Comparing Vesicular-Arbuscular Mycorrhizal Colonization in Conventional versus Organic Strawberry Production Systems

Final Report for GNC03-023

Project Type: Graduate Student
Funds awarded in 2003: $10,000.00
Projected End Date: 12/31/2004
Region: North Central
State: Minnesota
Graduate Student:
Faculty Advisor:
Dr. Emily Hoover
University of Minnesota
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Project Information

Summary:

An on-farm comparison showed no differences in arbuscular mycorrhizal fungal (AMF) colonization between conventional and organic strawberry management strategies. However, the high variability of on-farm systems may make small differences difficult to detect. A research plot study comparing simulated conventional and organic strawberry management strategies found that fungicide and insecticide sprays did not affect levels of AMF colonization. However, organic mulch materials in the establishment year translated to higher levels of AMF colonization and root biomass than herbicide or hand-weeded groundcover management strategies.

Introduction:

Strawberries are herbaceous perennials and can be an important part of a sustainable agroecosystem. Matted row systems in Minnesota allow growers to utilize minimal disruptive tillage, establish long-term pest control strategies and enhance soil quality by maintaining a vegetative cover. Strawberries also provide the grower with economic benefits because labor is confined to a relatively short season, often before other crops mature.

The current research on mycorrhizal associations is extensive yet inconclusive. The published literature repeatedly emphasizes the need for site- and crop-specific studies (Barea et al., 1993) to account for factors such as plant-fungus specificity and environmental adaptations of fungi. Many of the current studies have been conducted in greenhouses using inoculum to supply mycorrhizal fungi to the system. While these studies are effective means for gaining preliminary information, they are not representative of the highly complex interactions occurring in natural and agricultural systems. The potential benefits of mycorrhizae to sustainable agriculture have been repeatedly emphasized (Bethlenfalvay and Linderman, 1992; Gianinazzi and Schuepp, 1994) and warrant further research.

Research which applies the current knowledge base and moves ahead to affect real changes in agricultural management practices and agricultural system functioning is the next step. This project provided an opportunity to evaluate mycorrhizal associations at the systems level to make a clearer assessment of the benefits conferred to plants, and the factors which affect these associations. Our work was partially conducted on working strawberry farms with an emphasis on grower participation. This set-up allowed us to evaluate the costs and benefits of managing for these associations in a realistic way, considering grower acceptance and farm profitability in addition to environmental quality.

Literature Review
The soil biology within a strawberry field is an important, yet poorly understood contributor to the health and productivity of the system. To understand the physical role of the soil, we must also be aware of the many microbiotic associations occurring in the soil and how these affect plant health and crop yield. Other SARE-funded projects have recognized the importance of studying the whole soil system, and the roles of microorganisms in that system (LNC00-175, LS00-110). Possibly the most prevalent soil-plant interaction is with mycorrhizal fungi. Most horticultural crop plants form associations with mycorrhizae, and the majority of these with arbuscular mycorrhizae (Barea et al., 1993). These associations have demonstrated numerous benefits to plants in both natural and agricultural systems. In strawberries, these associations have been shown to increase nutrient uptake (Dunne and Fitter, 1989), protect against soil-borne pathogens (Norman et al., 1996), and generally improve soil aggregation (Schreiner and Bethlenfalvay, 1995). Other putative benefits include protection against drought and salinity stress, and increased nitrogen fixation (Barea et al., 1993).

However, much of the information on AMF interactions in strawberry crops remains incomplete. In other crops, soil type has been shown to affect fungal species diversity, fungal density, and mycorrhizal colonization (Land et al., 1989). Agricultural practices also affect the mycorrhizal colonies and symbioses. Many practices have demonstrated negative effects on fungal colonies, including belowground fumigation of white bean and soybean (Buttery et al., 1988), pesticide and fertilizer application to meadow plants (Titus and Leps, 2000), and tillage in maize (Hamel, 1996). Although there have been relatively few SARE-funded projects relating to mycorrhizae, several studies have investigated the relationship between mycorrhizae and management practices (LNC00-162, LNC91-041). However, this was the first SARE project to study mycorrhizae and strawberry crops, and to compare the dynamics of these interactions among conventional and organic management systems. Through proper management of AMF, the need for fertilizer applications and pesticide treatments may be reduced. These benefits represent a particularly promising addition to low-input, sustainable management plans (Azcon-Aguilar and Barea, 1997).

