Containerized Strawberry Transplants as a
Replacement for Methyl Bromide Soil Fumigation in California Strawberry Nurseries
Progress Report - August 2000
Principal Investigator:
Kirk D. Larson
Associate Pomologist and Associate CE Specialist, UC Davis Dept. of Pomology
Siskiyou Resource Conservation District
UC South Coast R.E.C.
7601 Irvine Blvd.
Irvine, CA 92618
(949) 857-0136, kdlarson@ucdavis.edu
Other Investigators:
Elizabeth E. Ponce and Curt Gaines, Lassen Canyon Nursery, Inc.
John Duniway, Dept. of Plant Pathology, UC Davis
Location of project: Siskiyou, Orange, & Solano Counties
Funding:
$118,766 for 3 year project
Table of Contents:
Objectives
Summary
Specific Results
Potential Benefits/Impacts on
Agriculture
Dissemination of Findings
Future Workplan
Tables
Annual plantings of pathogen-free strawberry transplants are the basis for high productivity and successful strawberry IPM programs in California, and over 900 million strawberry transplants are produced annually in California. To ensure the production of pathogen- and nematode-free transplants, strawberry nurseries preplant fumigate in each nursery cycle with mixtures of methyl bromide (MB) and chloropicrin (CP). The impending ban on MB requires that alternative technologies be developed for strawberry nursery production. Alternative fumigants are less effective in controlling common soil pathogens than MBCP, and crop rotations are ineffective for control of serious pests and pathogens of strawberry nurseries. The use of containerized transplants produced in disease-free, soilless media ("plugs") could obviate the need for nursery soil fumigation.
Strawberry plugs have been used in other regions for many years, but little information is available regarding propagation and use of plugs in California. Commercial strawberry plant propagation consists of at least three field propagation cycles, with the final propagation phase conducted in high elevation (HE) nurseries in northeastern California. In this region, exposure to chilling temperatures (< 7°C) and short days in early fall results in transplants that produce greater yields, larger, higher quality fruit compared to non-conditioned plants.
Although strawberry plugs have been used for many years in various regions of the world, plugs are not widely used in California, and information is lacking on plug propagation methods for California’s unique production system; 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 attain the highest possible yield and fruit quality, and 3) plug performance (yield, fruit quality) in the state’s major strawberry production regions.
Containerized strawberry plants ("plugs") can be produced without the use of fumigating soil with methyl bromide or other chemical fumigants, but little information has been developed for use of plug plants in commercial strawberry production in California. To develop protocols for producing high-quality strawberry plugs that have performance characteristics similar to, or better than, conventional nursery planting stock, we conducted research in a commercial strawberry nursery during the 1999 propagation season using the cultivar Camarosa. At two-week intervals from July 21 to August 18, stolons (runner tips) were obtained from containerized mother plants and propagated in soilless media using three container sizes.
In August 1999, most of the plugs propagated on July 21 were transported to the University of California Davis Department of Pomology Wolfskill Experimental Orchard near Winters, California and established alongside conventional (bare-root) transplants in a performance trial.
On September 7, all remaining plugs were transferred to either high elevation (>4,000 feet) or low elevation (400 feet) nursery locations in Macdoel and Redding, respectively, where they were acclimated under ambient (outdoor) conditions until September 28. During this same period, a subset of the plants kept in Redding was manually transferred to an unlighted walk-in cooler maintained at 40° F for 15 hours each night.
Plugs produced using the various nursery sites, container sizes and rooting dates were transplanted to a coastal fruit production field in Irvine California on October 2, 1999, and established alongside conventional bare-root transplants in replicated plots. Importantly, due to a scarcity of runner tips in the screenhouse propagation facility for the July 21 and August 4 propagation dates, only a limited number of plugs were obtained for the first two propagation dates. The lack of plant material prevented the use of the full range of container sizes and nursery locations for these first two propagation dates.
For the Winters (Central Valley) performance trial, fruit were harvested from early April through the end of May 2000. Plug plants produced 87% of the yield of conventional plants, and had significantly reduced fruit size and appearance scores compared to the conventional plants.
