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.
