Containerized Strawberry Transplants as a
Replacement for Methyl Bromide Soil Fumigation in California Strawberry Nurseries
Final Report - September 2002
Principal Investigator:
Kirk D. Larson
Associate Pomologist and Associate CE Specialist, UC Davis Dept. of Pomology
Siskiyou Resource Conservation District
University of California South Coast R.E.C.
7601 Irvine Blvd.
Irvine, CA 92618
Phone: (949) 857-0136 Fax: (949) 653-1800
Email: kdlarson@ucdavis.edu
Co-Principal Investigator:
Elizabeth E. Ponce
Lassen Canyon Nursery, Inc.
1300 Salmon Creek Rd.
Redding, CA 96003
Phone: (530) 223-1075 Fax: (530) 223-6754
Email: lcnci@snowcrest.net
Cooperators:
Douglas V. Shaw
Department of Pomology, University of California, Davis
Phone: (530) 752-0905 Fax: (530) 752-8502
Project Locations (2000-2001):
Lassen Canyon Nursery, 1300 Salmon Creek Rd, Redding, Shasta County, CA
Lassen Canyon Nursery, Macdoel Ranch, Macdoel, Siskiyou County, California
UC South Coast REC, 7601 Irvine Blvd., Irvine, Orange County, California
UC Davis Pomology Department Wolfskill Experimental Orchards, Solano County,
California
UC Davis (Pomology Department) Watsonville Strawberry Research Facility, Santa
Cruz County, California.
Commodity:
Strawberry
Funding:
| UC SAREP FUNDS | MATCHING FUNDS | |||
| 1999-2000 | $17,080 | Calif. Strawberry Comm. | $16,200 | |
| 2000-01 | $35,089 | Calif. Strawberry Comm. | $28,290 | |
| 2001-02 | $38,667 | Calif. Strawberry Comm. | $32,000 | |
| Total | $90,836 |
Table of Contents:
Objectives
Summary
Specific Results
Potential Benefits/Impacts on
Agriculture
Dissemination of Findings
Tables
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, soilless 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.
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.
Our 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, we 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, we 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 3-4 weeks prior to transplanting. In the third year
of trials, we 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 our 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 our 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 U.C. Davis Pomology Department's Wolfskill Experimental Orchards (WEO), 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 outyielded 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.
There are three objectives to the research conducted: 1) develop suitable methods
for propagating strawberry plug plants under California conditions; 2) determine
suitable methods for conditioning (temperature and photoperiod conditioning)
plug plants prior to transplanting in fruit production fields; 3) determine
plug plant yield performance (early season and total season yield, early and
total marketable yields, fruit size, and fruit appearance scores) in various
production environments in California.
Objectives # 1 and #3: Develop suitable methods and protocols for strawberry
plug plant propagation in California; specifically, determine the effect of
rooting date and propagation cell size on subsequent yield performance.
Propagation of containerized strawberry plants ("plugs") entails removing runner "tips" from the "mother" plant and rooting them under intermittent misting. In our trials, runner tips were rooted in a peat-vermiculite based potting media (Scott's-Sierra Premium Professional Potting Mix, Scott, Inc., Marysville, Ohio).
Runner tip rooting date. Previous research demonstrated that strawberry runner plant rooting date affects subsequent yield after transplant to fruiting fields (K.D. Larson, unpublished data, 1993). For this reason, we considered it necessary to examine the effects of plug plant rooting date on yield performance. We did this by harvesting runner tips from mother plants and propagating plugs at about 14-day intervals from early August to mid-August in 1999 and 2000.
We observed significant effects of rooting date on plug performance in both years. In 1999-2000, plugs rooted on August 4 1999 had greater early season (December - March) yield and early season marketable yield than plugs rooted two weeks later (Tables 1-3). In 2000-01, rooting date affected fruit size, with plugs rooted on August 2 having larger fruit than plugs rooted on August 16 (Tables 4, 5). However, we observed no effect of rooting date on yield in the second year.
Due to the difficulty in obtaining sufficient numbers of strawberry runner tips before mid-August, and due to the variable effect of runner tip rooting date across the two years of our study, we concluded that additional studies on the effect of runner tip rooting date not a priority, and we therefore terminated this line of research in 2001.