For the Irvine trials, fruit harvest commenced Dec. 13, 1999 and continued weekly until May 31, 2000. High-elevation plugs generally had greater early season (to Feb 28) yields and fruit appearance scores than low-elevation plugs and bare-root transplants. Also, plugs produced in larger containers had greater early season yield than smaller plugs, and mature (10-week old) plugs had greater early season yield than recently rooted (6-week old) plugs. There was little or no difference among plug treatments and conventional bare-root transplants in regard to total yields or fruit size, but many plug treatments produced fruit with inferior appearance scores.
In a separate Irvine trial, high elevation plugs and conventional, bare-root transplants were planted in fumigated and non-fumigated soil on October 4 and yield performance assessed from December 1999 through May 2000. In this trial, there was no significant effect of plant type (plug, bare-root) on yield or fruit size, but total yield and fruit size for plants grown in non-fumigated soil were 52% and 83%, respectively, of plants grown in fumigated soil.
Further research regarding runner plant age, container size and high-elevation plant conditioning is planned for the 2000-2001 season, with the goal of improving plug plant performance in coastal fruiting fields. Also, a plug performance trial will be conducted in Watsonville during the 2000-2001 production season. For the Central Valley trial, planting dates will be adjusted in an effort to obtain reduced vegetative growth and larger fruit size from plugs.
A. Methodology
Plug propagation: rooting date and propagation cell size. Containerized strawberry plants ("plugs") are easily propagated by removing developing runner plants (runner "tips") from the "mother" plant and rooting them under intermittent misting. In April 1999, 100 'Camarosa' foundation plants (the highest quality commercial planting stock available) were potted in sterile potting media in 3.8-liter containers and placed in a runner propagation shed at Lassen Canyon Nursery Headquarters in Redding, California. Runner tips were harvested from these plants and propagated as plugs at 14-day intervals from July 21 to August 18, 1999 in Redding, California. Different rooting dates were used to determine the effect of plug age on subsequent plant performance. Runner tips were rooted in a peat-vermiculite based potting media (Scott's-Sierra Premium Professional Potting Mix, Scott, Inc., Marysville, Ohio).
For all three propagation dates, plug plants were propagated in standard strawberry plug trays (Cooks Garden, Hodges, SC 29653) containing 50 round-conic cells, with individual cells measuring 5 cm wide x 6 cm deep (hereafter referred to as "#1 plugs") (Table 1). Unfortunately, few runner tips were available for harvesting on July 21 and August 4. Strawberry runner production requires long days (long daily photoperiods) and high temperatures, and thus the scarcity of runner tips in early summer is, to a certain extent, a biological limitation, but cooler than normal summer conditions in Redding throughout the summer of 1999 also may have affected mother plant runner development.
Due to insufficient runner tips for the first two propagation dates, the interactive effects of container size and nursery source could only be fully assessed for plugs rooted on August 18, when runner tip production was sufficient to permit use of all three cell sizes for plug plant propagation (Table 1). Due to the same limitations, the interactive effects of rooting date, container size and nursery location could not be determined this first year of the project.
Two larger cell sizes were used in our trials: a forestry propagation tray that had 36 round-conic cells per tray with individual cells measuring 5.5 cm wide x 11 cm deep (hereafter referred to as "#2 plugs"). Another type had 18 cells per tray, and individual cells 8 cm wide x 8 cm deep (hereafter referred to as "#3 plugs"). These various cell sizes were used to determine the effect of plug size on subsequent performance.
For all dates and plug cell sizes, trays were placed in a screenhouse immediately after planting and subjected to intermittent, overhead misting during a 3-week period. Regardless of cell size or planting date, all plugs had well-developed root systems within two weeks after runner tip planting.
Plug plant conditioning. On September 7, plug plants for the various and rooting dates were divided into three equal groups, and thereafter each group was exposed to a different temperature and light regime during a three-week period to Sept. 28. One group of plugs was placed outdoors in Redding under natural (ambient) temperature and daylength conditions. On a daily basis, a second group was exposed to identical Redding conditions during a 9-hour daily light period (0600 - 1500 hrs, PST), and to a 15-hour dark and cold period achieved by placing the plants in an unlighted walk-in refrigerator maintained at 5°C. The third group of plugs was transported to Lassen Canyon Nursery's (LCN) high elevation nursery in Macdoel, California (41.8 N, 1,200 m) where they were maintained outdoors and exposed to natural (ambient) light and temperature regimes during this same three-week period. Additional plugs (#1 cell size, Aug. 18 rooting date) were exposed to high-elevation nursery conditions at the Macdoel nursery site during this same 3-week period. These plants were later used to establish Irvine trials in fumigated and non-fumigated soil, and to establish the Irvine microbial inoculation trials conducted by Dr. John Duniway. From Sept. 7 to Sept. 28, plugs maintained outdoors in Redding received no exposure to chilling (< 7° C) temperatures, while plugs exposed to artificial and high elevation conditioning received 315 and 250 hours of chilling exposure, respectively.