Project Objectives:
  1. Compare effects of various strawberry management strategies on AMF

    Increase grower awareness of soil system and its biotic components, focusing on AMF

    Facilitate grower discussions

Cooperators

Click linked name(s) to expand
  • Bill and Nancy Bauer
  • Brad and Kristi Fernholz
  • Emily Hoover
  • Bill Jacobson
  • Steve Poppe
  • Mike and Susan Willis

Research

Materials and methods:

This project was conducted from May 2003 through December 2004 and consisted of two major studies. In both studies, we attempted to evaluate the entire strawberry production system, collecting data on percent mycorrhizal colonization, plant biomass parameters, leaf nutrient analyses, soil type and soil nutrient analyses.

In study one, hereafter referred to as the Farm Study, we compared levels of AMF colonization and plant growth parameters between conventional and organic farms. Additionally, we investigated differences among strawberry cultivars in AMF formation and functioning. Data collection for the Farm Study was conducted entirely on productive plots on working farms with four cooperating farm families. Participants were Minnesota and Wisconsin strawberry producers (one certified organic, one non-certified organic, two conventional). Preliminary farmer surveys provided information on management strategies, cultivars, soil texture, size and age of plantings. Farms were chosen based on established plantings (1+ year) of the cultivars, ‘Annapolis’, ‘Jewel’ and ‘Winona’ growing on similar soil types.

Soil and plant samples were collected in June prior to harvest; July post-harvest, pre-renovation; and September. At each site and date we randomly delineated two 6 x 6 meter plots of each cultivar. ‘Winona’ was substituted for ‘Annapolis’ on Farm 3. Due to the small size of the Farm 2 planting, the plots at this location were 6 x 2.4 meters. We randomly collected 6 to 8 soil cores, 15 cm deep, from within the strawberry rows. Vegetation and mulch materials were removed prior to obtaining soil cores. Soil samples were pooled, dried, and analyzed for P, K and pH for each date at the University of Minnesota Research and Analytical Laboratory. Only P was used in statistical analyses because it has a demonstrated role in the AMF symbiosis (Smith and Gianinazzi-Pearson, 1988; Smith and Read, 1997). Soil pH values were not compared statistically but were used to interpret soil P test results. Where pH was greater than 7.5, P was analyzed using both Bray and Olsen test methods (Frank et al., 1998).

We randomly collected four plants from each plot by removing the plant and soil with roots. Runner plants were separated from mother plants and not included in the sample. After collection, plants were stored in a 2ºC cooler for approximately 18 hours until processing. Individual plants were carefully washed using a spray attachment to remove all soil adhering to the root system. Detached root pieces were retained in a fine sieve. Plants were separated into below- and above-ground sections by cutting just below the crown, and fresh weight was recorded.

Six to eight sub-samples of actively growing root tips were randomly collected from each root system. Root sub-samples from each plant were frozen until mycorrhizal processing and scoring were possible. The remaining below- and above-ground parts were oven-dried at 65ºC and dry weights recorded. Percent water content was calculated for below- and above-ground sections.

All fully-expanded, non-senescent leaves from each of the four plants were pooled by plot and analyzed for nutrient content including Al, B, C, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, N, Na, Ni, P, Pb, S and Zn. All nutrient analyses except C, N and S were conducted at the University of Minnesota Research and Analytical Laboratory using standard ICP analysis (Dahlquist and Knoll, 1978). Analyses for C, N and S were conducted using combustion techniques (Horneck and Miller, 1998). Statistical analyses were conducted for N and P as these nutrients are known to be of primary importance in the AMF symbiosis (Dunne and Fitter, 1989; Mathur and Vyas, 2000).

For AMF scoring, thawed root sub-samples from each plant were cleared in 10% KOH using a 121ºC autoclave for 11 minutes, acidified in 1% HCl (Kormanik et al., 1980), stained with 0.05% Aniline Blue (Grace and Stribley, 1991) and destained in 70% glycerol. Stained roots were cut into 1 cm pieces and pooled by plot (4 root sub-samples/plot). Sub-samples were stored in 70% glycerol-water (v/v) until further processing (Koske and Gemma, 1989). Roots that appeared lightly stained were re-stained by soaking at room temperature in 0.05% Aniline Blue for 1 h.