Plug plant cell size. Previous research demonstrated that strawberry runner plant size affects fruit yield (K.D. Larson, unpublished data, 1993). Based on these previous observations, we wanted to examine the effects of plug plant size on yield performance. We created plug plants of various sizes by propagating runner tips in trays molded into different plug cell sizes. We use two different cell sizes in all three trial years: a standard strawberry plug tray (Cooks Garden, Hodges, SC 29653) containing 50 round-conic cells, with individual cells measuring 5 cm wide x 6 cm deep (referred to as "#1 plugs"), and larger, round-conic cells measuring 5.5 cm wide x 11 cm deep with 36 cells per tray ("#2 plugs"). In addition to these two cell sizes, we used large square cells (8 cm wide x 8 cm deep) (#3 plugs) to propagate plugs in 1999.
There were significant effects of cell size on early season yields in both 1999-2000 and 2000-01 production seasons, with plants produced in larger cells outperforming plugs produced in smaller cells (Tables 6-8). In addition, in 2000-01, large plugs had greater total yield than smaller plugs, and in 2002, large plugs had greater total season marketable yield than smaller plugs (Table 9).
However, propagation of large plugs requires considerably more resources (propagation space, potting media) than smaller plugs. Perhaps more importantly, the cost of transporting and planting large plugs, which may be four times larger and weigh 4-6 times more than the smaller plugs, could be prohibitive. Despite the benefits that consistently accrued from use of large plug plants, we consider that use of large plugs is probably not cost-effective.
Objectives #2 and #3. Based on yield performance in fruiting fields, determine suitable methods for conditioning (temperature and photoperiod conditioning) plug plants prior to transplanting in fruit production fields.
For all of our plug trials, runner tips were cut from mother plants, then planted
immediately in trays in potting media. 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. Three weeks after tip
cutting, rooted plugs were transferred to different environments where they
were exposed to natural or artificial temperature and daylength conditioning
treatments during a 3-6 week period.
Plug plant conditioning and yield performance. In 1999, newly-rooted,
3-week old plugs were transferred to three conditioning environments on September
7: 1) outdoor (ambient temperature and light) conditioning at low elevation
in Redding California; 2) ambient daytime conditions as in environment #1, but
with a 15-hour night period at 5° C achieved by placing the plugs in a walk-in
cooler each day at 4:00 p.m.; 3) exposure to ambient conditions at a high-elevation
nursery site in Macdoel, California (41.8° N, elevation 1,200 m). Plugs
were maintained in these environments until September 28, when they were transported
to Irvine and established in fruit production trials. Plugs conditioned at high
elevation had greater early season yields and fruit appearance scores than plugs
conditioned at LE, whereas artificially conditioned plugs had an intermediate
yield and appearance response (Table 10).
The impracticality of moving plugs from an ambient to an artificial environment
on a daily basis during a 3-week period precluded continued examination of artificial
conditioning methods for plug plants, and this treatment was discontinued after
1999. Also, the significant enhancements in yield and fruit quality obtained
by conditioning plugs in a natural HE environment prompted us to focus our efforts
on the use of a high elevation nursery site for conditioning plugs rather than
artificial methods.
In 2000, newly-rooted plugs were transferred to the HE nursery site in Macdoel, California or were maintained in Redding, California to undergo conditioning under ambient conditions. After about one month of conditioning, plugs were transplanted into yield performance trials in Irvine. High-elevation conditioning resulted in greater early-season and total season yields, and early-season and total season fruit size compared to plugs conditioned at LE (Table 11).
Recognizing that transport of newly-rooted plugs to HE conditioning site would be impractical on a commercial scale, we resolved to propagate plugs at both HE and LE sites. For propagating runner tips under mist, we established a small metal-framed structure and covered it with clear polyethylene; later, after plugs were completely rooted, this same structure was left open at both and served as the HE conditioning site. After six weeks of propagation and conditioning, plugs from both HE and LE sites were transported to Irvine and established in yield performance trials. Compared to LE conditioned plugs, HE conditioning resulted in greater early-season yields, and larger fruit with better appearance scores (Table 12). Compared to LE conditioning, HE propagation and conditioning also resulted in greater total yields, and larger fruit with better appearance scores (Table 13).
Performance of plug plants compared to bare-root transplants, 1999-2001.
The information presented above was derived from comparative studies that examined
the effects of plug rooting date, cell size and nursery conditioning environment
on plug plant performance. Our trials demonstrated significant differences between
HE and LE plugs; differences among plugs due to cell size and tip rooting date
were also observed. While these studies are useful for developing protocols
and methodology for propagating high-quality plug plants, ultimately, plug performance
must be compared with that of bare-root transplants, which is the present day
standard planting material.
