Methyl bromide alternative results for strawberries

by Sam Prentice and Jenny Broome, SAREP

Our last newsletter (Vol. 15, No. 2) outlined the results of three projects funded by SAREP in grapes, stonefruit, and ornamentals. In the second part of the series, we present results from three additional projects addressing the challenges facing the California strawberry industry. These projects focused on producing clean planting material as transplants, biological alternatives to pre-plant fumigation, and alternatives to post harvest fumigation.

In 1998 SAREP was awarded a one-time. legislative augmentation to support research into alternatives to methyl bromide. Six projects were funded with the special allocation (AB 1998) sponsored by Assemblywoman Helen Thomson (D-Yolo County) with a friendly amendment by then-State Senator Mike Thompson (now U.S. Congressman, Napa) and funded through the Department of Pesticide Regulation. Methyl bromide is a broad- spectrum fumigant that is widely used to control insect, pathogen, nematode, weed and rodent pests. It has also been identified as a Class I ozone-depleting substance. Under the Clean Air Act, the U.S. Environmental Protection Agency (EPA) has prohibited the production and importation of methyl bromide starting January 1, 2005. In addition, the United States has joined 140 other nations in signing the Montreal Protocol, which in 1994 froze production and importation of methyl bromide at 1991 levels, and which requires use to be reduced in developed countries by 25 percent in 1999, 50 percent in 2001, 70 percent in 2003 and 100 percent in 2005. According to EPA, continued use of methyl bromide as an agricultural pesticide may contribute five to 15 percent to future depletion of the ozone layer if it is not phased out.

This phase-out has significant implications for California agriculture, since methyl bromide is widely used as a pesticide for the production and export of high value crops and commodities produced statewide. Approximately 90 percent of the methyl bromide used in California is for pre-plant soil fumigation to control soil-borne pathogens and pests principally in strawberries, nursery crops, grapes, and tree fruits and nuts. When used in this manner, about 50 to 95 percent of the methyl bromide injected can eventually enter the atmosphere. Postharvest commodity treatment accounts for another five to 10 percent of the methyl bromide use statewide and is directed largely at insects that damage nuts, cherries, grapes, raisins, and imported materials. About 80 to 95 percent of the methyl bromide used in a commodity treatment eventually enters the atmosphere. Structural fumigation accounts for most of the remainder of the methyl bromide use in California.

Several potential chemical and non- chemical alternatives to methyl bromide have been identified nationally and internationally and some of these alternatives have been and are currently being evaluated in California. The previous issue of Sustainable Agriculture (Vol. 15, No. 2) publicized the results of three projects funded by SAREP on environmentally sustainable alternatives to methyl bromide. The three summaries presented here constitute the remainder of the projects funded through this effort. Overall, it appears that there is no single alternative for the use of methyl bromide that is both as effective and economical. Rather, the SAREP-funded research indicates that a matrix of alternatives is necessary to manage pests currently controlled by methyl bromide within California farming systems. In addition to the SAREP funded research, from 1993 through 2002, the USDA-ARS has estimated that they have spent $135.5 million to develop alternatives to methyl bromide. Through their competitive grants programs, USDA has provided an additional $11.4 million to state universities for research and outreach. This research is ongoing, as there is a continued urgent need to develop and evaluate effective, economical alternatives to the agricultural use(s) of methyl bromide as a pre-plant soil fumigant and postharvest commodity treatment.

For further information on research projects into alternatives, as well as an up-to-date summary of the science behind and the process for the phase out, please see the 2003 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions–Conference Proceedings, available in PDF format at mbao.org/2003/mbrpro03.html. SAREP partial funding supported the work presented in abstracts # 44, 44A, and 112.

Microbiological Improvement of Root Health, Growth and Yield of Strawberry
Principal Investigator: John Duniway, UC Davis plant pathology department. Cooperator: Kirk Larson, Submitted September 2002, Updated May 2003 and again in December 2003 from the abstract for presentation at the annual International Research Conference on Methyl Bromide Alternatives and Emission Reductions # 44-1.

