Table of Contents Introduction Planting Systems Varieties Fertility Weed Control Insect and Mite Control Disease Control Greenhouse Production Alternatives to Methyl Bromide Harvest and Postharvest Economics Marketing References Print Resources Electronic Resources Plant Sources Appendix: Recommended Strawberry Varieties by State

Introduction

Organically grown berries may command a price premium. Organic production excludes the use of synthetic fertilizers and pesticides, and requires soil building and biological pest control. The federal Organic standards restrict claims of "organically grown" to those farms that are certified organic by a USDA-accredited certification agency. For more information, request the ATTRA publication Organic Farm Certification and The National Organic Program.

Excellent cultural information for conventional strawberry production—planting systems, pest control, cultivar recommendations, etc.—can be obtained from the Cooperative Extension Service in most states (also see Print Resources below). ATTRA's Overview of Organic Fruit Production provides general information on organic weed control, organic fertilization, and some basic considerations for organic disease and pest control. This publication will cover problems specific to strawberries and will offer organically acceptable solutions, as well as some integrated pest management (IPM) strategies and other reduced-chemical options. We have not attempted to develop a one-size-fits-all prescription for organic (or other ecologically based) strawberry production. Rather we have introduced the most common challenges and offered some possible solutions and factors for consideration.

Many large-scale strawberry growers have begun reducing their reliance on synthetic pesticides by incorporating the latest advances in IPM. Innovative weed-control methods promise to reduce herbicide use while maintaining or increasing organic matter. These reduced-chemical systems will be discussed alongside organic systems in this production guide.

The Agriculture Land Base Training Association (ALBA) farm, Salina, CA

For many years, conventional strawberry growers have routinely used the soil fumigant methyl bromide to control weeds, soil-borne diseases, nematodes, and soil-dwelling insects. Current legislation calls for a 70% reduction in methyl bromide use by 2003, and a total phase-out by 2005 (1). This change promises to radically alter the way strawberries are grown by some of the nation's largest producers. Researchers and growers alike are experimenting with various chemicals and techniques to replace methyl bromide; some of these are described below in a section on Alternatives to Methyl Bromide.

Planting Systems

Raised Bed Plasticulture
Conventional growers in California and Florida, where most strawberries are produced, tend to favor this system. They grow plants as annuals, transplanting strawberry crowns in the late summer or early fall. Production starts in the late winter and continues through the summer and into late fall, depending on the area and the varieties grown. In some cases, the raised beds are fumigated with methyl bromide and chloropicrin, and the plastic mulch serves to seal the fumigant in the bed for three to four days. Next, planting holes are punched through the plastic, aerating the beds for a couple of days before the strawberry crowns are planted. Many organic growers in California and Florida also use this plasticulture system. Of course, methyl bromide is not allowed in organic production, so crop rotation is critical to control soil-borne diseases and pests.

Two types of raised beds are used in these intensive systems. Narrow beds have two rows of plants with one drip line running between them. The distance between beds averages 40 inches. Drip tape is buried at a depth of about 2½ inches. Wide beds usually have four rows of plants and two drip lines, with 64 inches between beds. Spacing between plants in both types of bed averages 12 to 14 inches.

Plastic mulch is used in both types of bed and can vary from a single strip of plastic laid between the plants to full bed coverage where holes must be punched for the plant to develop. Some conventional growers in California use clear plastic, which warms the bed faster, stimulating early-season growth; these growers use fumigation to control most weeds. Black plastic is used in organic production, primarily for weed control. Because the black plastic prevents the sun's rays from penetrating them, the beds remain cool, resulting in slower initial growth of the plants and reduced irrigation frequency compared to clear plastic mulch. There are dark plastic mulches on the market that selectively permit soil-warming radiation to penetrate while eliminating the light rays that promote weed growth. This type of plastic is preferred by growers in the Southeast.

The raised beds provide good drainage. Because they make the flowers and fruit easier to see and reach, raised beds also help growers to forecast yields and make harvesting easier and faster. Some growers dig the furrows between the beds deep so that harvesters will not have to stoop very low to search for the fruit. In cold climates, plants in raised beds may be prone to freeze damage. Still, raised beds will usually out-produce flat beds. Because of increased aeration and protection from splashing soil particles, plants in plastic-mulched raised beds have less disease.

Machinery is available that will shape the bed, lay out the irrigation line, and cover with plastic mulch all in one pass. Sources of bed-shapers and transplanters are listed in the ATTRA publication Season Extension Techniques for Market Gardeners.

Regarding varieties, recent research indicates that any variety that normally does well in a specific region will do well under plasticulture in that region (3). However, some of the cultivars that come from the California and Florida systems perform best at 12- to 14-inch spacing, while many northern cultivars do best at 8- to 10-inch spacing.

By now, growers and researchers in many states have adapted and validated at least parts of the production model described above. Growers should check with their state Extension Fruit Specialist to see if specific plasticulture guidelines are available for their area. Otherwise, a complimentary copy of Nourse Farms Success with Plasticulture can be obtained by calling Nourse Farms at 413-665-2658.

Matted Row System
In this system the crowns are planted in early spring. As the plants produce flowers, the blossoms are removed to encourage runner (or daughter plant) production. The daughters root on the bed and produce a crop the following spring. Weeds can be a problem in this system, and dead leaves and other debris must be removed to reduce disease and pest problems. However, once established, this system can produce for three to four years, depending on the pest pressures. Distance between plants is 18-24 inches, and distance between the rows varies from 36 to 50 inches depending on the cultivation equipment used. According to Marvin Pritts of Cornell University, the matted row system offers northern strawberry growers a low-risk system that requires less focus and time than annual plasticulture systems (6).

