Due to increasing restrictions on fumigant use, considerable research efforts in the last decade have focused on developing non-fumigant alternatives to chemical fumigation of soil. These alternative technologies place greater emphasis on an integrated approach to soil pest and plant-soil environment management compared to standard broadcast soil fumigation. The target of the alternative approaches is to sustain cost-effective fruit production systems without chemical fumigation, including organic production. The strategies combine the use of resistant cultivars and site-specific management with soil treatments such as soil solarization, steam application, anaerobic soil disinfestation (ASD), soil substitution with soilless media, and use of naturally produced biocides.
Using cultivars most fitted for production conditions and the soil pathogens present in the field is of great importance in fumigated soil and is absolutely critical in non-fumigated soils. The table CHARACTERISTICS OF PUBLIC STRAWBERRY CULTIVARS COMMONLY GROWN IN CALIFORNIA shows susceptibilities of current cultivars to key pathogens and their production characteristics. Breeding for soilborne disease resistance has become one of the primary objectives for the development of new cultivars due to methyl bromide's replacement with less effective fumigants and the greater focus on non-fumigant alternatives.
In summer, clear plastic applied to preshaped beds several weeks before planting will solarize the soil and reduce the number of soilborne disease organisms and weed seeds (by making them inviable). On the central coast, this practice requires at least 12 to 15 weeks in order to obtain pest management benefits; consequently, solarization is usually not practical in this region. Solarization is much more effective in areas of the state where temperatures are consistently (30–45 days) hot enough in summer to produce soil temperatures of at least 122°F. Solarization can be even more effective if the residue of a cruciferous crop (especially broccoli or mustards) is incorporated into the soil just before the plastic is installed or following an application of metam sodium. For more details on how to effectively solarize soil, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds , UC ANR Publication 21377. For optimum results, check the plastic for good adhesion to the soil and for any holes that might have developed and need repair.
Steam kills weed seed and soil pathogens by transferring heat from a steam boiler to soil particles and soil pests. Sufficient steam must be applied to maintain a temperature of approximately 160°F for a duration (dwell time) of 20 minutes. Steam application to soil has a similar effect to soil as soil solarization, although the length of time for treatment is much less for steam. Adoption of steam disinfestation of field soils has been hindered by high fuel consumption, labor, and application time required. Steam provides effective control of weeds and pathogens and yields similar to fumigated strawberries.
Applications of steam that applied 1.55x105 British thermal units (BTU) per cubic meter of heat raised the soil temperature to 160°F for 20 minutes. This results in control of annual bluegrass (Poa annua), common chickweed (Stellaria media), burning nettle (Urtica urens), and common purslane (Portulaca oleracea), as well as yellow nutsedge tubers (Cyperus esculentus). Application of steam to soil has also been shown to control sowthistle (Sonchus oleraceus), burclover (Medicago polymorpha), purple cudweed (Gamochaeta purpurea), lesser swinecress (Coronopus didymus), vetch (Vicia spp.), corn spurry (Spergula arvensis), and little mallow (Malva parviflora). Steam achieves greater than 90% control of these weeds, which is approximately the same as field fumigation with chloropicrin/1,3-dicloropropene.
Traditional sheet steam applications in greenhouses and fields use heavy tarps to cover soil while steam is injected under the tarp. It generally takes 8 hours to treat fractions of an acre. Mobile steam applicators are currently under development.
For more information on traditional sheet steam applications, see The UC System for Producing Healthy Container-Grown Plants. For information on mobile steam applicators, see Evaluation of a Mobile Steam Applicator for Soil Disinfestation in California Strawberry.
Anaerobic Soil Disinfestation (ASD)
Anaerobic soil disinfestation is a method of treating soil that utilizes a carbon source and moisture to produce anaerobic soil conditions over several weeks. These conditions change soil properties, making the soil environment less favorable for certain soilborne pathogens, thus providing some reduction in soilborne disease. This is a relatively new technique with benefits and limitations that are not well understood.