Percent AMF colonization of root subsamples was estimated using a modification of the magnified intersection method (McGonigle et al., 1990). Sub-samples were letter-coded to insure observer objectivity. Using 40x magnification, 150 intersections per root sub-sample were analyzed for presence of mycorrhizal structures including: hyphae, hyphal coils, vesicles, arbuscules and absence of structures (“absent” intersections). If a structure was questionable, observation was done using 100x. Total number of intersections was used to estimate root length.

Calculations of Mycorrhizal Colonization
Parameters of mycorrhizal colonization were calculated as follows:
% Arbuscular Colonization = (arbuscule intersects / total intersects) * 100
% Vesicle Colonization = (vesicle intersects / total intersects) * 100
% Hyphal Coils = (hyphal coil intersects / total intersects) * 100
% Hyphal Colonization = [(total intersects – absent intersects) / total intersects] * 100
% AMF Colonization =
[(total intersects – absent intersects – hyphae only intersects) / total intersects] * 100

Hyphae-only intersects were subtracted when calculating total AMF colonization because it is difficult to visually distinguish between AMF hyphae and hyphae of other soil fungi. We did use characteristics such as lack of septae and right angle branching patterns in our scoring. However, in order to obtain a more conservative estimate of total AMF colonization we subtracted this parameter (McGonigle et al., 1990).

Analysis
Sources of variation for this experiment included date, management type, farm, cultivar within farm and plot within cultivar within farm. Total percent AMF colonization, percent arbuscular colonization, plant biomass, leaf nutrients and soil nutrients were pooled across date and compared among cultivars within farms using ANOVA. ‘Annapolis’ and ‘Jewel’ were compared within farms 1, 2 and 4. On Farm 3, ‘Winona’ was substituted for ‘Annapolis’ for within farm comparisons. Total percent AMF colonization, percent arbuscular colonization, plant biomass, leaf nutrients and soil nutrients were pooled across date and compared between conventional and organic management types using ANOVA . Means were compared using pre-planned contrasts and Tukeys’ studentized range test in SAS. Percent total AMF was regressed individually on root dry weight, shoot dry weight, leaf P, leaf N and soil P to determine correlations between AMF and these factors.

In study two, the Management Strategy Study, we used research plots to compare simulated conventional and organic management strategies. We evaluated the effects of fungicides, pesticides and ground cover management strategies on AMF colonization and plant growth responses. This study was conducted at the West Central Research and Outreach Center at Morris, Minnesota (45.5 ° N x 95.88 ° E). Plots of the strawberry cultivar ‘Glooscap’ were established in 2002 in a randomized complete block with four replications and three ground cover treatments. The groundcover treatments applied in the establishment year were: 1) ‘Dwarf Essex’ canola plus wool mulch (wool-canola), 2) traditional herbicide (herbicide) and 3) hand-weeding (hand-weed). The wool-canola treatment included a wool-fiber mulch placed between strawberry plants and an interrow canola cover crop. In the establishment year, the canola was planted on 15 May, and killed with glyphosphate (Roundup) herbicide on 18 June. The establishment year herbicide treatment included two applications of DCPA (Dacthal W-75) herbicide and one application of napropamide (Devrinol 50-DF) applied between rows. The hand-weed treatment did not receive any herbicide treatments. Each treatment plot consisted of three rows, each three meters long.

In 2003, two pesticide spray treatments (sprayed, unsprayed) were overlaid on each of the three ground cover treatments for a 2X3 factorial design with four replications. The sprayed treatment consisted of three fungicide applications and two insecticide applications from 4 June to 26 June 2003. The unsprayed treatment did not receive any sprays in the 2003 season. In spring 2003, straw applied to all plots as winter mulch in Fall 2002 was removed and placed between rows. Canola residues and wool mulch materials remained in the wool-canola treatment plots. Each pesticide treatment (sprayed, unsprayed) was randomly applied to row 1 or 3 in each groundcover treatment. Soil and plant samples were collected in June, following pesticide application, prior to harvest; July post-harvest, pre-renovation; and late August.

On each sampling date we collected 8 to 10 random 15 cm deep soil cores from within the rows across the entire planting. Vegetation and mulch materials were removed prior to obtaining soil cores. Soil samples were processed as for the Farm Study. Soil samples were pooled, dried, and analyzed for P, K and pH for each date at the University of Minnesota Research and Analytical Laboratory. Only P was used in statistical analyses because it has a demonstrated role in the AMF symbiosis (Smith and Gianinazzi-Pearson, 1988; Smith and Read, 1997). Soil pH values were not compared statistically but were used to interpret soil P test results. Where pH was greater than 7.5, P was analyzed using both Bray and Olsen test methods (Frank et al., 1998).