Ribbon Row System
This system can employ high-density or low-density planting on a single row. With low-density planting, the spacing is 12-36 inches between rows and 14-18 inches between plants. With high-density planting, the distance between the rows is the same but the distance between plants varies between 4 and 12 inches. The crowns are planted in the fall. Once they start blooming, the flowers are not removed, and fruit is produced in the first season. Runners are removed to stimulate flower formation and increase fruit size. At the end of the second season the planting can be changed to the matted row system by letting the runners fill in empty spaces on the bed.

Varieties

Strawberry varieties are classified as either "June-bearing" or "Everbearing". June-bearing or short-day varieties start forming flower buds as the day length gets shorter and temperatures get cooler. Everbearing or day-neutral varieties are insensitive to day length and produce fruit throughout the season as long as night-time temperatures drop below 60°F (7).

See the Appendix for a list of recommended strawberry varieties by state.

Fertility

First, June-bearing strawberries set buds for the following year's fruit in the fall. (Most perennial fruit crops set their fruit buds in the spring or early summer.) To get a good bud set, the plants must have adequate chilling and not be nutritionally stressed. Therefore, a fertilizer application is usually warranted in the late summer, giving the organic fertilizer material enough time to break down and provide nutrients for the plants during the crucial fall bud-set. Day-neutral strawberries set fruit buds throughout the growing season, and therefore need to have adequate nutrition provided for the entire period. Producers using the annual raised-bed or plasticulture system must rely on the nurseries that supply the strawberry crowns to adequately fertilize the plants for fall planting. (The nurseries that provide strawberry crowns on a large scale use conventional pesticides and fertilizers in growing them; these crowns are acceptable for organic production only if no organic sources of strawberry crowns can be found. Certifiers require that growers document their search for certified organic planting material.)

Second, while all perennial fruit crops will benefit from the fertility provided by pre-plant cover-cropping and green-manuring, strawberries are so prone to weed problems that pre-plant preparations to reduce weed pressure are practically mandatory in organic production. A thick cover crop (or two in succession) of a grass/legume mix will help to smother out many weeds and will provide important long-term improvements in soil fertility and soil organic matter. In areas such as coastal California, long growing seasons and high rents may make the extended use of cover crops uneconomical. However, many growers believe that the long-term benefits of cover crops and rotations to fertility management and pest and disease suppression are worth the cost. Compost can be used as a supplement or an alternative. Spreading and incorporating the compost on the beds only, avoiding the furrows, will help concentrate fertility where it is most needed. Ten to 15 tons per acre of compost is recommended. If this volume is not available, more frequent supplemental fertigation will be necessary.

Organic growers using black plastic or fabric mulches will also find it useful to make a heavy application of compost during bed preparation to provide fertility. Subsequent fertility needs can be met through fertigating with appropriate soluble organic fertilizers such as seaweed-fish blends or, to a limited degree, with foliar applications of soluble organic fertilizers. For more information, request ATTRA's Sources of Organic Fertilizers and Amendments.

Weed Control

Cultural
Organic growers will find that some hand weeding is necessary. It is quite possible for weeds to become troublesome in plasticulture systems, especially in organic plantings that haven't been fumigated. In such situations, the weeds emerge from the same planting holes made for the strawberry plant. The rows must be straight and the plastic laid precisely to allow mechanical cultivation of the furrows without damaging the beds and plastic.

The matted row system (where plants from runners form a 6- to 30-inch-wide solid bed) is commonly used by strawberry growers in many regions of the U.S. This method precludes mechanical cultivation for weed control within the bed, though cultivation is commonly used to renovate or narrow a bed. Weed problems tend to increase with the age of the planting. Many organic growers have therefore chosen shorter fruiting rotations. That is, a bed may be allowed to fruit for two seasons before it is turned under and replanted to a cover crop.

Mechanical
European strawberry growers and researchers have led the way in innovations and research involving mechanical weed control in strawberries. Recent research in the U.S. has confirmed the usefulness of the flex-tine harrow, the brush hoe, and the finger weeder for weeding strawberry plantings. For instance, the brush hoe required only three passes per season plus two hand weedings for complete weed control, compared to standard cultivation with a rototiller, which required three passes and four hand weedings (8). Visit the European Weed Research Society's Physical Weed Control web page for more information on these and other mechanical cultivation tools. In plasticulture systems, harvest crews are sometimes used to weed when the weed pressure is high or when the harvest day is short.

Biological
Before the widespread adoption of herbicides, geese were commonly used for weed control in commercial-scale strawberry production. In areas of concentrated crop production, farmers often had the benefit of weeder-geese services for hire. Weeder geese can still be used to control grasses and a few broadleaf weeds, but close management of the geese is essential. Not every farmer will find the extra requirements suitable to his or her management regimen. The extra work may be offset to some degree by on-farm consumption of the geese or by sales of geese and their products. In any case, the geese must be removed before fruiting season, because they will eat strawberries before going after grass. Also, under the National Organic Program, raw animal manure must be composted unless it is incorporated into the soil not less than 120 days prior to harvest of a crop whose edible portion has direct contact with the soil surface or soil particles. Therefore, geese would need to be removed from the field and their manure incorporated at least 4 months prior to the beginning of strawberry harvest. ATTRA has more information on the proper management of weeder geese available on request.

Organic mulches
Strawberry plants, especially in the North, are commonly mulched with straw over the winter to minimize cold damage. In the spring, the straw is raked into the aisles where it provides some control of weeds and helps to keep the berries clean. Caution must be taken with some organic mulches in that they may harbor pests like snails, slugs, cutworms, earwigs, and sow bugs. On the other hand, straw provides excellent habitat for spiders and has been known to reduce diseases.

Research in West Virginia indicates that shredded or chopped newsprint makes an excellent, safe mulch (9). It can be applied over the top of the plants at the outset of winter, just like straw. Apply it 4-5 inches thick (this will require about 500-600 pounds of chopped paper per 1,000 square feet). The newsprint mulch is subject to being windblown until it is stabilized by rain or overhead irrigation. Only newspaper or other recycled paper, without glossy or colored inks, may be used as mulch under the National Organic Program standards.