The standard anaerobic soil disinfestation treatment involves
- uniform mixing of easily biodegradable carbon source (e.g., rice bran) into the top 24 inches of soil just before strawberry beds are listed,
- covering the new beds with plastic mulch,
- applying irrigation (if needed), and
- maintaining anaerobic conditions for 3 to 5 weeks.
Tears and holes in plastic will allow aeration and should be avoided or patched during the anaerobic process. One to two days before planting, holes can be cut to allow aeration. Then, planting can begin immediately.
Processes and changes that take place during and after anaerobic soil disinfestation application period that likely benefit the soil environment for plant growth include
- production of organic acids and volatiles,
- lowering of pH,
- increase in available nitrogen and phosphorus, and
- changes in physical and microbiological properties of soil.
Rice bran (at 9 tons/acre) has been the most common carbon source for anaerobic soil disinfestation with annual application to more than 1,000 acres in California, mostly in organic fields. Rice bran-based anaerobic soil disinfestation provides consistent improvements in plant productivity compared to non-treated soil and fruit yields similar to those obtained in fumigated soil. Anaerobic soil disinfestation shows good efficacy in reducing Verticillium wilt (caused by V. dahliae) levels in soil and provides significant control of many broadleaf annual weeds in Southern California. However, current anaerobic soil disinfestation technology does not control important strawberry pathogens like Fusarium oxysporum and Macrophomina phaseolina or perennial weeds.
Culture in soilless media (sterile substrates) is a very intensive system but nevertheless offers an alternative to farming in soil for high value crops such as tomatoes, peppers, and strawberries.
The advantages of the soilless system are clear: more control over the rooting medium, a theoretically pathogen free environment, and absolute control over irrigation and fertility. Gravel, sand, and rock wool have been used previously as substrates. Current research is evaluating organic substrates such as coconut coir and peat moss as a substrate for their superior nutrient and moisture holding capacity, aeration, leaching, and reusability. The substrate system also offers benefits to labor when beds are raised to waist or eye level, which prevents stooping and increases productivity, especially in older workers.
Soilless culture is almost always performed in greenhouses or high plastic tunnels (i.e., "hoops"), and planting configurations can vary widely. In greenhouses, strawberries can be planted in stepped, rotating, or fixed troughs, stacked in perforated cylinders, or built up in pyramids. The goal of these configurations is to achieve maximum exposure to sunlight. In the field, recent research on a raised-bed soilless media system shows significantly higher production costs compared to standard field soil production but no significant fruit yield improvements.
The holding pot of the substrate plant must be given careful consideration. Colors should be light so as to not accumulate heat too quickly, and it is highly recommended to use interlocking troughs of shorter length, which prevents transmission of any root pathogen over long stretches of planting medium.
Irrigation and fertility of strawberries grown in soilless media is the key to successful production. Watering must be frequent, and salinity must be monitored at all times to ensure that it does not reach harmful levels. Since managers of strawberries in soilless culture are injecting fertilizers and potential salts with the water into the substrate, it is very easy to exceed safe levels of electrical conductivity (EC) very quickly.
For more information on soilless substrate, see Technical Equipment in Soilless Production Systems .
Naturally Produced Biocides
Biocides are naturally produced pesticides that are most commonly comprised of seed meal or the aboveground biomass of high-glucosinolate plants from the Brassicaceae family (mustards). The breakdown of glucosinolate yields several biologically active compounds such as low levels of isothiocyanates, the active ingredients in fumigants such as metam sodium, potassium, and allyl-isothiocyanate.
These pesticides have not been observed to consistently control soilborne pathogens. Although on-site cover cropping with mustards and the use of seed meal have reduced numbers of weeds and nematodes at some sites, the time or cost needed to produce sufficient cover crop biomass or apply seed meal has been prohibitive for widespread application. Additionally, seed meal may be phytotoxic to strawberry if applied at high rates close to transplanting.
Even though biocides have limited uses for pest management, it is worth noting their potential benefits to strawberry production due to contribution of nitrogen and organic matter. In addition, mustard is a strong competitor with weeds when grown as a cover crop. When considering cover crop selection, consult with the UC Cover Crop Guide.
For current information about non-fumigant technologies in your production area, contact your local UCCE advisor or specialist.