On each sampling date, we also randomly collected one mother plant and three runner plants from rows 1 and 3 by removing the plant and soil with roots. Runner plants were pooled by treatment for all measurements. Plant samples were processed as for the Farm Study. After collection, plants were stored in a 2ºC cooler for approximately 18 hours until processing. Individual plants were carefully washed using a spray attachment to remove all soil adhering to the root system. Detached root pieces were retained in a fine sieve. Plants were separated into below- and above-ground sections by cutting just below the crown and fresh weight was recorded.

Root sub-samples were collected and processed as for the Farm Study. Six to eight sub-samples of fine, actively growing root tips were randomly collected from each root system. Root sub-samples from each plant were frozen until mycorrhizal scoring was possible. The remaining below- and above-ground parts were oven-dried at 65ºC and dry weights recorded. Percent water content was calculated for below- and above-ground sections.

Leaf samples were processed as for the Farm Study. All fully-expanded, non-senescent leaves from each plant sample were removed and analyzed for nutrient content including Al, B, C, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, N, Na, Ni, P, Pb, S and Zn. All nutrient analyses except C, N and S were conducted at the University of Minnesota Research and Analytical Laboratory using standard ICP analysis (Dahlquist and Knoll, 1978). Analyses for C, N and S were conducted using combustion techniques (Horneck and Miller, 1998). Statistical analyses were conducted for N and P as these nutrients are known to be of primary importance in the AMF symbiosis (Dunne and Fitter, 1989; Mathur and Vyas, 2000).

AMF scoring was conducted as for the Farm Study. For AMF scoring, thawed root sub-samples from each plant were cleared in 10% KOH using a 121ºC autoclave for 11 minutes, acidified in 1% HCl (Kormanik et al., 1980), stained with 0.05% Aniline Blue (Grace and Stribley, 1991) and destained in 70% glycerol. Stained roots were cut into 1 cm pieces and mother and runner root sub-samples were pooled by treatment. Sub-samples were stored in 70% glycerol-water (v/v) until further processing (Koske and Gemma, 1989). Roots that appeared lightly stained were re-stained by soaking at room temperature in 0.05% Aniline Blue for 1 h.

Percent AMF colonization of root subsamples was estimated using a modification of the magnified intersection method (McGonigle et al., 1990). Sub-samples were letter-coded to insure observer objectivity. Using 40x magnification, 150 intersections per root sub-sample were analyzed for presence of mycorrhizal structures including: hyphae, hyphal coils, vesicles, arbuscules and absence of structures (“absent” intersections). If a structure was questionable, observation was done using 100x. Total number of intersections was used to estimate root length (McGonigle et al., 1990).

AMF colonization parameters were calculated as for the Farm Study.

Calculations of Mycorrhizal Colonization
Parameters of mycorrhizal colonization were calculated as follows:
% Arbuscular Colonization = (arbuscule intersects / total intersects) * 100
% Vesicle Colonization = (vesicle intersects / total intersects) * 100
% Hyphal Coils = (hyphal coil intersects / total intersects) * 100
% Hyphal Colonization = [(total intersects – absent intersects) / total intersects] * 100
% AMF Colonization =
[(total intersects – absent intersects – hyphae only intersects) / total intersects] * 100

Hyphae-only intersects were subtracted when calculating total AMF colonization because it is difficult to visually distinguish between AMF hyphae and hyphae of other soil fungi. We did use characteristics such as lack of septae and right angle branching patterns in our scoring. However, in order to obtain a more conservative estimate of total AMF colonization we subtracted this parameter (McGonigle et al., 1990).

Analysis
Sources of variation were date, replications, plot, groundcover treatment, pesticide treatment and the groundcover by pesticide interaction. Total percent AMF colonization, percent arbuscular colonization, plant nutrients, and root and shoot dry weights were pooled across dates and compared among treatments using ANOVA. Percent total AMF was regressed individually on shoot dry weight, root dry weight, leaf P and leaf N to determine the relationship between AMF and these factors.