Living and killed mulches
Cornell small-fruit researchers Marvin Pritts and Mary Jo Kelly (8) have looked extensively at using cover crops for weed suppression in strawberries. They tried several species—including tall fescue, marigold, buckwheat, and ryegrass—but sudangrass seemed to combine the most desirable characteristics: rapid establishment, low water use, low nutrient use, and competitive displacement of weeds. Their research suggests that interseeding sudangrass and mowing it twice a year provides acceptable weed control without herbicides, especially when used in conjunction with a winter straw mulch.

Herbicide-killed mulches offer non-organic growers the opportunity to practically grow mulches in place. There are two basic types of desiccated mulch systems-one pre-plant and one post-plant. The pre-plant system employs a cover crop (grass, legumes, forbs, or a mix) and glyphosate (Roundup®) to create a no-till mulch, often just in foot-wide strips where the strawberry plants are to be set out.

The post-plant system employs grass cover crops between rows or even within the row and a grass-specific herbicide such as sethoxydim (Poast®) or fluazifop-butyl (Fusilade®) to kill the grass. The strawberry plants are unaffected by these herbicides. Chemically killed mulches can allow organic matter to be maintained and potentially increased during the cropping sequence. Another plus is that some grasses, including common ryegrass, suppress annual weeds allelopathically (in other words, these grasses exude chemicals that keep other plants from growing near them). The herbicidal desiccation apparently does not reduce the allelopathic effect.

In USDA zones 6 and colder, another option is to plant spring oats in the fall. Freezing weather will kill the oats, leaving a nice mulch. Yet another option is to plant sorghum-Sudan grass in the late summer; it is not at all cold tolerant, and will be killed by the first frost. For information on the USDA's hardiness zones check their website.

Steam
Technology is evolving that allows for weed management with steam. The tools are similar to those for flame weeding (see the ATTRA publication on flame weeding). Steam is directed onto weeds, and they wilt. This is distinct from steam sterilization of soil, discussed later in the section Alternatives to Methyl Bromide.

While the technology currently exists, it is still too expensive for most growers to consider. However, cooperative ownership or perhaps further technological innovation could render it more affordable. For a full discussion of the technology, applications, and economics, see the article Full Analysis: The Use of Steam as an Alternative Herbicide.

Corn gluten
Researchers at Iowa State discovered a few years ago that a by-product of corn milling, corn gluten, inhibited the germination of certain weed seeds, especially grasses. Corn gluten's high nitrogen content also makes it an effective fertilizer. Though ineffective against many perennial weeds, corn gluten has been shown to be helpful in controlling certain weeds in strawberry plantings.

However, research at Cornell University indicates that in wet years or predictably wet climates, corn gluten may actually favor weeds. Evidently, in wet conditions, the herbicidal component of the gluten is leached out of the soil, but the nitrogen remains long enough to fertilize the weeds (8).

Vinegar Herbicide
The use of vinegar for weed control has been the least-toxic choice of many home gardeners. Its effectiveness varies on the type of weeds sprayed and the concentration of acetic acid. Most vinegar available commercially is 5% acetic acid. Through distillation the concentration can increase to 15% and by other non-synthetic processes to 30% acetic acid. Caution must be taken with formulations greater than 5%. There are more concentrated solutions of acetic acid that are derived synthetically; these types are not allowed in organic production systems. Some commercial formulations of vinegar herbicide include lemon juice or citrus oil. The mode of action consists of the acidic solution degrading the leaf's waxy cuticle layer, causing desiccation. The thicker the cuticle layer on the weeds, the more frequent the applications or the more concentrated the solution should be. If preparing a homemade solution of vinegar herbicide, include citrus oil or juice along with a small amount of liquid soap as a surfactant. Be careful when spraying weeds and keep the spray off the strawberry plants. For more information on vinegar as an herbicide check the USDA website.

Woven synthetic fabric mulches
Synthetic fabric mulches (trade names: Weed Lock, Weed Barrier, Weed Stopper, et al.) offer some of the same weed suppression as regular plastic mulch, but add the advantage of being water- and air-permeable. Though initially more expensive than regular plastic, the higher-quality grades of the fabric mulches can be used year after year. These woven mulches are used in essentially the same way as the plasticulture systems described above. However, because they are water-permeable, it should not be necessary to add irrigation lines under the mulch in areas with adequate rainfall.

Insect and Mite Control

Beneficial-insect habitats planted alongside strawberry fields provide shelter, pollen, and nectar sources to predators and parasites of insect pests, and give them refuge when the fields are treated with a pesticide. When purchased beneficial insects are released, these habitats will encourage the beneficials to remain and continue their lifecycle, helping reduce the pest population. Some pests may also inhabit the refuge along with beneficials, so it's important to monitor these habitats. (Some growers have used a "bug vacuum" to remove lygus bugs that have concentrated in a habitat planting.) For additional information see ATTRA's Biointensive Integrated Pest Management and Farmscaping to Enhance Biological Control.

Although pest problems vary with the production location, common strawberry pests include white grubs, strawberry weevils, strawberry rootworms, lygus bugs, and spider mites. For more detailed information on the pests themselves, refer to the publications listed in the Print Resources section below (see especially the publications by Funt et al. [1997], Maas [1987], Strand [1993], and Kovach et al. [1990]).

White grubs
Primarily a problem in the eastern U.S., white grubs can cause serious damage if strawberries are planted immediately after a sod crop. White grubs are the larvae of May and June beetles and other beetles in the Scarabaeidae family. Late-summer or early-fall plowing destroys many larvae, pupae, and adults in the soil and also exposes these stages to predators. The milky-spore-disease bacteria Bacillus popillae and Bacillus lentimorbus are important natural enemies of Scarab beetles, killing the grub stage. Grubs ingest spores on the thatch or roots of the grass they eat. The bacteria multiply inside the grubs, which then die and disintegrate, leaving many viable spores to spread the disease to succeeding generations (10).

