC 1 Pyridate .7 EPOST — — — 88 94 50 +Atrazine + COC 1 Pyridate .9 EPOST — — — 86 94 45 +Atrazine + COC 1 Sethoxydim .2 POST 75 0 95 89 0 100 4-por Sethoxydim .2 POST 83 33 84 96 94 89 +Atrazine + COC 2 DIR Sethoxydim .2 POST 78 70 98 100 99 99 + Linuron + COC 1 DIR Linuron 1 POST 65 78 88 100 98 78 + COC DIR LSD (.05) 13 25 18 10 14 34 zRatings are 0 = no vigor (dead) or no control to 100 where no loss of vigor and complete control. See Table 1. y(AMACH) smooth pigweed, (PANDI) fall pancum, (POROL) puslane, (ECHCG) barnyardgrass. "Application - (EPOST) early postemergence, (Post DIR) post directed. Proc. Fla. State Hort. Soc. 105: 1992. 329 Reduction of control in these cases was primarily due to atrazine also increased its broadleaf activity. Application of lack of coverage and the size of the plants at treatment. At pendimethalin at the spike stage was safe and efficacious Zellwood in 1992, barnyardgrass was controlled very well, on mineral soils. On organic soils, preplant incorporation even though these grasses were very tall at the time of and preemergence applications should be tested.