The various combinations of rooting date, container size and nursery environment resulted in 14 different plug plant treatments (Table 1).
Nursery digging dates. Plug plants were "dug" or lifted from the various nurseries on two dates. For the Central Valley (Winters) performance trial, plugs were transported to Winters in the propagation trays on August 12, 1999. All other plugs were "dug" from the various nursery locations on September 28, 1999, stored for one day at 2°C, and then transported to the South Coast REC in Irvine. To facilitate transport to Irvine, these plugs were removed from the propagation trays and placed in polyethylene-lined boxes. Commercial (bare-root), field-grown plants also were dug on September 28 from LCN's Macdoel nursery, and exposed to identical storage and handling conditions; these plants were used as the control treatment. In Irvine, plugs and bare-root plants were stored at 1° C for 1-3 days before planting.
Central Valley Fruiting Field Performance Trial. This trial was established to compare strawberry plug performance with that of bare-root transplants in the Central Valley region of California. On August 12, 80 #1 plugs were transported to the U.C. Davis Department of Pomology Wolfskill Experimental Orchard (WEO) near Winters, California and planted in four replicate plots on August 13, 1999 (Table 2). These plug plants received no artificial temperature or photoperiod conditioning prior to planting. Plugs were established on two-row strawberry beds spaced 136 cm apart, with four replicate plots of 20 plants each. Plug plots were established adjacent to plots of Camarosa plants that had been planted on August 12 using cold-stored, bare-root plants, and following U.C. recommendations for commercial strawberry production in the Central Valley. Fruit were harvested weekly from April 13 to May 31, 2000, and yields determined on a per-plot basis.
Fruiting Field Performance Trials - Irvine Trials. Three separate plug plant trials were conducted in Irvine, California during the 1999-2000 production season.
1. Effect of plug container size, rooting date and nursery conditioning treatment on fruiting field performance; comparison with commercial, bare-root transplants. The largest plug plant trial was established on October 2, 1999 using two-row strawberry beds spaced 120 cm apart, 14 different plug plant treatments (Table 1) plus a bare-root control. There were 4 replicate plots per treatment, with six plants per plot in a randomized block design. All plots in this trial were treated following U.C. recommendations for strawberry production in southern California, including preplant soil fumigation with a mixture of methyl bromide and chloropicrin. Fruit were harvested weekly from Dec. 13, 1999 until May 31, 2000 and yields determined on a per-plot basis. Performance assessments included total and marketable yield, mean fruit size and a subjective appearance score.
2. Performance of plug plants and commercial, bare-root transplants in fumigated and non-fumigated soil. This trial was established on October 4, 1999 (Table 3) on a site that had a previous history of non-fumigated strawberry production, but that had been cropped in barley (Hordeum sativum L) during the last two strawberry cropping seasons. Within this site, two adjacent parcels, each measuring 6 X 36 m, were either preplant fumigated with a mixture of methyl bromide and chloropicrin or left untreated. Plugs and bare-root transplants were established in adjacent plots within each soil treatment using four replicate plots for each plant type in a randomized block design. Fruit were harvested weekly from Dec. 13, 1999 until May 31, 2000 and yields determined on a per-plot basis. Performance assessments included total and marketable yield and fruit size.