Beneficial nematodes are also effective against soil-dwelling grubs. Steinernema carpocapsae will infect its host near the soil surface while Heterorhabditus bacteriophora actively searches for its host below the soil surface (11). These nematodes and milky-spore bacteria are widely available through mail-order garden supply companies.

Strawberry clipper (strawberry bud weevil)
The strawberry clipper or bud weevil, Anthonomus signatus, occurs only east of the Rockies. Adult beetles emerge in the early spring, lay eggs in the buds, and then cut partly through the stem, causing strawberry buds to fall over or fall to the ground.

Though it was once considered a major pest, recent research indicates that this insect can be largely ignored in most situations (12). Even though the clipper is removing flower buds, it appears that the remaining buds compensate by producing larger fruit, resulting in no overall loss in yield. Moreover, the clipper moves at the very slow rate of 30 feet per season. In a new planting, it is unlikely that the damage would extend more than 30 feet from the perimeter into the planting. Damage may be somewhat more extensive in older plantings, but still limited by the rate of movement of the clippers (they would be approximately 60 feet into a 2-year planting and 90 feet into a 3-year planting). Organic growers can destroy damaged buds, which contain eggs; eliminate trash and nearby foliage that provide hibernation sites for adult weevils; and apply a botanical insecticide registered for use on strawberries.

Strawberry rootworm
Strawberry rootworm (Paria fragariae) adults feed on leaves, mainly at night, making holes in the leaves. The larvae feed on fine roots and eat off crowns close to the ground. Cultural control consists of plowing infested fields after harvest and setting new plantings away from woods (favorable hibernation sites) and from older strawberry plantings.

Apparently, IPM damage thresholds have not been established for the rootworm. If the grower feels that pesticide treatment is necessary based on past history, treatment should be aimed at the foliar-feeding adults, since there are no effective or registered insecticides available for control of the larvae.

Lygus bugs

Lygus nymphs

The tarnished plant bug or lygus bug (primarily Lygus lineolaris in the East and L. hesperus in the West) can be troublesome, especially in plantings of day-neutral types, which fruit throughout the growing season. Adults and nymphs (the nymphs cause the most damage) suck sap from the plant and inject a toxic saliva. This feeding results in a characteristic deformation of the fruits, called cat-facing, which makes the berries unusable and unmarketable.

Keeping the groundcover well clipped for a distance of five to ten yards around the production site, and otherwise destroying places favorable for hibernation, may help reduce lygus-bug populations. Adult lygus bugs hibernate under leaves, stones, and bark. They usually lay eggs in the stems of herbaceous cultivated plants and broadleaf weeds. Legumes (vetches, clovers, alfalfa, etc.) can harbor large populations of these pests. This must be considered if beneficial habitats using these plants are established near the strawberry planting.

Trap crops are also useful in lygus bug management. In California an annual trap crop mix of one dormant and one semi-dormant alfalfa variety, two radish varieties (Daikon and Cherry Belle) and sweet alyssum have been used with success. Lygus bugs move in from the surrounding fields and settle on the trap crops, which can then be treated with insecticides or vacuumed (13). (Bug vacuums are discussed below.)

Research conducted in New England found variation in susceptibility to the lygus bug among 20 strawberry cultivars (14). Honeoye, Sparkle, Veestar, and Canoga suffered the least from feeding, while Kent, MicMac, Scott, Blomidon, and Redchief suffered most.

A fungus, Beauveria bassiana, has some efficacy against lygus bugs. In New York, three years of tests concluded that the commercial formulation of B. bassiana, Mycotrol™, reduced lygus damage about 50% compared to untreated controls, but was still considerably less effective than synthetic insecticides, such as malathion (15). The product worked best when targeted at younger nymphs and when humidity levels were adequate. In combination with other cultural controls (choosing the right cultivar and close mowing near the planting), use of Mycormax™ (J.H. Biotech), Mycostop™ Biofungicide (Ag Biodevelopment, Inc.), or Mycotrol O™ Beauveria bassiana strain GHA (Mycotech Corporation) could be of significant help to organic growers in controlling lygus.

While the lygus bug has several natural insect enemies, none of the native ones has proved consistently effective in providing a commercial level of control in strawberries. A small (1/8th-inch-long) parasitic wasp (Peristenus digoneutis) introduced from Europe in 1984 has exhibited excellent control potential (16); however, it is difficult to rear, and is not commercially available. While it is spreading naturally in the northeastern U.S., it has not moved south of latitude 41° N (New York City). Anaphes ioles is another lygus egg parasitoid that has been used in California and in other states with some success. Researchers who released 15,000 A. iole weekly on one-acre strawberry plots observed a 64% suppression of Lygus hesperus compared to a 44.7% reduction achieved with a pesticide application (17).

A little-used innovation in non-chemical pest control is the BugVac® machine, which is actually a large vacuum cleaner for pests. The BugVac is tractor-mounted and can be set up to sweep as many as four beds in one pass. A trial by University of California researchers concluded that three similar grower-designed vacuum machines reduced lygus bug damage compared to untreated controls, but were not equal to chemical control with a pyrethroid insecticide. The damage, though reduced, was still considered economically unacceptable (18). Because of their marginal effectiveness, combined with the spread of disease spores like powdery mildew and botrytis resulting from their use, Bug Vacs have not been used much in California strawberries. For more information on Bug Vacs, see the ATTRA publication "Bug Vacuums" for Organic Crop Protection.