Objectives
The research objective was to find and effectively deploy microorganisms to improve root health, growth, and yield of strawberry plants without soil fumigation or with less than optimum soil fumigation treatments. While no individual microorganism or combination of beneficial microorganisms is likely to reproduce the large yield increases obtained by methyl bromide/chloropicrin fumigation of soil, evidence was found that inoculations with specific microorganisms are likely to increase yield significantly. These increases are most likely to be useful when combined with alternatives to methyl bromide, including fumigants other than methyl bromide. Candidate microorganisms are available commercially, but more likely to succeed are microorganisms isolated recently from roots of strawberry plants growing in fumigated soils in California. The approach was to use these microorganisms, which were found to promote growth of strawberry plants in the greenhouse, to inoculate plants grown for berry production in the field. Methods of field application were researched, and resulting growth and yield responses of strawberry measured, relative to those obtained by normal farming practices with and without fumigation. The educational objectives were to help demonstrate mechanisms by which strawberry responds to soil fumigation and to scientifically explore, with grower involvement, the feasibility of using biological agents to help improve strawberry health and yield.

Summary
Researchers continued to sample field sites throughout the project period for additional isolates of rhizosphere bacteria, and to test their effects on the growth and health of strawberry plants in the greenhouse and growth chambers. In the last two years, investigators improved their screening efficiency by testing bacteria for inhibition of several pathogenic fungi in culture. Several new isolates with beneficial activity were found and some were tested in the field.

In each of the three years of the grant, several bacteria (and sometimes specific fungi) were used to inoculate strawberry plants in replicated field experiments. These were done at the Monterey Bay Academy (MBA), Watsonville, and at the UC South Coast Research and Education Center (SCREC), Irvine. Three bed fumigation treatments were applied each year at MBA, i.e., a standard rate of methyl bromide/ chloropicrin (MBC), a low rate of chloropicrin (Pic), and not fumigated. Plants were root-dip inoculated at transplanting; some were reinoculated periodically during crop growth. While MBC fumigation approximately doubled strawberry yields, none of the inoculation treatments increased yields significantly in MBC-treated soil and very few did so in nontreated soil. In contrast, several of the bacteria tested increased yields in soil treated with a low rate of Pic at MBA, and some of these increases were statistically significant. Reinoculation during crop growth did not enhance the effects of the bacteria. Additional experiments were done in the last two years at MBA using the variety Aromas and nonfumigated soils. A few of the bacteria tested reduced the incidence of Verticillium wilt in 2001 and two isolates increased yields in 2002. Aromas appears to be more responsive to bacterial inoculations than Selva.

Sections of the ground used in 1999-2000 at SCREC were broadcast-fumigated with MBC or were left untreated. Bare-root Camarosa runner plants were obtained from a high elevation nursery and Camarosa plug plants were propagated by Kirk Larson. In 2000-01 ground at SCREC, which was new to strawberries, was bed-fumigated with MBC, Pic, or not treated, and in 2001-02 beds were fumigated with MBC, metam sodium, or were not treated. The field used in 2001-02 had a history of strawberry production. Rhizosphere bacteria were used to inoculate bare-root transplants only at the time of planting. The effects of the soil fumigation and inoculation treatments on plant size at SCREC were variable, but fumigation generally increased yields significantly on ground with a history of strawberries. One bacterium increased growth significantly in Pic- and non-treated soils, while two did so following metam sodium treatment of soil. The use of plug plants in 1999-2000 had only small and variable benefits relative to bare-root transplants.

The main aspects of these experiments were repeated at MBA in 2002-03 with additional bacterial isolates from strawberries, and with support from the California Strawberry Commission.

Fumigation treatments were applied to preformed beds and included shank- applied MBC at 325 lb/a, drip-applied Pic at 200 lb/a, drip-applied Vapam at 70 gal/a, and a nontreated control. Five isolates of bacteria were used to inoculate transplants, some of which were beneficial in previous field experiments and some that had been tested only in the laboratory and greenhouse. The strawberry varieties Camarosa and Aromas were used. On MBC-treated soil, most isolates increased the berry yields of Camarosa but had small effects on the yield of Aromas. Bacterial effects on berry yields on Pic- and non-treated soils were smaller than before in both varieties. On Vapam-treated soil, however, one isolate increased the yield of Camarosa and two isolates increased the yield of Aromas significantly.

In the 2002-03 crop cycle, researchers found that marked strains (antibiotic resistant) do colonize soil and roots following inoculations, with high numbers on both older and new roots up to two months after inoculation. Dispersal of marked strains appeared vertically downward from the points (tested at about 10 cm). At five months, plants were nearly fully grown and there were still fairly high numbers of inoculated bacteria on roots at shallow depths, but low numbers deeper in soil. There was no spread laterally at the 10 cm distance tested.

