Jan 2004


The Effect of Herbicide Treatment on Direct-Seeded Watermelon: 2003

Russell W. Wallace
Extension Vegetable Specialist
Dept. of Horticultural Sciences
Texas A & M University—Lubbock

A study was conducted to evaluate and compare the efficacy of postemergence herbicides on crop injury and yield to watermelons grown on the Texas High Plains.

The trial was conducted at the Texas A & M University Agricultural Research & Extension center located in Halfway on a Pullman clay loam soil with an average pH of 7.6 and 1.0% O.M. The trial site was plowed and prepared by applying a pre-plant fertilizer (60 lbs/A nitrogen) and then disking the soil. Watermelon (var. “Legacy”) seeds were planted by hand (3-4 seeds/hill) on May 21 at a distance of three feet in single row plots measuring 8’ x 30’. Plants were later thinned to two plants per hill. All herbicides were applies using a CO2- backpack sprayer equipped with a hand-held boom containing four Teejet 8002VS nozzles that sprayed at a rate of 20 GPA at 30 PSI. Plots were irrigated overhead as needed during the season. Treatments were compared to the hand weeded control. All disease and insect management practices were followed as needed to maintain pests. The plots were harvested by hand on August 18 and weighed accordingly. The experimental design was a randomized complete block with 3 replications.

Crop injury in the form of stunting recorded on June 19 was greatest in treatments of sulfentrazone (0.10 lb a.i.) and Aim 2EC applied with a directed-hood spray. Sulfentrazone injury resulted from preemergence applications, while that of Aim occurred as a result of post-directed sprays. The stunting associated with Aim treatments was likely a result of the leaf necrosis that occurred soon after those treatments were applied. Leaf necrosis ratings recorded June 19 showed that Aim treatments had significantly higher injury compared to all other treatments. The leaf necrosis likely occurred as a result of drift from under the hooded spray during periods of gusty winds.

Weeds present in the trial site included Palmer amaranth (pigweed) and common purslane. Control of pigweed was greatest (90%+) in treatments that included Prefar + Sandea, Dual Magnum, Outlook, Spartan ( 0.1 lb a.i.), and Dual Magnum + Aim. Poor control was observed with PRE applications of Curbit + Command, Define, Spartan (0.05 lb a.i.) and EPOST applications of Trifluralin + Sandea, or Curbit + Command (PRE) + Sandea. Control of common purslane generally followed the same trend as those treatments for pigweed with few exceptions. Poor control of common purslane was observed with Dual Magnum applied alone, Spartan, and trifluralin or trifluralin + Sandea treatments. No control was observed in plots treated PRE with Define.

As expected, yields were lowest where no weeds were controlled in the check and were the highest in the hand weeded control. Yields were reduced an average of 60% in plots treated with Define (PRE) and trifluralin + Sandea (E-POASR-Direct), most likely the result of poor weed control. Significant yield reductions also occurred in plots treated with Prefar + Sandea (EPOST-Direct) although good weed control was observed. Trifluralin and Trifluralin + Sandea (EPOST-Direct) applications failed to adequat6ely control both weed species resulting in an average of 51% yield reduction.

Preemergence applications of Sandea and Outlook gave good to excellent weed control in this study. Yields from these plots were not significantly different for the hand weeded check. Likewise the combination of Curbit + Command (PRE) + Sandea (EPOST-Direct), Prefar or Dual Magnum (PRE) followed with Aim (EPOST-Hooded) did not result in significant yield reductions. A trend to reduced yields with Prefar was seen. Although significant stunting occurred with the high rate, Spartan did not reduce yields.

The results of this study indicate the potential of several new herbicides for use in watermelon including Outlook, Spartan, and Aim as long as care is taken using hooded application. More research is needed to evaluate these herbicides at selected rates and timings in order to improve weed control in direct-seeded watermelons.