Research results and discussion:

Farm Study

All cultivars on all farms supported AMF colonization. Average total AMF colonization for ‘Annapolis’ ranged from 34% to 38% and average total AMF colonization for ‘Jewel’ ranged from 25% to 41%. On Farm 3, ‘Winona’ had 27% total AMF colonization. The levels of colonization we found are comparable to the results of other field-based studies. Dunne and Fitter (1989) found average colonization levels from 25% to 35% in 2-year old field-grown ‘Hapil’ strawberries. In first season ‘Chandler’ strawberries, Werner et al. (1990) found levels similar to this study with 23% colonization under conventional management and about 48% colonization under transitional organic management.

In contrast to Werner et al., (1990), there were no significant differences in total AMF or arbuscular colonization between ‘Annapolis’ and ‘Jewel’ compared between conventional and organic farms and within farm cultivar differences were not significant. Other researchers have found increased colonization in organic systems compared with conventional (Douds et al., 1993; Mäder et al., 2000; Werner et al., 1990). However, these studies were conducted on research plots with lower variability than our on-farm plots. Additionally, the conventional farms in this study were managed under reduced-input plans minimizing the detectable differences between the organic and conventional farms.

Root and shoot dry weights were not significantly different between cultivars within farms, but were significantly greater in conventional farms. This is likely due to the significantly greater root dry weights on Farm 1 (conventional) compared to root dry weights on Farms 2, 3 and 4. Shoot dry weights on Farm 1 were lower than shoot dry weights on Farms 2, 3 and 4 but differences were only significant between Farms 1 and 4. We did not find a strong positive correlation between total AMF colonization and plant biomass, although many inoculation studies with strawberry have shown significant increases in dry matter (de Silva et al., 1996; Robertson et al., 1988; Vestburg, 1992). However, our results concur with those of Niemi and Vestburg (1992) who found no increase in total dry weight of field-grown strawberries inoculated with AMF.

Leaf P was greater in organic farms while leaf N was greater in conventional farms and soil P did not vary between conventional and organic farms. Among cultivars within farms, leaf P was significantly greater in ‘Jewel’ compared to ‘Annapolis’ on Farm 2 and, ‘Winona’ on Farm 3. The correlation between Total AMF and leaf P, leaf N and soil P varied by cultivar/farm combination but total AMF was generally negatively correlated to leaf P, leaf N and soil P.

In this study, the benefits of AMF to plant growth and leaf nutrients may have been minimized by the agricultural conditions, where nutrients are carefully managed and deficiencies are rare (Smith and Read, 1997). The benefits of mycorrhizal-increased P uptake are greatest in low-P environments (Miller et al., 1986; Smith and Gianinazzi-Pearson, 1988; Smith and Read, 1997). In Minnesota strawberry production, P additions are recommended for soil test values less than 41 ppm (Rosen and Eliason, 1996). The average soil P concentration of our plots ranged from 43 ppm to 219 ppm. Furthermore, leaf N concentrations for all cultivars were also within the sufficient range for strawberries (Rosen and Eliason, 1996). Adequate N in the plants, along with sufficient soil P may have masked any potential mycorrhizal contribution to biomass. This is further evidenced by the fact that the only positive correlation between AMF and leaf P was found in ‘Annapolis’, Farm 2 which had the lowest soil P levels.

The lack of detectable differences may also be attributed to variation in the formation and functioning of the symbiosis. Compatibility between strawberry and AMF may vary by species (Norman et al., 1996). Additionally, fungal species may vary in their efficiency as symbionts (Niemi and Vestburg, 1992; Ruiz-Lozano et al., 1995) and in their tolerance to high P conditions (Stribley et al., 1980). These factors may be masking the detectable AMF differences in this study.

Field grown strawberry plantings in Minnesota and Wisconsin supported natural populations of AMF. However, there were no significant differences in levels of colonization between conventional and organic farms. External factors outside the control of the experiment such as weather and varied management strategies may have affected plant nutrient levels and biomass production to a greater extent than AMF colonization, thus minimizing the effects we were able to detect. Understanding the functioning of this symbiosis in an agricultural setting requires investigation into many interacting factors. Long-term, site-specific and cultivar-specific studies may best address these needs by allowing researchers to collect information on the AMF species present and their interactions with strawberries under various conditions.

Management Strategy Study

All treatments supported AMF colonization. The average total AMF colonization across all treatment combinations was 34%, ranging from 28% in the unsprayed/hand-weed treatment to 43% in the unsprayed/wool-canola treatment. Average arbuscular colonization ranged from 17% in the unsprayed/wool-canola to 21% in the unsprayed/hand-weed treatment. There were no significant interactions for percent colonization between pesticide and groundcover treatment factors.