Bacterial growth and yield promotion of strawberry following inoculation in the field was variable and depended on soil fumigation treatment, as well as isolate, strawberry variety, and probably location. The researchers are continuing to further characterize bacterial isolates from strawberries with the greatest beneficial activities, and to further optimize bacterial colonization and yield promotion of strawberries in field experiments.

Containerized Strawberry Transplants as a Replacement for Methyl Bromide Soil Fumigation in California Strawberry Nurseries

Principal Investigators: Kirk Larson, UC Davis pomology department. Submitted September 2002.

Objectives
Annual plantings of pathogen-free strawberry transplants are the basis for high productivity and successful strawberry IPM programs in California, and the state produces more than 900 million strawberry transplants annually. In California, propagation of strawberry transplants for fruit production entails at least three field propagation cycles, with the final propagation phase conducted in high elevation (HE) nurseries in northeastern California. In this HE region, exposure to chilling temperatures (< 7°C) and short days in late summer and early fall results in transplants that produce greater yields and larger fruit with better appearance scores compared to low elevation (non-conditioned) plants. To ensure production of pathogen- and nematode-free transplants, strawberry nurseries fumigate the soil prior to each propagation cycle with mixtures of methyl bromide (MB) and chloropicrin (CP). The impending ban on MB requires development of alternative technologies for strawberry transplant production. Compared to MBCP, alternative fumigants are more difficult to use and less effective in controlling soilborne pathogens, and crop rotations provide ineffective control of serious pests and pathogens in strawberry nurseries.

The use of containerized transplants (“tray plants,” “plug plants,” or “plugs”) produced in disease-free, soil-less media has been suggested as an alternative to MB nursery soil fumigation, but information on plug propagation methods for California’s unique production system is unavailable. In addition, because plugs are not widely used in California, information on plug productivity and fruit quality is also lacking. Research is needed to determine: 1) cost-effective methods for strawberry plug propagation, 2) appropriate methods for conditioning strawberry plugs to maximize fruit quality and yield, and 3) plug performance (yield, fruit quality) in the state’s major strawberry production regions.

Summary
Containerized strawberry plants (“plugs”) are readily produced without soil fumigation, but little information is available for optimizing plug plant production and performance under California conditions. Although strawberry plug plants can be established with less irrigation water and enter into fruit production sooner than bare-root plants, plugs have relatively high production and transportation costs, and plug plants in California often produce a high proportion of off-grade (small and misshapen) fruit late in the season. This inferior quality fruit has low market value and high harvest labor costs.

This research has focused on developing protocols for producing high-quality strawberry plugs that have performance characteristics similar to, or better than, conventional (field-grown) nursery planting stock. By propagating runner tips at about two week intervals from mid-July to mid-August and using different container (cell) sizes, researchers have been able to compare the effects of plug plant size and plug physiological maturity on plug plant yield performance. To compare the effect of conditioning environment on yield performance, investigators propagated plug plants at a low elevation (LE) nursery site in Redding, Calif. in 1999 and 2000, and then conditioned a subset of these plugs at a high elevation (HE) nursery site (Macdoel, Calif.) for three to four weeks prior to transplanting. In the third year of trials, researchers propagated and conditioned plug plants at both HE and LE, thereby lengthening the HE conditioning period. Yield performance for all plant material then was assessed under commercial strawberry management systems typical of the farming practices in those regions.

In these trials, the effects of cell size and nursery environment on plug yield performance varied somewhat from year to year, but results demonstrated significant effects of rooting date, plug cell size and nursery environment on early season (December-March) yield performance, and early and total season fruit quality (fruit size and shape) in most years. Early rooting date (July), use of a large plug cell size, and HE conditioning generally maximized early season yields compared to later rooting dates, smaller cell size and LE conditioning. Compared to LE conditioning of plugs, HE conditioning also resulted in increased fruit size and fruit appearance scores. Compared to conventional bare-root transplants, HE plugs generally produced greater early-season yields but had reduced fruit quality (i.e., reduced size and appearance scores). However, in the third year of the investigations, propagation and conditioning of plugs at HE resulted in fruit quality equal to that of conventional transplants and yields that were superior to either conventional transplants or LE conditioned plugs. There was little or no difference in total yield (December-June) among bare-root plants and plugs in most years.

Also during two years (1999-2001), yield performance of plug plants vs. bare-root transplants was assessed in the Central Valley at the UC Davis Pomology Department’s Wolfskill Experimental Orchards in Winters. In both years, plug plants yielded less than conventional plants, and had significantly reduced fruit size and fruit appearance scores.