Because the nymphs are most troublesome, aim your lygus scouting efforts at this life stage. Start checking for nymphs as soon as flowers appear. Tap 10 to 15 flower clusters over a white plastic saucer so that the bright green nymphs can be seen and counted. Figure the average number of nymphs per cluster (total number of nymphs divided by total number of clusters). If sampling is concentrated near weedy borders, the action threshold is 1 nymph per cluster, but if done randomly throughout the planting, .5 nymphs per cluster should be considered adequate to prompt a pesticide treatment (19). However, Cornell researchers caution that growers who intend to use the slow-acting biological insecticide B. bassiana discussed above may need to use a lower threshold (15). If other natural enemies of lygus are present—such as spiders, bigeyed bugs (Geocoris species), assassin bugs (Zelus and Sinea species), damsel bugs (Nabis species), and lacewing larva (Chrysoperla species)—you might want to consider adjusting the threshold numbers accordingly.

Mites

Minute Pirate Bug UC Statewide IPM Project © 2001 Regents, University of California

The web-spinning spider mites are in the Tetranychus genus, which includes the two-spotted spider mite, Pacific spider mite, and strawberry spider mite, among others. These plant-feeding mites consume juices from strawberry leaves. Large populations can reduce photosynthetic capacity, resulting in weakened plants and reduced fruit yields. Organic growers who do not often use botanical pesticides may see very few mites—if not reduced by pesticides, the natural enemies of the mites will usually keep them in check. These include other mites such as Phytoseiulus persimilis, Metaseiulus occidentalis, and Neoseiulus californicus, and insects like bigeyed bugs, damsel bugs, minute pirate bugs, lacewings, spider mite destroyers, and six-spotted thrips. Growers can buy some of these predators from commercial insectaries to release on the farm. The predators can also be attracted and conserved naturally through the use of insect habitats.

Two-spotted Spider Mites UC Statewide IPM Project © 2001 Regents, University of California

If mites do become a problem, soap sprays are of some help. Sprays for mites, organic or otherwise, need to reach the undersurface of leaves, because this is where the mites feed. This is especially important with soap sprays, which have no residual activity. Sprayers, therefore, need to be powerful enough to blow the leaves around, exposing the undersides.

A scouting method for two-spotted spider mites has been developed in British Columbia and successfully implemented both there and in New York (19). To sample for these mites, walk diagonally across the planting, randomly picking one mature, fully expanded leaflet from every other row, until you've collected 60 leaves. If 25% are infested with mites (about 5 mites per leaflet), a miticide may be in order. Again, the number of natural enemies should also be considered when determining a threshold for chemical treatment. While this scouting method is probably applicable to most areas, growers outside the New York region should check with their local Cooperative Extension Service for scouting guidelines.

Some of the mites you see when scouting may be predator mites. You may need a magnifying glass to distinguish between these beneficial mites and the pest mites. One key to telling them apart is that the beneficial predator mites are generally more active than the two-spotted mites-they are typically moving quickly about the leaf surface looking for prey. Depending on your geographical area and the species involved, the recommended ratio of beneficial mites to pest mites varies, but seems to average approximately 1:10. That is, if there appears to be at least one beneficial mite for every 10 pest mites, control of the pest mites will probably be achieved naturally without the intervention of miticide sprays.

Dust that accumulates on the spider mite's webbing creates an ideal shelter for the mites and their eggs. These little dust "tents" discourage predators and prevent the miticide from reaching the mites and their offspring. California growers commonly water roads, post "slow" signs, plant windbreak crops, and use fencing to decrease dust in strawberry fields.

Other Pests
Other arthropod pests that will occasionally reach pest status include aphids, spittlebugs, whiteflies, Cyclamen mite, various caterpillars, earwigs, and leafhoppers. If they become a problem consult your local farm advisor, visit the numerous websites listed below under Electronic Resources, or call ATTRA's toll-free number for information.

Disease Control

Because soils high in organic matter tend to be inhospitable to soil-borne pathogens, incorporating cover crops and/or compost in strawberry fields can aid significantly in suppressing root and crown diseases such as black root rot and red stele. Also, strawberries that are grown on the same land year after year tend to suffer chronic, severe disease problems—well-designed rotations can help to suppress some of these diseases. (Rotations also help control weeds and insects, provide organic matter, and improve the physical structure of the soil.) The best rotations for strawberries include a legume (such as soybeans, alfalfa, cowpeas, or vetch) with rye. Solanaceous crops such as tomatoes, potatoes, peppers, and eggplants should not be grown preceding strawberries, because of the potential introduction of pathogens such as Verticillium.

Shredding the strawberry leaves immediately after harvest—with a rotary mower, sickle-bar mower, or scythe—can significantly reduce diseases such as leaf spot, gray mold, and leaf scorch. Rotary mowing shreds leaves, hastening the breakdown of the plant matter on which the pathogens' survival depends. Since some pathogens will survive this treatment, removal of the plant residue from the field will further aid in minimizing inoculum. If you use a scythe or sickle-bar mower, definitely rake and remove the clippings. Take care that mowers are set high enough to avoid damage to the plant crowns but low enough to remove most of the leaves.

Compost teas and other innovative concoctions such as yeast-sugar solutions, sodium bicarbonate, and milk have become popular as foliar disease preventives among many organic growers. The concept behind their use is to make the host plant inhospitable to the potential pathogen. Compost teas and yeasts introduce non-plant-pathogenic microorganisms that compete with and antagonize the disease spore as it tries to establish itself on the host. Baking soda works at the chemical level, interfering in spore germination . For more information request ATTRA's publications Notes on Compost Teas and Use of Baking Soda as a Fungicide.

Elemental copper and sulfur have long been used by conventional and organic growers as pesticides for bacterial diseases and powdery mildew respectively.

Botrytis (gray mold)
Gray mold, caused by the fungus Botrytis cinerea, is one of the most common and serious fruit rot diseases. The fungus grows best in cool damp weather, and gray mold can be devastating if rainy weather coincides with harvest, when the fruit is at its ripest and most susceptible. Pickers handling infected berries can spread the infection to healthy berries, causing them to rot within two days after picking. Control of gray mold is aided by removing infected debris from the field and by providing good drainage. Infected fruit can be picked off the plants and placed in the furrow as long as a cultivator can go through the field and bury this fruit. Clean mulch, which keeps fruit off the ground, is also highly recommended. Removing leaves from the field as soon as the harvest season ends can significantly reduce the incidence of gray mold on fruit in June of the following year (21).

