Microbial changes associated with use of brassica cover crops in a strawberry production system

View the project final report

Commodities

• Fruits: berries (strawberries)

Practices

• Crop Production: application rate management, cover crops
• Education and Training: extension
• Pest Management: biological control, mulching - plastic, soil solarization
• Production Systems: organic agriculture
• Soil Management: organic matter, soil analysis, soil microbiology

The objectives of this project included characterizing plant pathogen and general microbial population changes in soils under different sustainable management practices for strawberry production systems at two different locations in Arkansas. A brassica cover crop, mustard seed meal, solarization or a combination of the cover crop and solarization were compared to no soil treatment prior to establishing the strawberry crop. General microbial and suspect pathogen populations from soils were quantified by plate count methods. Additional soil samples were taken after cover crop incorporation to generate denatured gradient gel electrophoresis (DGGE) profiles for bacterial and fungal populations. Roots of strawberry plants, including the initial transplants, were also analyzed for the isolation frequency of plant pathogens.
Pythium populations were numerically higher in soils following the brassica cover crop compared to the control, but results were only significant at one site where Pythium was also increased when brassica cover crop was followed by solarization and in mustard seed meal amended soils compared to the control. Rhizoctonia was only isolated from soil at one location, where binucleate and multinucleate Rhizoctonia populations were higher in soils that received a brassica cover crop compared to no soil treatment. However, soil treatments usually did not significantly affect the frequency of Pythium, Rhizoctonia or Colletotrichum isolation from the roots of strawberry. The only exception was in plots that had been planted with a brassica cover crop and then solarized prior to strawberry planting. In these plots, 15.3% percent of strawberry plants were found to be colonized by Rhizoctonia solani compared to only 5.0% in control plots. Approximately 80% of strawberry transplants were colonized by Pythium, Rhizoctonia or Colletotrichum before they were planted into the test plots. Root ratings, which were used to monitor disease severity based on the percent discoloration of roots, did not differ among the treatments. Along with contamination of transplants, unusually high rainfall amounts prior to and shortly after strawberry transplanting likely contributed to a compromised root system for all plants.
Soil treatments did affect the level of bacterial, fungal and actinomycete populations in the soil at the time of brassica cover crop termination and at strawberry transplant. In all cases, there was a trend for higher bacterial, fungal and actinomycete populations in brassica, brassica plus solarization and mustard seed meal amended soils compared to solarized only and control soils, yet results were not always significant at both test locations. Total culturable, bacterial populations were significantly higher in soils that had been planted with a brassica cover crop followed by solarization and soils receiving mustard seed meal amendments at both locations at the time of strawberry transplanting. From soil samples taken at 7 and 25 days after the brassica cover crop was incorporated into the soil, denatured gradient gel electrophoresis (DGGE) produced unique profiles of bacteria and fungi compared to that of control soils. At the time of strawberry transplant, all bacterial DGGE profiles of soils from both locations receiving different treatments were still distinct and grouped separately in dendograms, yet fungal DGGE profiles were not as consistently distinct among treatments.
This project has successfully proven how including soil treatments such as a brassica cover crop, solarization or mustard seed meal application as a practice in annual strawberry production can enhance the soil microflora, especially the bacterial community. Since changes could be observed in both the bacterial and fungal communities throughout the sampling times, this system has the potential to produce a soil that is more diverse and possibly reduce populations or colonization of roots by soilborne pathogens. The impacts of these shifts in the soil microflora for soilborne diseases should be compared to chemical fumigants in soil with a history of strawberry production to examine their value in developing a sustainable strawberry production system.

Introduction

Soilborne pathogens are a limiting factor for strawberry production and historically have been managed through the use of fumigants, especially methyl bromide. Common soilborne diseases of strawberries are black root rot, crown and leather rot, red stele and Verticillium wilt (Guerena and Born, 2007; Maas, 1988). It has been shown that rotation with a brassica crop, broccoli, led to a reduction in Verticillium wilt of strawberry and microsclerotia in the soil compared to a non-brassica crop, lettuce (Subbarao et al., 2007). Additionally, in vitro studies have shown that macerated roots of brassica suppressed six different soilborne pathogens of strawberry, including Colletotrichum dematium, Cylindrocarpon destructans, Fusarium oxysporum, Pythium ultimum, Phytophthora cactorum and Rhizoctonia fragariae (Mattner et al., 2008). Another study has shown that a brassica-strawberry cropping sequence resulted in increased fruit yield compared to a non-treated control and showed no significant difference in yield from methyl bromide treated plots (Lazzeri et al., 2003).
Brassica spp. produce glucosinolates which decompose into chemicals that are inhibitory to a range of microorganisms including nematodes, fungi and bacteria. A proposed mechanism of disease suppression has been based on the release of these chemicals. However, these volatile chemicals of glucosinolate hydrolysis are detected in soil for only short periods of time, with reports ranging from 24 hours (Mazzola et al., 2007) to 12 days (Njoroge et al., 2008). When brassica seed meal was incorporated into soil four weeks prior to infestation with Rhizoctonia solani, there was a significant reduction in apple seedling infection, indicating other factors besides glucosinolate hydrolysis products play a role in suppressing disease (Mazzola et al., 2007). The authors of this study concluded that decreased root infection by Rhizoctonia was associated with increased populations of Streptomyces. Earlier research reported brassica seed meal amendments increased fluorescent Pseuodomonas, actinomycetes, and total bacterial populations (Mazzola et al., 2001), suggesting that benefits of using brassica cover crops could be associated with enriching the microbial population resulting in suppression of pathogens of the crop of interest.
This initial study investigated changes in the soil pathogen and general microbial populations over time after a brassica cover crop, mustard seed meal application, solarization and a combination of a brassica cover crop and solarization compared to control soils. Additionally, strawberry roots were examined for disease symptoms and colonization by potential pathogens. The goal of this research effort is to contribute to the development of a sustainable production system by suppression of soilborne strawberry diseases without the use of chemical soil fumigants.

This research is part of a larger project to develop a sustainable system for annual strawberry production using a brassica cover crop for soilborne disease suppression and will be done according to the following objectives:
1) Quantify changes in disease incidence and severity and soil pathogen populations in a strawberry cropping system that includes a brassica cover crop, mustard seed meal application, solarization or a combination of a brassica cover crop and solarization as preplant treatments
2) Quantify and characterize soil microbial changes in different strawberry production systems that include a brassica cover crop, mustard seed meal application, solarization or a combination of a brassica cover crop and solarization as preplant treatments
Monitoring changes in the soil microbial composition that result from the adoption of a brassica cover crop will help indicate whether this is a factor contributing to disease suppression. Strawberries are a high value crop identified as an attractive option for limited resource farmers or farmers with limited acreage. This research will aid in determining if brassica cover crops are a desirable option in strawberry disease protection for those farmers adverse to the high input, chemically intensive strawberry production system.