Strawberry growers seek methods to increase yields and to manage soilborne diseases and pests. There is increased interest in eliminating fumigants or transitioning to organic practices. We explored the on-farm utility of using a method called “anaerobic soil disinfestation” (ASD) to treat soils. The method includes the use of a highly labile carbon source incorporated into soil within beds covered with plastic mulch. Then the beds are flooded, using the drip tape assembly, to induce anaerobic conditions that are speculated to result in a decrease in strawberry pathogen and weed seed populations. This on-farm research assessed the ASD method and took data on microbial communities, plant growth and yield.
Organic growers, and conventional growers who desire to eliminate soil fumigants, have very few options for managing soilborne diseases in high value specialty crops. Crop rotation and host resistance play a critical role in managing soilborne pests. Many strawberry growers do not have opportunity to rotate crops for the length of time that is best and/or need to grow their “money” crops on their “money land” e.g. superior soil types or under high tunnels. Likewise, many specialty crops do not have advanced host resistance to the major and often too common soilborne diseases in NC and the South. Therefore, alternative management practices need to be developed and incorporated into a sustainable IPM-based management plan. We seek to research a method called Biological Soil Disinfestation (BSD) or Anaerobic Soil Disinfestation (ASD) to manage soilborne diseases in strawberry production.
Anaerobic soil disinfestation (ASD) methods were first developed in Japan where the researcher sought an alternative soil treatment in areas not conducive to soil solarization or other economical soil disinfestation processes (Shinmura, 2000; Momma, 2008). A group in the Netherlands also developed a modified protocol in field production and obtained superior control of Verticillium wilt, reducing inoculum levels by up to 85% for up to 4 years after soil treatments, compared to non-treated soil. In the Netherlands, they called it Biological Soil Disinfestation (BSD), which emphasizes the interaction between soilborne pathogens and beneficial or antagonistic microbial communities (Blok et al., 2000; Goud et al., 2004; Messiha et al., 2007). Since then, both regions have seen extensive expansion of the method to control a range of soilborne pathogens in many specialty crops, particularly strawberries and solanaceous and cucurbit vegetables. Recent research in the U.S., Florida vegetables and California strawberries, has shown promising results when adapting this technique to raised-bed, plasticulture production systems (Butler et al. 2010; Shennan et al. 2010). In California ASD in strawberry production using flat field production or raised beds (Shennan et al., 2010) proved successful. They found mortality of the Verticillium dahliae microsclerotia was 98% in ASD treated areas, whereas the counts increased in non-treated areas. This work is now supported by the California Strawberry Commission – as well as the USDA-NIFA MBT program (J. Muramoto and C. Shennan, personal communication; Shennan et al., 2007; Butler et al., 2010; 2012). The system relies on incorporation of a labile carbon source (e.g. wheat bran, rice bran, molasses, fresh plant residues potentially grown on site) to stimulate soil microbial growth and respiration, saturation of the topsoil to reduce soil oxygen, and tarping with plastic to limit gas exchange during a three to six week pre-plant treatment period. Control of plant pathogens and to some extent, weeds, is largely due to the formation of organic acids and volatile compounds during the anaerobic breakdown of the added C source, as well as through potential biocontrol mechanisms occurring during shifts in microbial community composition (Momma 2008). Therefore, the research and practical experience developed in the Netherlands and Japan and the current work begun in California and Florida offers substantial promise that the ASD system for managing soilborne diseases (and other pests) will be effective in Southeast strawberry production systems. We believe this system is highly adaptable to our strawberry production systems because growers generally do not grow an economic crop during the summer prior to fall planting, providing an ideal window for advanced farming system management practices. Also, most of our growers desperately need a systematic and systems-based mechanism to enhance the organic matter content of their soils.
We proposed to implement and develop anaerobic soil disinfestation (ASD) as a mechanism to suppress soilborne pathogens, enhance soil health and increase crop productivity in strawberry production systems.
Two on-farm research projects were initiated. The first was conducted during the 2009-2010 strawberry season in Bunn, NC on certified organic land owned by Vollmer farm. The standard practice was to incorporate compost in the field just prior to forming the strawberry beds (Beds were formed by pulling a bed forming machine through the soil that simultaneously formed an 8 inch high bed 24 inches wide, covered the bed with a plastic mulch and laid one or two lines of drip tape beneath the plastic mulch). Mr. Vollmer had a sudex cover crop that was harvested as a source of residue needed for the ASD process. The ASD beds were established in July. We compared the utility of virtually impenetrable film (VIF) to standard LDPE mulch. (Mr. Vollmer uses VIF as his standard practice). These films have recently emerged in the methyl bromide alternatives market. (Work in the Netherlands used plastic made for silage production). Freshly harvested sudex (highly labile, low lignification) was broadcast applied and rototilled into the soil (Figure 1). Subsequently, soils were cultivated for smoothing and standard beds were formed and covered with the plastic mulch. Drip irrigation (via the buried lines) was immediately applied to induce high moisture content in the soils. This farm has shallow topsoil and a hard clay pan 8-12 inches below the topsoil. We employed this clay hardpan to effectively trap moisture resulting in soil saturation and thereby it was anticipated that anaerobic conditions would be induced. The 3 treatments were LDPE (standard practice) with no green residue addition, VIF with no green residue added, and VIF plus green residue (~17.8 t/A). Plot size was 50 feet long and three beds wide (24 in beds on 5-ft centers) and arranged as a randomized complete block design with 3 replications.
A second experiment was done in Spring City TN on land with a long history of conventional strawberry production. In this case dried molasses was used as the carbon source at a rate of 5000 lb/A. The molasses were broadcast applied, incorporated into the soil and then beds were pulled and flooded as described above. The ASD was compared to standard methyl bromide (MeBr) fumigation and PicClor-60 (Pic60) fumigation treatments, as well as a non-fumigated control using a randomized complete block design with 6 replications.