Total AMF and arbuscular colonization were not significantly different between the sprayed and unsprayed treatments. Comparing groundcover treatments, average total AMF colonization in the wool-canola groundcover treatment was 42% and was significantly greater than either the herbicide treatment (30%) or the hand-weed treatment (28%). There were no significant differences in average arbuscular colonization among groundcover treatments.

Leaf P was not significantly different among groundcover or pesticide treatments and there were no significant interactions between groundcover and pesticide treatments for leaf P. Leaf N was significantly greater in the wool-canola treatment compared to the other groundcover treatments. There was significant interaction between groundcover and pesticide treatments for leaf N. The only significant differences in leaf N between pesticide treatments were found in the herbicide groundcover treatment. In this case, the sprayed treatment had significantly greater leaf N.

Percent water did not differ significantly among plant samples. Therefore, dry weights were used for the remainder of the biomass analyses. Average root and shoot dry weights of mother plants across all treatment combinations ranged from 6.08 grams and 21.2 grams, respectively, in the unsprayed/herbicide treatment to 12.5 grams and 32.6 grams, respectively, in the unsprayed/wool-canola treatment. Runner plant root dry weights ranged from 1.54 grams in the unsprayed/herbicide treatment to 2.57 grams in the unsprayed/wool-canola treatment. Runner plant shoot dry weights ranged from 5.78 grams in the unsprayed/herbicide treatment to 8.90 grams in the sprayed/hand-weed treatment. There were no significant interactions between pesticide and groundcover treatments on plant biomass parameters for either mother plants or runner plants.

There were no significant differences in any parameters of plant biomass among pesticide treatments for mother plants or runner plants. However, there were significant differences in plant biomass among groundcover treatments. Average root dry weight of mother plants was significantly greater in the wool-canola treatment. Average root dry weight of runner plants was significantly greater in the wool/canola treatment compared to the herbicide treatment but not the hand-weed treatment. Average mother plant shoot dry weight was not significantly different among groundcover treatments. Average runner plant shoot dry weight was significantly greater in the hand-weed treatment compared to the herbicide treatment but not the wool/canola treatment.

There was a significant positive correlation between total AMF colonization and shoot dry weight in the herbicide and hand-weed treatments for mother plants only. Total AMF was negatively correlated to root dry weight in both mother and runner plants in the wool-canola treatment. However, root dry weight of mother plants was positively correlated to total AMF in the herbicide treatment.

Total AMF colonization was only significantly correlated to leaf P in the herbicide treatment and the correlation was negative. In all groundcover treatments there was a significant positive correlation between leaf N and total AMF colonization.

Discussion
In this study, pesticides did not have a major impact on AMF colonization. Previously, in studies using white bean and soybean, fungicides caused significant decreases in AMF populations (Buttery et al., 1988) but not in winter barley (Land et al., 1989). Miller and Jackson (1998) also found a negative correlation between AMF colonization and fungicide, herbicide or insecticide application in lettuce. The method of application, especially for fungicides, may be important, however. The significant effects of fungicides found by Buttery et al. (1988) might be attributed to the use of below-ground fumigation while Land et al. (1989) and this study used foliar applications. The management of strawberries as a perennial crop with minimal tillage might also support a greater inoculum potential than the annual crops in the previous studies. Studies have shown mycorrhizal inoculum to be decreased by tillage (Hamel, 1996) and increased by cover cropping (Boswell et al., 1998; Kabir and Koide, 2000).

The wool-canola groundcover treatment had significantly greater total AMF colonization than the other two groundcovers. Although plants in the Brassicaceae family are known to be non-mycorrhizal (Glenn et al., 1988), we did not see negative effects of establishment-year canola (Brassica napus L.) on AMF colonization in the subsequent growing season. Many studies have shown that non-host plants do not affect AMF colonization of nearby host plants (Glenn et al., 1988; Kabir et al., 1996; Vierheilg et al., 1995). Researchers have speculated that the non-mycorrhizal status of the Brassicaceae may be controlled by the absence of fungal growth stimulators, rather than the presence of fungal growth inhibitors (Glenn et al., 1988). Kabir et al. (1996) also speculated that the presence of a host plant was a more important factor in hyphal spread than the presence of a non-host plant. In this study, conducted in the second year after planting the strawberries and killing the canola, any negative effects of the canola may have been minimized. The canola planted between strawberry rows was only actively growing in the first month of the establishment year, after which it was killed with herbicide. The canola was also planted in the aisles, possibly minimizing strawberry-canola root contact.