Yield performance comparisons of plugs and bare-root transplants were made in
each of the three years of investigations. Plug plants, and in particular, plug
plants conditioned and/or propagated at high elevation, usually produced greater
early season yield than bare-root transplants (Tables
14, 16, 18),
but often had inferior fruit appearance scores. However, for the 2001-02 trial,
HE plugs had the highest early season appearance scores, LE plugs had the lowest
scores, and bare-root transplants had intermediate scores (Table
18). For the 1999-00 and 2000-01 trials, there was little difference in
total yield between plugs and bare-root transplants (Tables
15, 17), but bare-root plants generally had the highest fruit appearance
scores; only a few of the HE plug treatments had fruit appearance scores similar
to those of the bare-root plants. In the 2001-02 trial, plugs had greater total
season yields than bare-root plants, but only the small HE plugs had greater
total marketable yields than bare-root plants (Table
19). Also, for this trial, there was no difference between bare-root plants
and HE plugs in regard to fruit size and fruit appearance scores, but LE plugs
had reduced size and appearance scores compared to bare-root plants (Table
19).
Early season yield is an important consideration for fruit growers in southern California, since early season (December-March) fruit commands relatively high prices in the marketplace: market prices are high because fruit production is limited. It is important to recognize that the use of plug plants by a large portion of the industry will result in increased early season production and lower early season fruit prices; the point at which market saturation occurs and prices drop is unclear, but the early season market is limited.
The previous sections summarize the performance trials conducted in Irvine to assess the effects of propagation methodology and plug plant conditioning environment. Additional trials were conducted to assess plug plant performance in the Central Valley (Winters) and the Central Coast (Watsonville) regions. Results of trials in these regions are presented below.
Central Valley Fruiting Field Performance Trials, 1999-2001. Trials were conducted in 1999-2000 and in 2000-01 to compare performance of strawberry plugs with that of bare-root transplants in the Central Valley region of California. For both trials, plantings were established using plug plants that had not been exposed to any artificial temperature or photoperiod conditioning. For the 1999-2000 trial, 80 #1 plugs were established at the Department of Pomology Wolfskill Experimental Orchard (WEO) near Winters, California on August 13. On September 7, 2000, 80 #1 plugs were transported to the U.C. Davis Department of Pomology Wolfskill Experimental Orchard (WEO) near Winters, California and planted on 2-row raised beds in four replicate plots of 20 plants each. Plug plots were established adjacent to plots of Camarosa plants that had been planted on August 11 using cold-stored, bare-root plants, and following U.C. recommendations for commercial strawberry production in the Central Valley. Visual observation indicated that plug vegetative growth was greater (i.e., more crown development) than that of conventional (bare-root) plants, and that plug plants over-wintered well. Fruit were harvested weekly from April 13 to May 31, 2000. Compared to conventional plant material, plugs produced 87% of the yield and had significantly reduced fruit size and appearance scores (data not shown).
We considered that the greater crown development observed in plug plants in the 1999-2000 trial may have resulted in a greater number of branch crowns that produced more flowers but smaller fruit and consequently reduced yield. In a second trial conducted at WEO in 2000-01, and in an effort to reduce the excessive crown development, plugs were established on September 7, 2000, more than three weeks later than in 1999. The delayed planting date resulted in a plant that was equal in size and vigor to the bare-root controls, but yield performance was similar to the previous year's results: compared to control plants, plugs produced less fruit and with reduced fruit size (data not shown).
Watsonville Fruiting Field Performance Trials, 2001-02.
This trial was conducted to ascertain the feasibility of using plug culture
to generate properly-conditioned plug plants of 'Diamante', the leading day-neutral
strawberry cultivar used in California. Like all other day-neutral cultivars,
'Diamante' benefits from rather extensive exposure to chilling temperatures
in the nursery prior to digging in mid-October, as well as a brief period of
supplemental chilling (about two weeks) in a cooler after digging the plants.
Here our concern was that Diamante plug plants left at HE until mid-October
to accumulate chilling could be subjected to damaging low temperatures in the
nursery prior to "digging" the plants. Diamante plugs were propagated
at HE using the tunnel structure described previously. Runner tips were rooted
in mid-August, and were left at HE until mid-October, 2001. Despite minimum
temperatures of -8° C (18° F) during the second week of October, plug
plants maintained at HE did not suffer freeze damage. Plugs were lifted from
the nursery on October 15, then transplanted into fruit yield performance trials
in Watsonville either on October 23, or on November 6, after two weeks of cold
storage at 1° C (33° F). Plugs developed well during fall and winter,
but suffered significant plant die-out in spring due to infection with Phytophthora
cactorum. As a result, meaningful data was not obtained from this study.
