James W. Shrefler

Effects of Halosulfuron on Weed Control in Commercial Honeydew Crops1

Studies were conducted at four sites during a 2-yr period in Oklahoma, Texas, and Arkansas to determine effectiveness and safety of halosulfuron in honeydew crops. Halosulfuron applied postemergence at 26.3 to 78.8 g ai/ha controlled yellow nutsedge 85 to 97%, golden crownbeard 100%, and tumble pigweed 83 to 95%. Control of yellow nutsedge continued to increase for 3 to 6 wk after treatment. Golden crownbeard and tumble pigweed efficacy increased to its highest levels after 4 and 3 wk, respectively. Reduced crop growth and yellowing of foliage did not exceed 13%. No differences were recorded for yield, earliness, or percentage of marketable fruit. Nomenclature: Halosulfuron; golden crownbeard, Verbesina encelioides (Cav.) Benth. & Hook. f. ex Gray #3 VEEEN; tumble pigweed, Amaranthus albus L. # AMAAL; yellow nutsedge, Cyperus esculentus L. # CYPES; honeydew, Cucumus melo L. Inodorus group ‘Honeybrew’. Additional index words: Melon crops, mulch, plasticulture, plastic mulch. Abbreviations: POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

POST Weed Control Using Halosulfuron in Direct-Seeded Watermelon

Watermelon growers rotate crops to prevent problems, but weed populations in new fields may hold unexpected control challenges. Having effective POST herbicides would provide growers an opportunity to respond to emerging weeds on an as-needed basis. To address this need, field studies were conducted over 4 yr in Oklahoma to determine efficacy and crop response of POST halosulfuron applications to direct-seeded watermelon that received PRE application of ethalfluralin at 840 g/ha. At 5 wk after crop emergence (WAE) halosulfuron was applied at 18, 27, 36, and 54 g/ha. The 27 g/ha rate was also applied at 1, 2, 3 and 7 WAE. Halosulfuron applications made 5 WAE did not provide acceptable (> 80%) control of pigweeds and cutleaf groundcherry regardless of rate. Applications made 1 WAE provided significantly better control of pigweeds and cutleaf groundcherry than did later applications. Halosulfuron treatments of 36 and 54 g/ha made 5 WAE and of 27 g/ha made 1, 2 and 3 WAE did not result in significant yield increases compared with the hand-weeded check. These studies show that POST halosulfuron application may be a useful treatment for direct-seeded watermelon. This option would enable more judicious use of herbicides and possible reduction in production costs. Nomenclature: Halosulfuron; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; tumble pigweed, Amaranthus albus L. AMAAL; cutleaf groundcherry, Physalis angulata L. PHYAN; eclipta, Eclipta prostrata L. ECLAL; watermelon, Citrullus lanatus (Thunb.) Matsumura & Nakai var. lanatus ‘Jubilee’, ‘XIT 101’.

Injury Potential from Carryover of Watermelon Herbicide Residues

Studies were conducted to determine injury potential to rotational crops from carryover of herbicides used in watermelon production. Treatments included halosulfuron, ethalfluralin, and sulfentrazone alone; halosulfuron in tank mixtures with bensulide, clomazone, ethalfluralin, and naptalam; and a tank mixture of naptalam and bensulide. Sulfentrazone applied at 224 g ai/ha to watermelon severely reduced spinach emergence, but did not reduce emergence of broccoli, cabbage, or wheat. Residues of sulfentrazone applied to watermelon at 450 g/ha stunted growth of broccoli and cabbage and was the only treatment that reduced wheat stand. Injury to broccoli, cabbage, and spinach increased as the halosulfuron rate increased. Ethalfluralin did not reduce stand or cause injury to any of the four rotational crops. Naptalam plus bensulide did not reduce stand of the four crops and caused either slight or no injury. Residues of sulfentrazone and halosulfuron can injure vegetables following crops in which these herbicides are used, and caution should be taken particularly with spinach, broccoli, and cabbage in this respect. Nomenclature: Bensulide; clomazone; ethalfluralin; halosulfuron; naptalam; sulfentrazone; broccoli, Brassica oleracea var. botrytis (L.) ‘Everest’, ‘Green Sprouting Calabrese’; cabbage, Brassica oleracea var. capitata (L.) ‘Early Jersey Wakefield’; spinach, Spinacia oleracea (L.) ‘Cypress’, ‘F-380’; watermelon, Citrullus lanatus (Thunb.) ‘Jubilee’, ‘XIT 101’; hard red winter wheat, Triticum aestivum (L.) ‘Jagger’.

Preemergence Weed Control in Direct-Seeded Watermelon1

Studies were conducted at eight sites during a 3-yr period in Oklahoma and Arkansas to determine the effectiveness and safety of preemergence applications of halosulfuron both alone and in tank mixtures with bensulide, clomazone, ethalfluralin, and naptalam. Ethalfluralin, naptalam plus bensulide, and sulfentrazone also were applied alone. Although halosulfuron caused up to 20% seedling stunting, watermelon plants recovered by 5 to 7 wk after planting, and yield was similar to that of hand-weeded plots. Halosulfuron treatments controlled hophornbeam copperleaf, Palmer amaranth, carpetweed, and cutleaf groundcherry 80 to 100%. Control of goosegrass was at least 97% with clomazone plus ethalfluralin plus halosulfuron. Injury to watermelon treated with sulfentrazone ranged from 76 to 98% at 2 to 4 wk after treatment. This was reflected by yields that were lower than any other herbicide treatment in the studies. Nomenclature: Bensulide; clomazone; ethalfluralin; halosulfuron; naptalam; sulfentrazone; hophornbeam copperleaf, Acalypha ostryifolia Riddell #3 ACCOS; Palmer amaranth, Amaranthus palmeri S. Wats. # AMAPA; goosegrass, Eleusine indica (L.) Gaertn. # ELEIN; carpetweed, Mollugo verticillata L. # MOLVE; cutleaf groundcherry, Physalis angulata L. # PHYAN; watermelon, Citrullus lanatus ‘Jubilee’, ‘XIT 101’, ‘Crimson Sweet’. Additional index words: Broadleaf weed control, watermelon injury. Abbreviations: PRE, preemergence; WAT, weeks after treatment.

Organic Weed Control

Organic vegetable producers rank weeds as one of their most troublesome, time consuming, and costly production problems (OFRF, 1999). Because there are only a few organically approved herbicides, optimizing their application may increase their potential usefulness for organic production systems. Inter-row cultivation for the purpose of weed control is not always the ideal choice for organic vegetable production or due to soil and weather conditions may not always be an available option. Additional cultivations can decrease soil organic matter (Dick, 1983; Gallaher & Ferrer, 1987; Johnson, 1986) and soil water holding capacity (Johnson, 1986), increase soil erosion (Logan et al., 1991; McDowell & McGregor, 1984) and nutrient loss (McDowell and McGregor, 1984), and stimulate new weed growth (Pekrun et al., 2003). In conventional, non-organic production systems, herbicides are increasingly used to avoid the detrimental impact of soil erosion from weed control from cultivation. Preventing soil, nutrient, and organic matter losses due to tillage are a fundamental tenant of certified organic production (USDA National Organic Standards Board, 2010).