The greater colonization in the wool-canola treatment could be due to greater soil organic matter from the canola residues, though this parameter was not measured. Some organic amendments such as chicken litter, leaf compost and sheep or cow manure have been shown to increase mycorrhizal colonization (Boswell et al., 1998; Muthukumar and Udaiyan, 2000). The higher levels of colonization could also be attributed to the benefits of mulch. Although there have not been studies specifically examining the effects of mulches on AMF, the well-known benefits to plants, including moisture retention and temperature regulation might positively affect soil organisms as well.

In many cases, researchers have found greater strawberry plant growth in response to greater mycorrhizal colonization (de Silva et al., 1996; Vestburg, 1992). Although the wool-canola treatment had significantly greater root dry weights and total AMF colonization, the correlation between root dry weight and total AMF colonization was negative in this treatment. However, total AMF colonization was positively correlated to leaf N concentration in the wool-canola treatment indicating possible secondary effects of AMF on root biomass due to improved N uptake. Although P uptake is considered the primary benefit of AMF colonization (Smith and Read, 1997), researchers have documented increased N uptake in other mycorrhizal crops (Mathur and Vyas, 2000; Ruiz-Lozano et al., 1995; Tobar et al., 1994). The fact that the wool-canola treatment exhibited the positive correlation between total AMF and N without the accompanying positive correlation to biomass may also be attributed to the effects of the wool mulch. The temperature regulation and weed control provided by the mulch (Forcella et al., 2003) may have benefited plant growth while masking effects of AMF colonization.

Shoot dry weight was greatest in the hand-weed treatment and was positively correlated to total AMF in both the herbicide and hand-weed treatments. These correlations support the research of Vestburg (1992) and Robertson et al. (1988) who found significantly higher shoot dry weights in mycorrhizal strawberries compared to non-mycorrhizal controls. Furthermore, in all treatments, AMF was positively correlated with leaf N. These correlations imply an AMF-mediated increase in shoot dry weight, possibly as a result of increased N uptake.

The fact that we did not find a significant positive correlation between AMF and leaf P in any groundcover treatment may be due to the high P levels in the research plots. The benefits of mycorrhizal-increased P uptake are greatest in low-P environments (Miller et al., 1986; Smith and Gianinazzi-Pearson, 1988; Smith and Read, 1997). In Minnesota strawberry crops, P additions are recommended for soil-test values less than 41 ppm (Rosen and Eliason, 1996). The average P concentration of the research plots was 172 ppm.

We conclude that in strawberry production systems, establishment year groundcover treatments may be a more important factor than fruiting year pesticide sprays in determining AMF colonization levels and plant responses in the fruiting year. Further research with mulches and cover crops could result in mulch recommendations for encouraging AMF symbiosis in field-grown strawberries.

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Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Thesis:
Arbuscular Mycorrhizal Fungi in Field Grown Strawberries. University of Minnesota, November 2004.

Publications:
“Cultivar Differences in the Arbuscular Mycorrhizal Symbiosis in Field Grown Strawberries (Fragaria x ananassa)” has been submitted to Biological Agriculture and Horticulture December 2004.

“The Effects of Groundcover Strategy and Spray Treatment on the Arbuscular Mycorrhizal Symbiosis in Strawberries (Fragaria x ananassa)” has been submitted to Biological Agriculture and Horticulture December 2004.

Outreach Events:

February 2004: Minnesota Fruit and Vegetable Growers Association (MFVGA)-seminar presentation. Symbiosis: Strawberries and Mycorrhizal Fungi

February 2004: Participating grower feedback session-share preliminary results with growers and discuss implications

November 2004: Wrap-up Meetings with participants-share final results with growers, discuss conclusions, implications and future research needs

Evaluation Results:

At the 2004 MFVGA meetings we provided a survey for presentation attendees. Additionally, MFVGA conducted an independent survey.

University of Minnesota Grower Survey
Surveys were distributed to all attendees at the Minnesota Fruit and Vegetable Growers Association (MFVGA) conference in February 2004, and specifically to those attending the Berry Session.