Grey Mold on Strawberry © 2003 ohioline.ag.ohio-state.edu

Beneficial fungi antagonistic to Botrytis species are being developed for commercial use and may eventually provide effective biological control of gray mold. Gliocladium roseum has shown the most promise for gray-mold control in Canadian research (22). Sprays of Gliocladium spore suspensions are effective, but the researchers are developing a more efficient system using honeybees to deliver the beneficial spores. The researchers place Gliocladium spores in a shallow tray at the hive entrance. The bees walk through the tray, pick up the spores, and deliver them to the flowers during pollen and nectar collection. Control of gray mold in these trials is excellent.

Although there is not a high level of gray-mold resistance in any one strawberry cultivar, Earliglow is relatively resistant compared to most cultivars (23).

Leaf spot
Leaf spot diseases-identified by the presence of spots on leaves and stems-can be caused by the fungi Mycosphaerella fragariae, Ramularia tulasnei, or Phomopis obscurans, or by the bacterium Xanthomomas fragaiae. These pathogens are spread by splashing water and harbored by dead leaves and other plant debris. The recommendations above on foliar disease preventives apply to leaf spot.

Powdery Mildew
Powdery mildew is a fungal disease that infects foliage, flowers, and fruit. The disease spore prefers intermittently moist conditions and will not germinate in free-standing water. The practically year-round production season, foggy cool nights, and warm days make the disease a major, very persistent problem in coastal California strawberry fields. Sulfur is the most common control on both conventional and organic farms. Milk has been used successfully against powdery mildew on cucurbit crops (24). The fungus Ampelomyces quisqualis has been developed commercially as AQ 10™ for the biological control of powdery mildew on numerous crops including strawberries.

Red stele
Red stele, so named because the disease results in a reddening of the root's center, is caused by the fungus Phytophthora fragariae. The pathogen can remain viable in the soil for up to 20 years.

Where soil moisture is high, such as in low or poorly drained sites, the pathogen will spread to infect new areas. Growers can prevent the disease by planting strawberries in a pathogen-free well-drained area, avoiding over-watering, and setting out only certified disease-free plants. It is possible to select cultivars with resistance to all five races of the red stele fungus.

Anthracnose
Anthracnose can be very serious, causing plants to die out in midsummer. The disease produces a rust color throughout the crown and eventually stops the plants from growing. Symptoms are most noticeable during summer dry spells.

Since high soil fertility favors anthracnose, little or no fertilizer should be applied when disease pressure is strong. However, resistant cultivars can be grown successfully under much higher fertility levels (25). Anthracnose is more prevalent in the Southeast than elsewhere. Commercial growers in the Southeast should avoid planting on former strawberry sites and check with the local Cooperative Extension office for the prevalence of anthracnose in the area and for names of locally adapted resistant cultivars.

Greenhouse Production

Lighting is critical for winter production. The day-neutral cultivars (e.g., Tribute and Tristar) or the short-day types (e.g., Jewel) are much easier to grow during the short days of winter than most of the traditional June-bearing types. It is difficult and expensive to get the June-bearing types to fruit out of season. Even with the day-neutral types, some supplementary lighting will be necessary to get high-quality fruit.

Supplementary heat will have to be available (in some cases the lighting will provide enough heat). While some non-fruiting vegetables (e.g., leafy greens, such as spinach) can produce well in unheated greenhouses, strawberry plants need about 68 and 54 degrees Fahrenheit day and night, respectively, to produce high yields of high-quality berries.

The grower will also have to provide pollination. Bumblebees are probably the best pollinators in a greenhouse environment. GB Systems (26) and The Green Spot (27) are two commercial sources of bumblebees and bumblebee nesting boxes.

Certain pests (usually the larger ones, e.g., tarnished plant bugs) can be effectively excluded from greenhouses, but others, such as mites, aphids, whitefly, thrips, and fungus gnats are likely to thrive and proliferate. Because of the need for bumblebees for pollination, controlling these pests with conventional pesticides is not a good idea. Fortunately, these pests can be effectively managed with biological controls, such as beneficial mites and lacewing larvae. For the details of greenhouse pest management, contact ATTRA for our series of publications on greenhouse IPM.

Finally, prospective greenhouse growers should spend some time exploring local markets (restaurants, groceries, etc.) Off-season greenhouse growers will be competing with strawberries from California, Mexico, Chile, and Florida. Prices will have to offset the costs of production, so growers will have to be certain to produce an outstanding product. Cornell researcher Marvin Pritts found that the break-even price for greenhouse-grown strawberries was $3/pint. He reports, however, that a small but significant number of consumers are willing to pay that price for high-quality berries.

For more detailed information on greenhouse strawberry production, go to The Berry Diagnostic Tool web page.

Alternatives to Methyl Bromide

Finding a single substitute that can do all that methyl bromide does will be difficult. Chloropicrin, telone, metam-sodium, methyl iodine, and dazomet are some chemicals that are being used and investigated, alone and in combination, as methyl bromide substitutes. These chemicals are similar to methyl bromide in toxicity to soil life—the basis of sustainability—and represent a "silver bullet" approach to pest management. Production and use of these chemicals also entails environmental and worker-safety hazards.

Ironically, ozone gas is being investigated as a substitute for the ozone depleter methyl bromide to control soil pests (and as a post-harvest fumigant for some commodities). It is an effective sterilant, but because of its instability it cannot be stored and must be produced on-site. The advantage of ozone is that its decomposition by-product is oxygen—it leaves no toxic residues. Research done by Soilzone Inc. on strawberries, tomatoes, and carrots looked at marketable yields compared to untreated controls (29). More research is needed on the specificity of ozone against soil pests and other micro-organisms.