In additional trials conducted over a two-year period (1999-2001), yield performances of plug and bare-root transplants were evaluated in fumigated and nonfumigated soil in Irvine. In the 1999-2000 production season, plants established in fumigated soil out-yielded plants in nonfumigated soil, and there was no effect of plant type (plug vs. bare root) on yield, and no interaction between soil treatment and plant type. In the 2000-01 production season, an identical trial was established on a site that had been cropped only in barley during the previous 20 years. For this trial, both plug plant and bare-root plant yields were identical, and there was no effect of soil fumigation.

 

Acetaldehyde and Carbon Dioxide for Postharvest Control of Arthropods on Strawberry Fruit
Principal Investigator: Elizabeth Mitcham, UC Davis plant pathology department. Submitted October 2001.

Objectives
1. Determine the efficacy of acetaldehyde fumigation alone and in combination with carbon dioxide to kill western flower thrips and two-spotted spider mites..

2. Determine the affect of fumigation with acetaldehyde and carbon dioxide on strawberry fruit quality and postharvest life.

3. Demonstrate the commercial feasibility of the treatment within existing methyl bromide fumigation facilities.

Revisions to Original Objectives
1. In addition to acetaldehyde, researchers included tests with ethyl formate on both western flower thrips and two-spotted spider mites.

2. A repeated exposure technique with acetaldehyde was developed to determine if target pest mortality could be enhanced without significant fruit quality loss. Strawberry fruit was also exposed to ethyl formate, and effects on fruit quality were evaluated.

3. Research has not yet resulted in a commercially feasible treatment, and therefore has not been tested in a large-scale fumigation facility.

Summary
Methyl bromide fumigation is used prior to shipment of California strawberries to Japan and Australia. Methyl bromide will be phased out for soil fumigation in 2005 under the Clean Air Act and the Montreal Protocol. While there is currently an exemption for postharvest and pre-shipment uses, methyl bromide will likely be more difficult and expensive to use in the future. The value of the export market to Australia is more than $1.3 million and to Japan is more than $18 million. Alternatives to methyl bromide for postharvest insect and mite control on strawberry fruit are limited because of the perishable nature of the commodity. Natural fruit volatiles have been tested for efficacy against various insect pests.

Plant volatiles such as acetaldehyde (Aa) and ethyl formate (EF) have been shown to have varied effects on fruit quality parameters and have been demonstrated to have fungicidal and insecticidal properties. This study explored the possibility of using Aa and EF for postharvest disinfestation of western flower thrips and two-spotted spider mite on harvested strawberries.

Dose response curves for western flower thrips and two-spotted spider mites were developed for exposure to Aa. Strawberry fruit treated with 0, 1, 2, 3, or 4% Aa in air or in CO2 and stored at 0ºC or 20ºC were evaluated for changes in fruit quality. Volatile compounds in strawberry juice after treatment were also quantified. A repeated exposure technique was developed to determine if low concentrations of Aa had less impact on fruit quality.

Western flower thrips were susceptible to Aa; however, quarantine levels of control were not achieved. Two-spotted spider mites were more resistant to Aa than western flower thrips and concentrations necessary to elicit high mortality were well above those tolerated by strawberry fruit.

Acetaldehyde concentrations >3% caused calyx browning and drying. Initially, fruit exposed to 2, 3, or 4% acetaldehyde in the presence of 20% CO2 showed slightly less calyx damage than fruit exposed to acetaldehyde in air, however, after 24 hours, there were no significant differences.

Repeated exposures to low concentrations of Aa improved fruit tolerance to the treatments but did not maintain the same level of target pest mortality as a single, high dose of Aa. Acetaldehyde is readily absorbed and metabolized by strawberry fruit and was rapidly reduced to ineffective concentrations for control of target pests in the presence of strawberry fruit under the conditions of the experiments.

Strawberry fruit and target pests were exposed to varying concentrations of EF
in treatments utilizing both single and multiple exposures. Although EF was toxic to both target pests, concentrations necessary for complete control of two-spotted spider mite were well above those tolerated by strawberry fruit.

While neither Aa or EF appear particularly promising for postharvest insect control in strawberry, the information gained in the research may lead to a new quarantine treatment for other commodities.

 

[Note: Results of Mitcham’s work on post- harvest strawberry pest control have been recently published. Please see: Simpson, T., V. Bikoba and E. Mitcham. 2003. Effects of acetaldehyde on fruit quality and target pest mortality for harvested strawberries. Postharvest Biology and Technology 28(3):405-416.