Fall Weed Control in Apples and Peaches - 2006
Joseph Masabni, Department of Horticulture

Introduction
Fall-applied herbicides are an important component of a comprehensive weed control regimen, especially to control perennials such as honeyvine milkweed, quackgrass, and johnsongrass. Growers are often busy in the fall with harvest and fruit sales and neglect the importance of weed control after harvest. In order to assist fruit growers with their weed control options, two experiments, one in an eight-year old Golden Delicious apple orchard and the other in an 11-yr old Red Haven peach orchard, were conducted. The purpose of these experiments is to determine the residual control and benefits of various herbicides applied in the spring and fall of 2005 on weed pressure in spring of 2006.

Materials and Methods
Herbicides were applied using a CO₂-pressurized backpack sprayer with a four-nozzle boom calibrated to spray a 5 ft band at 30 psi and 3 mph walking speed. The 8002-nozzles were set at 17 inches above ground to obtain good spray overlap and complete weed coverage. The spray boom was moved in and out of the tree row to avoid spraying tree trunks. Therefore, weeds at the bases of tree trunks were taller throughout the season and did not reflect the effectiveness of the applied herbicides. Plots were 10 ft x 66 ft long for peach and 10 ft x 27 ft long for apple. The experimental design consisted of a randomized complete block with three replications.

The preemergence (PRE) treatments were applied on 14 April 2005. Peach and apple trees were at 100% and 50% full bloom, respectively. As weeds had been growing since early March and were 3-4 inches tall, Roundup WeatherMax at 16 oz/A (0.68 lb ai/A) was included with all treatments. The postemergence (POST) treatments were applied on 15 June 2005 when peach fruits were 1 to 2 inches in diameter. Roundup at 16 oz/A was also included with the POST treatment. All treatments were applied early in the morning when the average wind speed was 2.5 mph.

This experiment included labeled and non-labeled herbicides for apple and peach in an effort to support their possible registration. Chateau is labeled on non-bearing fruit trees with a one-year pre-harvest interval, and should not be used in bearing orchards. Readers are reminded that all experimental herbicides tested in this report would not be legal applications in commercial or residential settings, and UK does not recommend their use until they are labeled.

This experiment also evaluated the benefits of ‘Attach’, an additive that improves weed control. In the peach experiment, PRE and POST applications of treatment 2 included Attach while those of treatment 3 did not. In the apple experiment, treatments 2, 4, 6, 10 included Attach while treatments 1, 3 , 5, 7, 8, and 9 did not.

The fall treatments listed in the tables were applied December 17, 2005, when soil temperatures were below 55°F but before soil freezing. Roundup was included with all treatments for control of existing weeds.

Visual weed control ratings were made on 6 May and 15 June. Ratings were on a 1 to 10 scale, with 1 = no control and 10 = complete kill or no weeds present. A rating of 7 (70-75% control) or more is considered a commercially acceptable value.

In the peach experiment, three weeks after PRE treatments (6 May), all treatments resulted in desirable weed control on all weeds (rating of 7+) when compared to the Roundup-only control (Table 1). Princep was weakest on dandelion, with or without Attach. However, adding Attach to Princep improved control of purple deadnettle three weeks after treatment, but was weaker on clover and marestail. By 15 June (two months after treatment), Attach improved Princep activity on large crabgrass by about 10% and on shepherdspurse by about 30%. Chateau at 6 oz. controlled weeds better than Princep by the first evaluation date (6 May). However, its benefit was exhausted by 15 June, with best weed control achieved with treatment 2.

Treatments 4 and 5 had the best weed control but also stunted current season shoot growth. Karmex is a currently available herbicide but not labeled for use on peaches. The observed stunting on peaches is obviously the reason why it is not labeled on this crop.

Similar results were observed with the apple experiment (Table 2). The addition of Attach improved weed control, even two months after PRE application for all herbicides tested (Princep, Karmex, and Devrinol). Chateau at 6 oz (the low end of the labeled rate) gave better weed control initially but lost its effectiveness after two months. The experimental formula (treatments 8 and 9) gave good to excellent weed control but also resulted in stunting. With stunting observed on both peach and apple, it is doubtful that this herbicide will get registered in the near future.

In the apple experiment (Table 3), on April 17, 2006 (about 4 months after fall herbicide application) Chateau 12 oz had the fewest number of weeds in the sample area with 5 weeds/sq.ft. compared to 31 and 36 weeds/sq.ft. for Casoron and Gallery, respectively. By May 18 or 142 days after treatment, Chateau continued to show significant weed growth suppression with only 3.5 weeds/sq.ft. compared to about 30 for the other two herbicides.

Similar results were observed in the peach experiment at 142 days after treatment with Chateau showing the best weed suppression while Casoron and Gallery performed as well as the non-residual Roundup herbicide (Table 4). It appears that fall-applied Chateau has at least five months residual activity, whereas Casoron and Gallery run out of weed control suppression at least a month earlier.