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Potato Response to Post-Emergence Herbicides in Cool Weather
K.A. Rykbost l and K. Locket
INTRODUCTION
An experimental post-emergence herbicide with reduced toxicity to herbicide
sensitive potato varieties and improved activity against nightshade species is in
advanced stages of evaluation. The product, Matrix, combines the numbered
compound, E9636, with low rates of metribuzin. The product was evaluated at KES
in 1992 on the metribuzin-sensitive Shepody variety. Excellent control of nightshade,
Indian lovegrass, and redroot pigweed was achieved with little crop injury. A 1993
study evaluated crop injury potential for this product when application coincided
with near-frost conditions, a common occurrence in the Klamath Basin.
PROCEDURES
Shepody potatoes were planted at 8.7-inch seed spacing in 32-inch rows June 9.
Standard fertilizer, and disease and insect control practices were followed. Preemergence herbicides applied on June 15 included Prowl at 0.75 lb ai/A and Dual
at 1.5 lb ai/A. Four row plots, 20 feet long were established to accommodate 10
treatments and four replications in a randomized complete block design. Treatments
included an untreated control and three herbicide combinations applied the day a
frost or near frost was expected, the day following a frost or near frost, and several
days after a frost or near frost, when minimum air temperatures would remain above
40 °F. Herbicide combinations included: E9636 at 0.25 oz ai/A + Lexone at 2.0 oz
ai/A; E9636 at 0.50 oz ai/A + Lexone at 4.0 oz ai/A; and E9636 alone at 0.50 oz
ai/A. Treatments were applied with a backpack sprayer at 20 gpa of solution.
Treatment dates were July 15 (minimum air temperature at canopy level was 31 °F
at 6:00 am on July 15), July 16, and July 26 (minimum air temperature at canopy
level was 40 °F on July 27). Moderate pre-emergence herbicide injury was
experienced and pre-treatment crop stress was evident in approximately 50 percent
of plants prior to application of post-emergence treatments.
Weather conditions from July 14 to July 24 were cool and overcast through
much of the period. Minimum air temperatures recorded at the KES weather station
reached 34 or 35 °F on July 15, 16, 17, 18, and 22. Maximum air temperatures were
65, 65, 69, 70, and 70 °F on these dates, respectively. Rainfall ranging from a trace
to 0.04 inches was recorded on 4 days during the period. Sprinkler irrigation was
used for frost protection on July 15.
/ Superintendent/Professor, Klamath Experiment Station, Klamath Falls, OR.
2/ Klamath County Cooperative Extension Agent, Klamath Falls, OR.
1
Acknowledgment: E.I. du Pont de Nemours and Co., Inc. provided the experimental
herbicide E9636 and partial funding. Partial funding was also provided by the
Oregon Potato Commission.
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RESULTS AND DISCUSSION
Serious crop injury was experienced in over 50 percent of the study, with plant
death the end result. The injury was first evident shortly after emergence, indicating
that the pre-emergence herbicides, Dual and Prowl, were at least partially
responsible. Plants were not beyond hope of recovery in mid-July, and treatments
were applied as planned. All treatments applied on July 15 and 16 resulted in plant
death when prior crop injury was evident. Plants that were relatively vigorous prior
to treatment were not injured by any of the treatments applied on July 26. Due to
the confounding influence of injury from pre-emergence herbicides, the study was
abandoned and no data was obtained. However, the difference in plant response to
herbicides applied at near-frost conditions on July 15 and 16, and at warmer
conditions on July 26, demonstrated that the risk of crop injury from post-emergence
herbicides is much higher when plants are subjected to near-frost minimum
temperatures.