For broad-scale pest control similar to methyl bromide, steam and solarization are two possible non-toxic alternatives. Steam sterilization in large fields is currently considered prohibitively expensive and impractical (the equipment necessary to cover large acreages efficiently is simply not yet available). However, equipment is available to treat smaller acreage for high-value crops like strawberries (30).

Where summers are hot and sufficiently sunny, solarization is effective against many weed species, most nematodes, and most serious pathogens. A full description of soil solarization is available in ATTRA's Overview of Organic Fruit Production.

One of the problems with solarization and many of the other alternatives—such as rotations, specialty cover crops, fallow periods, etc.—is that they tie up the crop site for all or a significant part of the growing season. This may seem particularly onerous to growers who are used to planting strawberry crop after strawberry crop with between-crop fumigation. The current recommendation for solarization in California, for example, is to leave the soil covered for 4-6 weeks. The cost of solarization is considerably lower ($150-200/acre) than the cost of methyl bromide ($2,000/acre) and metam sodium under plastic ($400-600/acre) (31), but these savings must be weighed against the production opportunity lost during solarization.

The organic system requiring the least time between crops may be one relying on 1) soil amendments for disease and nematode control and 2) fabric mulch for weed control. Research consistently shows that adding various types of organic matter—compost, fish by-products, chitin, manure, etc.—can aid greatly in disease and nematode control; however, such control will never equal that of methyl bromide fumigation. Most of these soil amendments work by fostering microbial antagonism. That is, the increase of organic matter in the soil promotes the population and activity of various microorganisms, many of which are antagonistic to plant pathogens and nematodes.

SoilGard® and DiTera® are two soil amendment products that have proved effective against certain soil-borne strawberry diseases (SoilGard) and nematodes (DiTera). Both products are based on naturally occurring microorganisms. Other such products are in the registration pipeline.

All the other available alternatives—rotations, "specialty" cover crops, fallow, etc.—are essentially cultural manipulations, and each has its relative strengths and weaknesses when it comes to controlling the various disorders that methyl bromide controls. For instance, grass cover crops are generally non-hosts for nematodes and are therefore an effective rotation crop for helping to control them. However, grass sod is attractive to June and Japanese beetles, which could lead to grub problems. It is probably fair to say that each region, and possibly each grower, will have to work out the "perfect" rotation system for strawberries.

Nonetheless, it is known that chemicals exuded by certain cover crops such as marigolds, castor beans, chrysanthemums, sesame, and mustard suppress nematodes. Some cover crops have exhibited nematode-suppressive characteristics equivalent to aldicarb (32). Almost all of these cover crops are more effective as nematicides when incorporated into the soil. For more information on nematode control request ATTRA's Alternative Nematode Control publication.

Of all these "specialty" cover crops, those in the Brassicaceae (mustard family) seem to be the most effective in combining control of weeds, pathogens, and nematodes (and probably some of the soil-dwelling insect pests). This may be attributable to the release of sulfur compounds from the decomposing residues. Dr. Jack Brown (33), a plant breeder at the University of Idaho, has identified several factors in brassicas that support this "mustard effect" hypothesis. Plant chemicals produced by brassicas, and subsequently released from the decomposing plant, include glucosinolates and isothiocyanates. Breakdown products from glucosinolates are similar to the chemical fumigant VAPAM®. Sulfur, a natural component of isothiocyanate, is also released during decomposition. Dr. Brown has worked on breeding lines of brassicas that contain high levels of glucosinolates; he released the rapeseed cultivar Humus as a product of this research. Applying basic soil solarization techniques after tilling in a brassica cover crop would likely increase the "mustard effect" significantly, resulting in "biofumigation."

Frank Sances, director of Pacific Ag Research, and Elaine Ingham, a soil ecologist and president of Soil Foodweb Inc., have been collaborating on developing organic alternatives to methyl bromide for strawberry production (34). In 1995, they tested alternative chemical treatments alongside organic treatments combining broccoli residue, high rates of compost, and added mycorrhizal fungi. In soils that had been repeatedly fumigated, the organic materials could not regenerate the soil to the extent needed to suppress root pathogens and achieve commercially acceptable yields. However, in soil that had never been in strawberries and had lain fallow for three years, organic amendments produced yields that were only slightly lower than the methyl bromide standard.

In 1996, Sances and Ingham added another variable to their experiments—plug plants. In contrast to the traditional bare-root, field-grown strawberry nursery plant, a plug plant is an individual plant grown in sterile potting media. Such plants offer growers a healthier start for new plantings. The plug plants produced 38.5% higher yields than the bare-root plants in non-fumigated soils (35). The 1997 trial plots were amended with compost and inoculated with vesicular-arbuscular mycorrhizae (VAM); yields were similar to methyl bromide plots, producing 216 more flats than untreated plots. In 1998 there were smaller differences than in previous years between the treated plots with compost and the controls and almost no benefit on either bare-root or plug plants was demonstrated from inoculation with VAM (36).

The California departments of Food and Agriculture and Pesticide Regulation have formed a task force, including the USDA's Agricultural Research Service, to find alternatives to methyl bromide. For the most recent research regarding methyl bromide alternatives check the United States Environmental Protection Agency website.

Harvest and Postharvest

The Agriculture Land Base Training Association (ALBA) farm, Salina, CA

Proper postharvest handling of strawberries is essential. Cooling the berries will remove field heat and increase shelf life. Harvesting early in the day while temperatures are cool and then precooling the fruit before shipping will extend the shelf life significantly.