Performance of conventional transplants and plug plants in fumigated and
nonfumigated soil.
Trials were conducted in 1999-2000 and 2000-01 to compare the performance of
plugs and bare-root (conventional) transplants established in fumigated (methyl
bromide + chloropicrin) and non-fumigated soil. In both years, results of ANOVA
indicated that conventional transplants and plug plants responded similarly
to soil fumigation treatment. In 1999-2000, we observed significant yield reductions
due to non-fumigation for both types of plants, and with no fumigation x plant
material interactions (Tables 20, 21). In
2000-01, on "new" ground (ground cropped only to winter barley for
20 years) we observed no yield differences between fumigated and nonfumigated
soil environments, and no differences in yield performance between conventional
and plug plants (Table 22).
Potential Benefits/Impacts on Agriculture
Grower experience with plug plants in California has been highly variable. Some growers are pleased with the relatively high early-season yields that are possible with plugs, while other growers have experienced significant fruit quality (poor appearance and small fruit size) in mid- and late-season. Although use of strawberry plug plants appears to be increasing, particularly in southern California where early fruit obtains a premium price, plug plants still account for considerably less than one percent of the strawberry transplants used in California.
Currently, all commercial strawberry plug plants are produced in mild climate areas of southern California in Santa Barbara and San Luis Obispo Counties. These areas are close to the major southern California strawberry growing districts, an important factor in reducing the high cost of plug plant transport. However, the mild climate in these areas is an obstacle to plant conditioning by means of exposure to low temperature and short daylength in late summer and early fall. For decades the California strawberry transplant industry has recognized the importance of a high- elevation environment for producing quality transplants that produce high quality fruit, and the industry has sacrificed productivity while incurring greater production costs at high elevation to do so.
The results of our research demonstrate consistently significant benefits of high-elevation plug propagation and conditioning in regard to early and total season yields, fruit size and fruit appearance scores. Not surprisingly, the best quality strawberry plug plant is one that is produced in a high-elevation environment. However, although high-elevation plugs consistently outperformed low-elevation plugs, high-elevation plugs generally had lower fruit appearance scores than bare-root transplants, as well as smaller fruit size. Only in the third year of our studies, when plugs were propagated and conditioned at high-elevation, did HE plugs have fruit quality similar to that of bare-root transplants. The importance of fruit quality for California strawberry growers cannot be ignored, as the market recognizes and pays a premium for quality fruit: California strawberries typically obtain market prices that are at least 50% greater than that paid for fruit from other US production regions.
A major constraint to producing strawberry plugs at high-elevation is the distance from market and the associated high transportation cost. Our attempts to artificially condition plugs in walk-in coolers resulted in plants that were inferior to those conditioned at high elevation, and artificial conditioning of plugs by use of coolers and darkrooms is probably impractical on a commercial scale.
Strawberry plug plants require considerably less irrigation during the initial planting and establishment phase, and savings that accrue from using less irrigation will offset the higher cost of plugs. Water cost, quality and availability are expected to become increasingly important issues in the near future, particularly in southern California.
Additional, large-scale research research needs to eb conducted to determine
the costs and benefits of producing high-elevation and low-elevation plug plants,
as well as the economics of plug transport and fruit production. The main limitation
to conducting this research is that no commercial high-elevation strawberry
nursery currently appears interested in plug plant production. Most commercial
nurseries view plug plants as a niche market, rather than as a viable system
for producing about one billion runner transplants per year.
Presentations of the project's research objectives and results have been made at numerous UCCE/CSC-sponsored strawberry grower meetings in Irvine, Watsonville and Santa Maria, with an estimated total attendance of over 1,400 people. Additional presentations were made to 400 fruit and nursery growers in Zamora, Mexico (December 2000), and in Huelva, Spain (February 2001). There is a definite need for additional educational efforts in the California strawberry plug and fruit industries regarding improved plug plant propagation methods and high elevation conditioning to enhance early yield, total yield and fruit quality. With the project successfully completed, we intend to further highlight our research findings at nursery and fruit grower meetings statewide and worldwide.