Soils were sampled for nutrient status and microbial communities including prevalent strawberry fungal pathogens. Whole plant analysis was conducted in the late fall and at peak harvest to assess plant growth parameters and root rot severity. Whole plant samples are a good indicator of soil treatment effects and we have documented that these dates are representative of seasonal changes (Fernandez et al. 2001; Butler et al 2002). Whole plants were partitioned into crowns, roots, leaves and reproductive structures and documented by dry weights. Roots were assessed for root rot severity (Black root rot) on each sampling date and pathogens were isolated from roots to determine the impact of soil treatment on root disease incidence and pathogen frequency. Yield was collected through biweekly harvests from early April till early June.
The on-farm protocols for utilizing this method were implemented well using on-farm capacity and, in the case of NC, a local green manure source. In NC, plant growth data were not impacted by treatment compared to the standard organic practices (cover crop, compost, formation of beds, no bed saturation). Thus, the use of the green manure crop and subsequent soil saturation did not offer an advantage at this site (data not shown).
In TN, a highly labile source of carbon was used (dried molasses) and anaerobic conditions were achieved based soil probe measurements (data not shown). At peak harvest (mid-May), plant vigor was substantially suppressed in plots not treated (controls; Figure 2). The ASD plant vigor values were similar to ratings for plants in the MeBr and Pic60 plots (Figure 2). Likewise, dry weight of foliage (a productive indicator of plant growth) and root dry weights were similar in all the treated plots and these were superior to values in the control plots, where plants were clearly stunted. At harvest, control plants had severe black root rot (BRR) symptoms and root rot severity ratings were significantly higher in the control plots compared to all other treatments. The ASD resulted in modest BRR symptoms and MeBr treated soils had the best root systems (Figure 2).
Microbial community analysis was performed approximately 1 month after planting and again in May during peak harvest. ASD resulted in high respiration rates in November compared to all other treatments (Figure 3). Interactions were significant and by May no differences in respiration were observed. Potential labile carbon was highest in the control and ASD plots compared to the fumigated plots and increased in May compared to November (Figure 3). Microbial biomass carbon and nitrogen (MBC, MBN) followed a similar pattern (Figure 3). A high rate of colonization in May in strawberry roots by arbuscular mycorrhiza fungi (AMF) was observed in all treatments and the extent of colonization was highest in plants from the ASD and non-treated plots (Figure 3).
Fumigation dramatically suppressed Pythium and Fusarium populations in bulk rhizosphere soils (Figure 4) but had limited effect on Rhizoctonia and Trichoderma populations (values presented represent means on multiple sampling dates; individual sampling dates have not been analyzed to date). Fumigation also tended to suppress total fungal counts but had no impact on total bacterial counts based on culture-based assays (Figure 4). Frequency of root colonization by (potential) pathogens of strawberry was impacted by soil treatment. Pic60 dramatically decreased Pythium colonization of roots and all treatments suppressed Fusarium colonization. Rhizoctonia and Trichoderma populations, based on the preliminary analysis in this report, were not impacted by soil treatments (Figure 4).
Finally, yield was dramatically reduced in non-treated plots (Figure 5). The ASD treatment generated modest yields compared to the highest yields observed in the fumigated plots. Inorganic N tended to be low near the end of the season in the ASD plots (Figure 5).
Implementing alternatives to fumigants and the development of systems-based approaches to manage pests, increase plant health and enhance crop productivity is a high priority for designing future farming systems. Most strawberry growers (organic and conventional) tend to be constrained by availability of land suited to strawberry production. The anaerobic soil disinfestation system (ASD) is an approach that has high potential to fit within the cropping cycle of strawberries and can (potentially) be accomplished by using on-farm inputs and capacity. In NC, measurable responses to the ASD system were not observed and the grower cooperator observed the ASD tended to decrease overall yield compared to the standard organic management system used (data not captured). In contrast, in TN the ASD system appeared to result in plant growth parameters very similar to the values observed in the fumigated plots and ASD suppressed root rot severity compared to the control but not as well as MeBr fumigation. However, total marketable yield, while much higher than yield obtained from non-treated plots, was compromised compared to the yield obtained from fumigated plots. (Pic60 functioned as well as MeBr demonstrating it is a viable alternative to MeBr and it is not an ozone depleting compound). We noted N levels tended to be depleted in the ASD treatments. Managing ASD systems will take more research and experience to fine tune decisions related to application rates of carbon sources, methods and duration of saturating soils, fertility requirements and timing of application, and other key factors.
Data were logged, strawberry production budgets are being developed, and tools will be available for economic analysis of ASD systems for strawberries in the Southeast.
Additional research station and on-farm research will be conducted. The participatory role of farmers is critical since (in our experience) innovation and management solutions often originate through these collaborative experiments. To date, we are not aware of growers in the Southeast who have adopted the ASD method. In contrast, on-farm driven research by numerous farmers in the Southeast has enabled the widespread adoption of alternative fumigants (e.g. Pic60) and IPM-based tactics to replace MeBr as a soil fumigant. Emerging regulations and cost of fumigation have driven more growers to seek a broader systems-based approach to managing soilborne pests and to sustain profitability of strawberry production systems. This continued interest should enable advancements in systems-based research and extension of more complex management approaches such as the ASD system.
Areas needing additional study
More research and practice is needed to optimize ASD systems and discern site-specific issues that may arise. Also, ASD may generate gases, such as nitrous oxide, and these need to be quantified or managed to limit additional issues with their impact on the ozone layer or as greenhouse gases.
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