Summary of responses to preliminary strawberry grower survey. Sample size is 12 surveys.

1-In what county/state is your farm located?

9 Minnesota Counties
1 Wisconsin County
1 Indiana County
1 Iowa County

2-On a scale of 1(not important, low priority) to 5 (high priority) how important is soil management in your operation?

Nine – 5 (high priority)
One – 4
One – 3
One –N/A

3-How do you define the “soil community”?

Five respondents indicated: microorganisms, air, water, minerals, seeds, plants, etc.
Six respondents indicated: clay, sandy loam, well-drained, etc.
One respondent indicated: “As a poorly understood resource that few ‘farmers’ try to understand.”

University of Minnesota Grower Survey (cont.)

4-What management steps do you take to benefit the soil community?
The following responses were given on 10 surveys, 2 did not respond:

Plow down crop blends
Vertical farming
Less petroleum-based fertilizers
Fertilize (2)
Natural compost
Soil sampling (3)
Lime
Cover Crops (2)
Compost
AgroK-Symbex
Limit tillage (2)
Add organic matter
Biologic additives
Manure
Limit herbicide use
Careful fertilizer choice
Crop rotation
Deep fracturing for drainage
Cultivation
Fungicides
Mulch
Nutrition
Leaf analysis
Erosion control

MFVGA Evaluation: Berry Production Session
Attendance: 56
Evaluations: 23

Seminar Title:
Symbiosis: Strawberries and Mycorrhizal Fungi
Scores (on a 5 point scale):
Information: 4.58
Presentation: 4.47
Speaker Preparation: 4.53
Speaker Quality: 4.47
Usefulness of Topic: 4.28

Did this program include new information? 20 yes 0 no 0 maybe
Will this program help you initiate new practices 15 yes 1 no 4 maybe
Was there a high quality of instruction? 20 yes 0 no 0 maybe
Was the facility conducive to learning? 20 0 no 0 maybe
Was the total program well organized? 20 0 no 0 maybe

Project Outcomes

Project outcomes:

Implementation of, and results from this project provided strawberry growers with more insight into the soil ecosystem and how it is affected by agricultural practices. Through the example of AMF in strawberries, the project’s educational components highlighted the presence and importance of the soil community, which many growers may not have considered in overall farm management plans. Although the information was focused on AMF in strawberry systems its applicability to other production systems was emphasized.

While new management strategies may take infinite forms based on individual farms some examples might include reduced tillage and subsequent reduced erosion and compaction; increased mulch applications or cover cropping; or reduced pesticide use. These changes will benefit the farmer in terms of reduced costs and potential higher returns. In addition, the environmental benefits will serve both consumers and the farming community.

This research contributes to the body of knowledge related to the community of soil organisms in agricultural settings. Further research and outreach in this area, using this study as a starting point, will provide farmers with more insight into this ‘unseen’ yet very important component of the agroecosystem.

Economic Analysis

The most definitive new approach examined in our study is the use of inter-row wool mulch in perennial strawberry systems. Costs of adopting this technology may be initially high due to the materials and labor costs. However, these materials need only be installed in the establishment year, while their benefits continue through the life of the planting. With many strawberry farmers extending strawberry plot rotations beyond three years, these initial costs would depreciate over the life of the planting. Besides the initial cost of installing wool mulch there are few risks to adopting this technology. The demonstrated benefits of mulch including improved plant growth, increased AMF colonization and decreased erosion should outweigh the investment of the initial cost.

Farmer Adoption

We directly impacted the four farm families and one horticultural research associate/farmer with whom we worked. These participants have a new understanding of the soil community in their strawberry fields. They were an active part of the research process and therefore have a closer connection to University research and its relation to their agricultural operations. Additionally, study initiatives and research results were presented to 56 members of the Minnesota Fruit and Vegetable Growers Association attending the Berry Production session.

Based on the results of this research, we would recommend that strawberry farmers use inter-row mulch materials to increase plant growth and AMF colonization. Additionally, because we now know that AMF are present in agricultural strawberry plots, we would recommend that farmers consider AMF and other soil organisms when making management decisions.

Recommendations:

Areas needing additional study

This research raised several new questions. Specifically, we would like to know the impacts of various types of mulches on plant growth and AMF colonization in strawberry production systems. Additionally, it would be interesting to assess the species composition of AMF in different strawberry systems, and measure the effectiveness of those species as mutualists.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.