Forced-air cooling is the most common method used on strawberries. The flats are stacked parallel to each other in a cold room with an open space between the flats. A tarp is then placed over the top and ends of the stacked cartons, with a fan located between stacks. The fan pulls cold air between the gaps of the stacked flats, removing the field heat from the berries. It is vital that the fruit be cooled as soon as possible. The more the delay between harvesting and cooling exceeds one hour, the greater the losses to deterioration (37). Water loss from the strawberries can be a problem, so it's critical to maintain high humidity in the cooling facility. Avoid wetting the fruit, which can cause decay problems.

Fresh-market strawberries are usually sold in pint or quart baskets covered with plastic wrap. However, one-piece molded-plastic containers called "clamshells" are rapidly replacing this packaging. The time and labor involved in packing the fruit in the traditional pint-size plastic baskets is considerable, because shippers and buyers grade fruit packed in this manner by the arrangement of the fruit in the flat. This puts additional burden on the farm worker to pack the fruit correctly. The use of clamshells makes the strawberry pickers' job a little easier; the wholesalers are not as concerned with the appearance of the fruit pack since it looks uniform with the clear lid. Many of these clamshells are recyclable. A drawback to the clamshells is the greater difficulty of cooling the fruit. The holes in the containers are not big enough to allow for rapid cooling, so extra time in the forced-air cooler is necessary. The clamshell containers also hold less fruit than the pint baskets and sometimes are sold at a lower price. If you sell wholesale or direct to stores, the buyers may require this type of packaging.

Wholesale strawberries that are shipped long distances are placed on pallets, in bags that are injected with carbon dioxide after the fruit is thoroughly cooled. This modified-atmosphere process is patented by Transfresh Corporation of Salinas, California and is known as Tectrol® Atmosphere Pallet System. The process extends the shelf life of the fruit, allowing for transport and marketing. It is also accepted in organic production. It should be noted that large volumes need to be shipped to make this process economically feasible. For more information on the Tectrol® system go to the Transfresh web site.

Economics

Strawberries are a high-value crop, but they also have special production requirements, a short shelf life, and a brief marketing season. Initial investment in land preparation, irrigation and other equipment, and so on, can cost from about $3,400 per acre for a matted row system, and up to nearly $20,000 per acre for a plasticulture system (39).

Organic strawberries are in demand across the country. Recently, strawberries made it to a publicized list of top-ten foods to avoid because of high pesticide residues; this may have helped fuel interest in organic strawberries. Because they face higher costs of production, organic growers must secure a premium price in order to make a profit. Continuous cropping of strawberries is not possible in a system that relies on crop rotations; the production cycle is shorter (1-2 fruiting years); yields are both lower and more variable than in conventional systems; and labor requirements may be as much as twice those of a conventional system (39).

Organic and conventional annual strawberry production systems were examined in California over a three-year period (see Table 1). The organic yield was about two-thirds the conventional yield, but prices were 50% higher, making the organic system more profitable over the three-year period (40).

Organic and conventional production were also compared in New York State (39). The breakeven price—or the price at which costs of production are covered—for the conventional system was $1.10/qt. The breakeven price for the organic strawberries was 34% higher at $1.47/qt. By the fourth bearing year, organic strawberries were losing money. It appears that fields should be rotated out of strawberries after 2 years to keep the operation profitable. The researchers found that a price premium of 35% to 40% was required for the organic system to be as profitable as the conventional system. They concluded that there is opportunity for profit in organic strawberries in market niches, but that the size of the market is limited.

Prices for fresh-market strawberries have been relatively stable in recent years because of increasing demand. However, organic price premiums are declining as larger growers get into organic production, marketing and distribution systems improve, and a larger supply of organic berries reaches the market. In areas where local market demand is strong and a high proportion of the crop is sold directly to the consumer, prices tend to be much higher. For example, Pritts found that in New York state in 1996 (39), prices for direct-marketed organic strawberries ranged from $2 to $3.50 per quart.

Farmgate prices for organic strawberries in 1999 ranged from about $11 to $17 per 12-pint flat, with wholesale prices ranging from $12 to $23 per flat (41). Wholesale prices for conventional strawberries are relatively low because of the large volumes of inexpensive strawberries available in April and May from the southern and central-coast areas of California. In 2001, wholesale prices received by conventional strawberry farmers averaged about $65/cwt, which translates to about $7.81 per 12-pint flat (42). Organic wholesale prices in 2001 averaged $8.00 for a flat of clamshells and $11.00 for a flat of pint baskets (43).

Assistance with financial calculations for strawberry growers is available in Pritts's Strawberry Production Guide (see Print Resources), which includes budget spreadsheets on a floppy disk that allow the grower to calculate his or her own budgets and estimate returns under a range of production, pricing, and marketing scenarios.

Marketing

In wholesale marketing, either you or a shipper can take your crop to the market. Shippers generally sell and transport strawberries for a predetermined price. Wholesale marketing is subject to price fluctuations and is not usually very profitable, compared to direct marketing. Jim Cochran of Swanton Berry Farm in California, says, "I consider myself lucky to get five percent of gross. So, on a twenty-dollar flat of strawberries, (there is) a dollar for the company to keep" (44). Marketing cooperatives generally use a daily pooled cost and price, which spreads price fluctuations among all participating producers. Depending on your location and size, processors may or may not be a marketing option. Processors are less likely to contract with small-acreage growers.

Strawberries are successfully direct-marketed in a variety of ways, including farmers' markets, roadside stands, and pick-your-own (PYO) operations. Local retailers, such as grocery or health-food stores, are another possible market, but you must take the time to contact produce managers and provide good-quality strawberries when stores require them.

For more information on direct-marketing options, see the ATTRA publications Direct Marketing, Farmers' Markets, CSA, and Entertainment Farming and Agri-tourism. For information on organic markets, see ATTRA's Organic Marketing Resources.

Other ATTRA Publications of Interest:
	Overview of Organic Fruit Production Fertilization, pests, weed control, obstacles
	Biointensive Integrated Pest Management Uses, benefits, monitoring, economic thresholds, planning, tools & options, microbial pesticides