3. Performance of plug plants and bare-root transplants in fumigated and nonfumigated soil with and without bacterial inoculations. Dr. John Duniway is the principal investigator for this project, established on October 4, 1999 using the same fumigated and non-fumigated parcels as the above trial, as well as the same plant materials. The objective of this trial was to determine growth and yield responses due to inoculation of soil and field-grown plants with bacteria previously demonstrated to enhance strawberry plant growth in greenhouse trials. Bacterial inoculation and rhizosphere colonization may be improved with use of plug plants, due to the well-established, functional plug plant root system. As cooperators, we provided significant support for this project in regard to field site preparation, plant propagation and planting, all cultural practices, plant growth measurements, fruit harvesting, and yield determinations.
B. Results
Central Valley plug plant performance trials. Plug plant vegetative growth through summer, autumn and winter was equivalent to or greater than that of conventional plantings made at the same time (data not shown). All plants over-wintered equally well, and plug plants commenced fruit production at the same time as conventional, bare-root transplants. Fruit were harvested on a weekly basis from April 13 until May 31. Plug plants produced 87% of the yield of conventional plants, and had significantly reduced fruit size and appearance scores compared to the conventional plants (Table 4).
Irvine plug plant performance trials - effect of rooting date, container size, and nursery environment. Fourteen different types of plugs (Table 1) were produced through use of various rooting dates, container sizes and nursery environments. To the extent possible, performance of all 14 types of plugs was statistically analyzed and compared with that of conventional (bare-root) transplants (Tables 5-14). Analysis of variance (ANOVA) reveals significant effects of plant treatment on yield, marketable yield, fruit size and appearance score for both early season (Tables 5-8) and total season (Tables 9-12) performance periods. Compared to bare-root transplants, all plug treatments had comparable early season yields and marketable yields (Table 13), and various plug treatments (6, 7, 10, 12, 13 and 14) had greater early season yields and /or marketable yields than bare-rooted plants. In regard to early season performance, the best plug treatments were either: those propagated at early dates (trts 10, 12, 13, 14); those grown in larger containers (trts 6, 7, 14); or those conditioned at high elevation (trts 6, 12, 13, 14). Although fruit size for these plug treatments was generally similar to that of the bare-root plants, they tended to have lower early season fruit appearance scores, with the notable exception of plug treatments #12 and #14, which were both conditioned at high elevation (Table 13).
There was little or no difference among the various plant treatments in regard to total yields (Table 14). Most plugs had reduced total season fruit appearance scores compared to bare-root plants, but, for plugs, total season fruit size and appearance scores were generally enhanced with high elevation nursery conditioning (Table 14).
Additional statistical analyses were performed to determine the influence of rooting date (plug age) and nursery location on fruiting field performance for plugs propagated on August 4 and August 18, 1999 and then conditioned in three nursery locations (Tables 15-20). ANOVA indicates significant effects of propagation date and nursery conditioning environment, but no interaction between these two factors (Tables 15, 18). Plugs rooted on August 4 had increased early season yield and marketable yield compared to plugs rooted on August 18 (Tables 16, 19). Plugs conditioned at high elevation (Macdoel) had greater early season yield and marketable yield than plugs kept at low elevation (Redding), while plugs artificially conditioned in a cooler had intermediate performance (Tables 17, 20). As early season (to March 1) fruit typically commands a significantly higher market price, plug propagation date and nursery conditioning environment appear to be important factors in creating quality plug plants.
A detailed statistical analysis was also conducted to determine the influence of container size and nursery conditioning environment for plug plants propagated on August 18, 1999 (Tables 21-23). ANOVA indicates that both container size and nursery location have significant effects on for early season marketable yield of plugs, but that there were no significant size x nursery interactions (Table 21). The larger container size resulted in plugs that produced more early season fruit than plugs grown in the smallest container, and intermediate-sized containers produced intermediate yields (Table 22). Nursery location also affected fruit yield, but in this case, plugs exposed to artificial chilling had the lowest yields, while low elevation conditioning resulted in an intermediate early season yield response (Table 23).
Performance of conventional transplants and plug plants in fumigated and nonfumigated soil. Use of ANOVA revealed that conventional (bare-root) transplants and plug plants responded similarly to soil fumigation treatment, with significant yield reductions due to non-fumigation for both types of plants, and with no fumigation x plant material interactions (Table 24). Yield reductions due to non-fumigation were substantial (about 50%), with negative effects of non-fumigation on fruit size as well (Table 24). It is our intent to repeat, and hopefully expand, this trial in 2000-2001, although based on this study there is no evidence that plug plants per se afford any distinct advantage for non-fumigated environments.
