On-Farm Study Results: Biofumigation and Soil Solarization
prepared by research
assistant Brian Geier
to share preliminary findings of an ongoing
In the summers of 2007-08 we conducted on-farm trails at Au Naturel Farm to monitor the effects of biofumigation and soil solarization on Sclerotinia sclerotiorum, the fungus that causes white mold of lettuce in high tunnels growing winter vegetables across Kentucky. We compared four treatments:
- Untreated control
- Biofumigation and solarization
We replicated each treatment four times in a randomized complete block design.
|Sclerotinia sclerotiorum causes white mold of lettuce in the high tunnels at Paul and Allison Wiedigers' Farm. University of Kentucky Plant Pathologist Paul Vincelli observes. Mike Bomford photo.||Plots inside the high tunnel at Au Naturel Farm: Biofumigation, Soil Solarization, Biofumigation plus Soil Solarization, and Control plots were replicated and randomized. Brian Geier photo.|
We raised thousands of S. sclerotiorum sclerotia in Dr. Vincelli's lab by infecting a sterilized grain mixture with fungal spores. We separated the sclerotia from the grain, then made mesh bags containing 40 sclerotia each. We buried 24 bags in each plot: Three at each of four soil depths (0, 5, 10, and 15 cm) at the middle and edge of each bed.
|Left: Sterile milk jugs filled with a grain mixture were inoculated with spores from S. sclerotiorum. Right: After a few weeks, thousands of sclerotia were formed. Ed Dixon photos.|
|Left: REAP student Aminata Kalley separates and counts sclerotia. Ed Dixon Photo Right: Forty sclerotia inside a mesh bag are ready for burial in our experimental plots. Mike Bomford photo.|
|Plant pathologist Paul Vincelli (left) and farmer cooperator Paul Wiediger bury bags of sclerotia in experimental plots. By retrieving bags of sclerotia from different treatments, we can determine what effects these conditions have on survival of S. sclerotiorum sclerotia. Mike Bomford photo.|
Biofumigated plots were treated in late July by spreading Indian mustard, Brassica juncea, which has a high glucosinolate content. Glucosinolates (GSL's) are compounds produced by plants in the mustard family, as a natural defense mechanism. When mustard tissue is broken (for example, by harsh winds or an insect bite) glucosinolates are converted to a variety of volatile compounds, creating a kind of toxic soup, in an effort to make the wound less attractive to pests. Some of these compounds, called isothiocyanates (ITC's), are toxic to S. sclerotiorum. (Interestingly, ITC's are the compounds responsible for broccoli's cancer-fighting quality in humans.) We chopped mustards grown on the KSU farm by hand and with a food processor, then applied them to our biofumigation plots at rate of 900 grams of stems and leaves (fresh weight) per square meter of land. Plots were immediately tilled with a roto-tiller and watered to ensure rapid, thorough incorporation into the soil.
|Research assistant Brian Geier tends to 40 accessions of mustards at the KSU Research farm. Accessions were obtained from the USDA germplasm repository and tissues were evaluated for glucosinolate content by Dr. George Antonious. Extracts of GSL's from accessions with high levels are being tested for toxicity to S. sclerotiorum. Mike Bomford photo.|
|Biofumigated plots were treated by spreading 900 g/m2 chopped Brassica juncea leaves and stems, and incorporating this into the soil with a roto-tiller. Mike Bomford photo.|
Solarized plots were covered with sheets of clear plastic in late July to trap solar radiation in the soil and create temperatures that are lethal to S. sclerotiorum sclerotia. Solarization is often used with synthetic fumigants such as methyl bromide to kill weed seeds and soil-borne pathogens. Efforts to combine biofumigation and solarization have, in part, been inspired by the need to find alternatives to methyl bromide, which has been banned because of it destroys the ozone layer. Solarizaton enhances the effect of biofumigation on some pathogenic nematodes. We wanted to see if solarization, alone or in combination with biofumigation, could control S. sclerotiorum. Solarization is acceptable under national organic standards.
We buried temperature probes at the same depths and locations as the bags in solarized and unsolarized plots. The probes were wired to a datalogger that recorded soil temperatures hourly throughout the study.
|Farmer Paul Wiediger, left, and Research Assistant Brian Geier bury the edges of plastic on a solarized plot. