Weed Management in Strawberry

Weed Science Weed Management in Strawberry Principal Investigator Dr. Steven Fennimore Dept. of Plant Sciences University of California, Davis 1636 East Alisal Street Salinas, CA 93905 (831) 755-2896 safennimore@ucdavis.edu Cooperating Personnel Dr. Jayesh Samtani, Jonathan Hunzie, and John Rachuy Dept. of Plant Sciences - UC Davis Salinas, CA Dr. Krishna Subbarao Dept. of Plant Pathology - UC Davis Salinas, CA Dr. Oleg Daugovish UCCE Ventura County Ventura, CA Summary Alternative weed management tools are needed by growers to supplement fumigants like chloropicrin. Where particular local weed problems can no longer be managed by fumigants, continued studies from our team are evaluating nonfumigant alternatives. Continuing work in the 2010-2011 strawberry growing season showed the weed control efficacy afforded by the herbicide GoalTender in combination with the herbicide Devrinol (by initial tank mix pre-transplanting), or adding Devrinol at later application timings did not extend the length of weed control. Additionally, soil pest control was evaluated in a study comparing the fumigants methyl bromide with chloropicrin and Pic-Clor 60, both applied through the drip lines, to cultural treatments of mustard seed meal, steam disinfestation, and steam plus mustard seed meal. Compared to the non-treated control, the fumigants, steam, and steam plus mustard meal all offered disease and weed control, but mustard meal alone did not. Another study involved dose response and feasibility of applying chlorine dioxide (ClO2) through the drip system. While there was some weed suppression and no injury to the crop, chlorine dioxide did not suppress Verticillium or improve strawberry yields compared to the control. A rate study of Dual Magnum revealed stunting and yield reduction at rates over 0.63 lb ai/a and requires further work to determine safe rates balancing nutsedge control with crop injury. 101

Introduction High weed management expenses increase strawberry production costs. Greater use of methyl bromide alternative fumigants and non-fumigated areas such as buffer zones has created new weed control problems. Herbicides can be used to control weeds that alternative fumigants do not control. Regulatory actions have increased restrictions on fumigant applications to reduce the risk of bystander exposure to fumigant emissions. The result of these restrictions is increased buffer zones and either outright prohibitions on fumigant use or severe limitations. The outcome of current legal and regulatory considerations on fumigants is uncertain, therefore, growers need more options besides fumigants. Field studies were conducted to address the increasing difficulties with management of nutsedge and other weeds in strawberry. This research could result in lower weed control costs for strawberry producers and develop weed control tools that do not depend on fumigants. Weeds must be effectively managed to maintain high strawberry production. Strawberries are extremely sensitive to weed competition, and weeds can harbor harmful diseases and insects. Methyl bromide has long served as the foundation for strawberry weed control, and kills most weed seed in the soil except hard-seeded species like California burclover (Medicago polymorpha), and common mallow (Malva neglecta) (Fennimore et. al. 2003). The adoption of fumigants such as chloropicrin, Inline/Telone C35, Pic-Clor 60 and metam sodium/potassium probably has increased emerging weed control challenges such as yellow nutsedge (Cyperus esculentus). The use of herbicides to provide supplemental weed control is an effective strategy to hold down weed control costs with the adoption of alternative fumigants. Evaluation of Devrinol Applied by Drip Chemigation in Strawberry GoalTender (oxyfluorfen) is an effective application pretransplant in strawberry; however the rates of Oxyfluorfen do not allow it to persist long into the season. Devrinol (napropamide) can be used post transplant in strawberry; however the only way to apply herbicides under plastic mulch is by drip chemigation. Napropamide has the unique characteristic in that it has some soil mobility, and can be applied by drip chemigation with a reasonable expectation that it will spread laterally and control weeds across the bed. Supplementing oxyfluorfen with in-season applications of napropamide to extend the period of weed control was evaluated in a field trial located at the Spence Road research station in Salinas, CA, where each treatment was replicated four times in a randomized complete block experimental design (Table 1). Prior to transplanting, on October 26, 2010, Oxyfluorfen at 0.25 lb ai/a was sprayed on all plots except the untreated. Napropamide at 2 and 4 lb ai/a, i.e., treatments 2 and 3, were applied in combination with oxyfluorfen (Table 1). Sequential applications of napropamide at 2 and 4 lb ai/a, i.e., treatments 4 and 5, were applied by drip chemigation on January 4, 2011. A double sequential application of Napropamide at 2.0 lb ai/a, i.e., treatment 6, was applied on January 4, 2011 and again on February 4, 2011. Plots were covered with clear TIF (Raven) plastic. Paladin (dimethyl disulfide, or DMDS)-Pic (79/21) was drip applied at 50 GPA on November 8, 2010. Albion strawberry was transplanted on November 19, 2010. Plots were one 52 inch bed wide by 50 ft long with two plant lines per bed. Crop injury (0 = no injury, 10 = dead) was assessed December 21, 2010, January 10, 2011, February 10, 2011 and March 14, 2011. Weed densities were measured February 22, 2011, April 11, 2011 and May 4. 2011. Plant diameters were measured on March 3, 2011. Harvest stations were established in each plot with 35 plants per station and harvesting was conducted April 14, 2011 through October 3, 2011. Fruit were harvested and sorted by marketable and culls at regular intervals. 102 CALIFORNIA STRAWBERRY COMMISSION ANNUAL PRODUCTION RESEARCH REPORT

Weed Science Table 1. Napropamide applied with or following Oxyfluorfen applications in strawberry: weed densities, plant diameters and fruit yield App. Time Weed Crop Fruit Rate Pre Or Post Densities Diameters Yields Trt# Treatment (lb ai/a) Transplant No./1,000/A Inches Oz/Plant 1. Control 1 19.2 a 5.6 33.9 2. Oxyfluorfen + 0.25 + 30 day Pre 9.7 b 6.1 34.9 Napropamide 4.0 3. Oxyfluorfen + 0.25 + 30 day Pre 10.9 b 5.9 34.8 Napropamide 2.0 4. Oxyfluorfen fb 2 0.25 fb 30 day Pre fb 9.3 b 5.5 35.5 Napropamide 4.0 30 Day Post 5. Oxyfluorfen fb 0.25 fb 30 day Pre fb 8.7 b 5.2 33.3 Napropamide 2.0 30 Day Post 6. Oxyfluorfen fb 0.25 fb 30 day Pre fb 9.7 b 5.1 33.2 Napropamide fb 2.0 fb 30 Day Post fb Napropamide 2.0 60 Day Post 1 Control also received Paladin 2 Followed by (fb) i.e., a sequential application P Value <0.0001 0.49 0.81 Conclusion: Fruit yield up to October 3, 2011 and plant diameter results suggest that all treatments were safe to strawberry (Table 1). Weed control data suggest no advantages to sequential post transplant applications of napropamide compared to an application prior to transplanting. 103

Evaluation of Mustard Seed Meal and Steam for Soil Disinfestation in Strawberry Mustard seed meal (MSM) is compatible with soil disinfestation practices such as steam and anaerobic soil disinfestation. Originally investigated as a biofumigant for disinfestations of soil, MSM is also a slow-release fertilizer (6-1-1) well suited for strawberry production. This study was conducted at the Spence Road research farm at Salinas, CA, where each treatment was replicated four times and arranged in a randomized complete block design (Table 2). Treatments included a non-treated control, Pic-Clor 60 at 300 lbs/a, methyl bromide + chloropicrin (MBPic, 350 lbs/a), steam applied to raise the soil temperature to 158 F at 8-inches, mustard seed meal (MPT, Saskatoon, SK) 3000 lbs/a alone and in combination with steam. Mustard meal was applied with a Clampco shank injector at 6 inches deep on October 17, 2010. Soil samples infested with Verticillium dahliae were bagged and buried at 6 inches depth on October 21, 2010, in the bed center (two samples per plot) and retrieved post-treatment on November 15, 2010. Pic-Clor 60 and MBPic were applied by drip chemigation on October 28, 2011 in 1.5 acre inches of water. The steam was applied October 26 and 27, 2010 using hoses with 8 inch spikes. Plots were one 52 inch bed wide by 50 ft long and covered with clear mulch film. Albion strawberry was transplanted November 19, 2010. Crop injury, 0=safe, 10=dead, was evaluated on: December 21, 2010, January 25, 2011 and February 25, 2011. Weed densities were measured on: December 23, 2010 and February 9, 2011. Strawberry plant diameters were measured on February 28, 2011. One harvest station was established on each plot with 35 plants per station. Fruit harvest was conducted April 14, 2011 to September 29, 2011. Harvested fruit was graded as marketable and cull fruits. 104 CALIFORNIA STRAWBERRY COMMISSION ANNUAL PRODUCTION RESEARCH REPORT

Weed Science Mustard meal with steam disinfestation offered weed and pathogen control indistinguishable from methyl bromide with chloropicrin, and surpassed the old standard in plant growth, safety, and yield, at this Salinas site this season. Table 2. Verticillium sample viability, seasonal weed densities, strawberry diameters, fruit yields and injury estimates Weed Crop Injury Crop Fruit Yields Rate Verticillium Densities Estimates Diameters (4.14-9.29.11) Treatment (lb ai/a) Count/Gram No./1,000/A (2.25.11) (0 to 10) (2.28.11) Inches Oz/Plant Non-treated 1 61.3 a 181.4 a 1.4 a 4.7 d 18.8 e MBPic 350 2.8 b 8.3 b 1.8 a 5.7 c 27.9 bc Pic-Clor 60 300 5.1 b 11.5 b 1.8 a 5.9 bc 29.7 b Mustard Meal 3000 76.3 a 194.0 a 0.4 c 6.6 ab 26.8 c Steam 5.5 b 14.4 b 1.3 ab 5.6 c 23.3 d Mustard Meal 3000 6.0 b 7.2 b 0.5 c 6.9 a 33.1 a + Steam Treatment Prob (F) <0.0001 <0.0001 0.0030 <0.0001 <0.0001 Conclusions: The value of mustard seed meal is primarily as a fertilizer rather than as a biofumigant. 105

Evaluation of Chlorine Dioxide for Soil Disinfestation in Strawberry Chlorine dioxide is used primarily as a paper whitening agent; sodium chlorite generates chlorine dioxide and is registered as a disinfectant, sanitizer, fungicide, algaecide, slimicide, and deodorizer for various hygienic applications in clinics, industry, and horticulture. This dose study showed it also offers some weed control and strawberry safety at the rates examined. Prior to application the beds were formed and covered with clear tarp. Chlorine dioxide (Cl02) was applied by drip chemigation at 0, 10, 50, 100 and 1,000 parts per million (ppm) in about 1.5 acre inches of water on September 25, 2010 and Pic-Clor 60 was applied on October 28, 2010. Treatments were arranged in a randomized complete block with four replicates at Salinas, CA. Plots were one 52 inch bed wide by 50 ft long, with two plant lines per bed. Weed seed bags containing assorted weed species and nutsedge (C. esculentus) tubers were prepared. Mesh bags of soil samples infested with V. dahliae were also prepared. Weed bags were buried at 6 inches depth and Verticillium bags at 12 inches depth on September 22, 2010, in the bed center (two bags of each per plot) prior to treatment, and retrieved post-treatment on November 12, 2010. Albion strawberry was transplanted on November 19, 2010. Crop injury ratings (0=safe, 10 = dead) were taken on December 21, 2010, January 25, 2011, and February 24, 2011. Weed densities were measured on November 17, 2010, January 18, 2011, March 8, 2011 and April 11, 2011. The strawberry plant diameters were measured on February 28, 2011. Fruit harvest from one 35 plant harvest station per plot began April 14, 2011, and continued on a regular basis until July 25, 2011. Fruit were sorted as marketable and culls (nonmarketable). Chlorine dioxide treatments failed to suppress the Verticillium microsclerotia population. None of the weed seeds (data not presented) or nutsedge tubers were controlled by chlorine dioxide treatments. Chlorine dioxide at 100 and 1000 ppm did reduce weed densities compared to the control. The crop diameter and yield data suggests that chlorine dioxide up to 1000 ppm dose rates did not damage strawberry plant health compared to the untreated control (Table 3). Table 3. Verticillium and nutsedge tuber survival, weed densities, crop diameters, and fruit yields Nutsedge Weed Crop Fruit Yields Rate Verticillium Control Densities Diameters (4.14-7.25.11) Treatment (lb ai/a) Count/Gram % Tuber Controlled No./1,000/A (2.28.11) Inches Oz/Plant Untreated 0 ppm 24.1 b 13.7 c 199.8 a 4.8 b 10.0 b Chlorine Dioxide 10 ppm 82.5 a 18.7 bc 218.2 a 5.1 b 9.5 b Chlorine Dioxide 50 ppm 83.0 a 26.6 bc 212.4 a 5.0 b 9.8 b Chlorine Dioxide 100 ppm 77.8 a 23.8 bc 90.2 b 4.8 b 9.3 b Chlorine Dioxide 1000 ppm 67.7 a 29.5 b 63.5 b 5.2 b 8.3 b Pic-Clor 60 300 lb/a 3.3 b 90.1 a 22.7 b 6.5 a 14.5 a Treatment Prob (F) <0.0001 <0.0001 <.0001 0.0017 0.0007 Conclusion: Chlorine dioxide is not effective for soil disinfestation in strawberry at tested rates. 106 CALIFORNIA STRAWBERRY COMMISSION ANNUAL PRODUCTION RESEARCH REPORT

Evaluation of Dual Magnum Herbicide for Nutsedge Control in Strawberry Dual magnum (S-metolachlor) was evaluated in fall-planted Ventana strawberry at Santa Paula, CA during the 2010-11 season. The trial was conducted at the Hanson Research Center where S-metolachlor was applied at 0.63 and 0.95 lb ai/a 30 days prior to strawberry transplanting. These rates effectively controlled yellow and purple nutsedge in previous evaluations, however crop safety has not been adequately determined. Data taken were strawberry injury estimates (0 = safe, 10 = dead) and plant canopy area on November 22, 2010. Fruit yields were measured January to April 2011. The visual injury from S-metolachlor was not high in any treatment, but the canopy size was suppressed and fruit yields were reduced by S-metolachlor at 0.95 lb ai/a (Table 4). S-metolachlor rate 0.63 lb ai/a resulted in similar plant growth and fruit production to untreated check. This suggests that rate below 0.63 lb ai/a should be further evaluated. Table 4. Injury estimates, crop canopy and fruit yields through May 1, 2011 at Santa Paula, CA Rate Injury Canopy Fruit Yield Treatment (lb ai/a) (0=Safe, 10=Dead) Inches 2 (Oz/Plant) Untreated 0 2.0 29.8 a 209.5 a S-metolachlor 0.63 2.6 25.0 ab 148.9 a S-metolachlor 0.95 2.5 20.5 b 109.6 b 107