Potential Benefits/Impacts on Agriculture
There appear to be clear benefits in regard to early season yield and fruit quality enhancements due to plug plant conditioning in a high elevation environment. There also appear to be significant effects of plug plant age and plug container size on subsequent fruiting field performance, with the more mature or larger plugs outperforming younger or smaller plants. None of these results is unexpected, as the benefits of runner plant age and size, and a high elevation propagation environment have been demonstrated in previous nursery research. However, it is important to recognize that the added cost of producing a more mature, larger plug in a high elevation environment will be a significant factor, and one which may offset any yield or fruit quality advantage.
The 1999-2000 strawberry production season saw the largest-scale usage of plug plants in the history of the California industry, with several million plugs being used for fruit production. Use of plug plants is most common in southern California, where growers desire early season fruit production at a time when market prices are highest. However, plug plant performance has been highly variable, with individual grower experiences ranging from moderately positive to extremely negative. Understanding the basis for these varied experiences has proven difficult, since commercial plug plants producers are unwilling to reveal their propagation and production methods.
To our knowledge, no commercial plug producer in California propagates plugs at high elevation, or exposes plug plants to effective artificial conditioning treatments in an attempt to improve performance. Indeed, there appears to be considerable ignorance in the plug industry regarding strawberry plant physiology and the benefits that accrue from a high elevation (or similar) environment. The need for additional research in strawberry plug propagation and plug plant conditioning, and for extension education delivery programs related to this topic, appear to be greater than ever.
Presentations of the project’s research objectives and preliminary results of the work in progress have been made at five UCCE/CSC-sponsored strawberry grower meetings in Irvine, Watsonville and Santa Maria, with an estimated total attendance of over 500 people. Additional presentations have also been made to fruit and nursery growers at the California Strawberry Commission annual Research Committee Meeting. Research objectives and preliminary results also have been reported at two strawberry grower seminars in Auckland and Kati Kati, New Zealand (December, 1999), at the VI Annual Meeting of Strawberry Plant Producers and Exporters in Seville, Spain (February, 2000), and at the National Strawberry Congress in Metaponto, Italy (April, 2000). Attendance at these international meetings totaled over 320 people.
As the final results of this year’s research trials become available in early July, at least one report will be submitted to the California Strawberry Commission for publication as a CSC "Pink Sheet" Research Report. Upon successful completion of the first year’s field trials, we anticipate making additional, more substantive, presentations to grower audiences in California and abroad.
Based on the results of this year’s trials, we intend to expand our studies of plug plant maturity and high elevation conditioning, with the specific goal of improving plug plant fruit quality and fruit size. We will continue to test the effects of plant age, and intend to propagate additional runner tips at an earlier date compared to last year, as well as extend the period of time plug plants are conditioned at high elevation. Thus, in 2000-2001, our principal target date for cutting runner tips has been advanced from August 18 to August 1, which will allow us to produce a more mature plug plant, as well as test various durations of high elevation conditioning prior to sending plants to coastal fruiting fields. We will continue to test the effects of container sizes, but will confine our studies to the #1 and #2 cell sizes used this past year. We will forego artificial chilling and dark exposure treatments, as these treatments were time-consuming, expensive, and impractical. It is our intent to produce a sufficient number of plug plants to establish trials in Irvine, Watsonville and the Sacramento Valley.
In an attempt to increase plug plant fruit size and quality, we intend to study the effects of reducing early season plant vegetative growth by means of leaf removal at the time of transplanting (in effect, planting a rooted, leafless crown, rather than a green-leafed plug). We hypothesize that leaf pruning will alter photosynthetic source-sink relationships as well as the relationship between vegetative and reproductive growth in plug plants, which consistently produce large quantities of smaller, more poorly shaped fruit. Whether these forced alterations in carbon balance and growth can favorably affect fruit quality remains to be determined.
To meet the increased demand worldwide for California strawberry plants, Lassen Canyon Nursery expanded its cooler and freezer facilities in Redding in 2000. This expansion eliminated the screenhouse runner propagation facility that was used to grow last year’s plug plants. We plan to construct a small, portable screen house that will serve as our runner and plug propagation facility during the next two years.