Evaluation of Milestone Herbicide for Early Post-emergence Weed Control in Citrus on the Yuma Mesa

David L. Kerns, Assistant Specialist, Entomology, Yuma Agricultural Center
Tony Tellez, Research Specialist, Yuma Agricultural Center

Abstract

When applied early-postemergence, Milestone at 0.75 and 1.0 lbs-ai/A provided excellent control of summer annual grasses and broadleaf weed for as long as 4 months, equivalent to Karmex. Princep and Krovar also provided good weed control but appeared to offer slightly shorter residual against some broadleaf species. Solicam, although it provided good weed control, particularly of the grasses, was the weakest material evaluated in this trial. Although Karmex is notorious for causing crop injury on the sandy soils of the Yuma Mesa, we did not detect any injury from this herbicide or any others in this trial.

Introduction

Weed control in citrus grown in Arizona is an important consideration for every citrus grower. Not only do weeds rob trees of water and nutrients, but many also harbor serious insect pests such as citrus thrips and eye gnats. It is not a question as to whether weeds in citrus should be managed, but how. Currently in Yuma most citrus growers control weed through mechanical cultivation and Round-up. Mechanical cultivation can be an effective tool for weed control, but presents several problems. Frequent cultivation is necessary to maintain weed densities at acceptable levels. Also, damaging the trees by striking the trunk is of concern, particularly on small trees. Cultivation can also damage trees by destroying shallow roots. Additionally, disking weeds into the soil provides a breeding ground eye gnats. Presently, to avoid eye gnat problems, Yuma County citrus growers are offered a financial incentive not to disk under weeds. Round-up is by far the most popular alternative to cultivation, however many growers are concerned with chronic phytotoxicity problems from drift of Round-up on the lower leaves of the trees. The only other alternatives to cultivation and Round-up for weed control in citrus are the pre-emergence herbicides. Although several pre-emergence herbicides have proved effective in Yuma citrus, many growers have chosen to avoid these materials. Most citrus in Yuma County is grown on extremely sandy soil, which increases the probability of crop injury from herbicide. In the past, growers have reported crop injury when using Karmex, Krovar or Princep. Concerns of crop injury with Karmex have resulted in this product being made unavailable for use in Yuma County. Although other products such as Solicam and Surflan, have not been associated with extensive crop injury in Yuma County, many growers choose not to use these products due to the cost of the material, short residual on sand, or lack of fit due to weed spectrum. There is great potential for a long lasting citrus herbicide that has a broad spectrum of activity on sand, and a minimal risk of crop injury.

Milestone (azafenidin) is a new herbicide being developed by DuPont. Milestone is a unique chemistry that acts by inhibiting the enzyme protoporphyrinogen oxidase which is involved in the biosynthesis of chlorophyll. Milestone has primarily been tested pre-emergence, is used at low rates, and has proven to offer 4 to 10 months control of a variety of annual broadleaf and grass weeds with little risk of crop injury. The purpose of this study was to investigate the efficacy and crop safety of Milestone when used early post-emergence on flood irrigated lemons grown on sandy soil on the Yuma Mesa.

Materials and Methods

This test was conducted on three-year old 'Limoneira 8A Lisbon' lemons on C. volkameriana rootstock grown at the University of Arizona, Yuma Mesa Agricultural Center in Yuma, AZ. Artificially seeded and naturally occurring weeds were evaluated. Artificially seed weeds included two annual broadleaf weeds: redroot pigweed (AMARE) and common purslane (POROL), and several annual grasses: green foxtail (SETVI), green sprangletop (LEFDU) and field sandburr (CCHIN). Naturally occurring weeds included broadleaf weeds: hemp sesbania (SEBEX), horse purslane (TRTPO) and hyssop spurge (EPHHS), and sixweeks grama grass (BOBBA). The test site was cultivated clean of standing weed 2 days before seeding with weeds. Artificially seeded weeds were broadcast over the test area on 04/23/97 and lightly incorporated to 0.5 inches using a rotor-tiller. Following seeding, this trial was irrigated every other week with about 3 inches of water per acre.

This trial was a randomized complete block design with 4 replicates. Treatments included an untreated check, Milestone at 0.5, 0.75 and 1.0 lbs-ai/A, Karmex DF at 3.0 lbs-ai/A, Princep 80W at 3.00 lbs-ai/A and Solicam at 4.00 lbs-ai/A. All treatments include Kinetic non-ionic surfactant at 0.25% v/v. On 05/05/97 when the herbicide applications were made the weed were 0.25-4.0 inches in height. Treatments were applied using a 7 ft. wide CO2 powered hand-boom sprayer with flat-fan nozzles calibrated to deliver 20.00 gallons per acre. Plots were 10 ft wide X 30 ft in length. Each plot was bordered on one side by a lemon tree and on the other side by a 3 ft running check. Evaluations were made by rating the % weed control and % crop phytotoxicity at 7, 14, 30, 60, 120, 240 and 360 days after treatment.

Results and Discussion

Seven days following application, Milestone at all three rates, Karmex and Krovar exhibited excellent activity on all broadleaf and grass weeds present (Table 1). Princep appeared to have slightly slower activity on CCHIN, and Solicam was weaker across all weed species. Although many weeds in the Solicam treated plots were alive, most showed signs of poisoning.

By 14 DAT, all products appeared equally effective towards the grasses, but Solicam appeared slightly weaker towards the broadleaf weeds except AMARE ((Table 2). Lower values in the Solicam treated plot may be slightly misleading, because one plot appeared to be subject to washing out during irrigation. Although weed control was evident in this plot, it was never equal to the other three Solicam-treated plots.

By 30 DAT, efficacy among herbicides was similar to the 14 DAT readings, all products except Solicam were providing greater than 90% control across all weed species (Table 3). At 60 DAT, all herbicides tested were still providing excellent control of the grasses (Table 4). Among the broadleaf weeds there was much more variability in control than at previous evaluations. The Solicam plot that was subject to wash, continued to lose soil and thus weed control in this plot continued to worsen. It was evident the Milestone and Karmex were still providing slightly superior weed control than the other products evaluated.

Late in the summer, weeds in the untreated plots and in the running checks and alleys had gotten too tall and dense. To preserve the integrity of the health of the grove, weeds in the test area were mowed to a height of approximately 4 inches on 08/23/97. Additionally, weed densities in the check area had dropped considerably due to shading and crowding.

At 124 DAT, or 4 months after treatment, weed control in the Solicam-treated plots was beginning to slip, especially among the naturally occurring weeds ((Table 5), probably because of the more extensive seed bank and non-uniform germination relative to the artificially seeded weeds. Additionally, control of TRTPO began to slip in the Krovar-treated plots, and Solicam, Krovar, Princep and the Milestone at 0.5 lbs-ai/A all appeared slightly weaker on SEBEX. AMARE control with the low rate of Milestone also appeared to begin to slip.

Initially we had intended to collect data at 8 and 12 months after treatment, but the summer annual we were evaluating early in the trial had cycled out by October. Although some winter annuals had emerged, they were not plentiful and differences among plots were not detectable.

At no time in this trial did we detect any evidence of crop injury by any of the products tested, although Karmex has a particularly bad reputation for crop injury on the Yuma Mesa. Overall, Milestone at 0.75 or 1.0 lbs-ai/A and Karmex appeared to offer the best weed control for as long as 4 months. Princep and Krovar also provided good weed control, but began to slip by 4 months. Although weed control with Solicam was not bad, it appeared to be the weakest product evaluated.


This is a part of publication AZ1051: "1998 Citrus and Deciduous Fruit and Nut Research Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1051/az10518.html
Return to Table of Contents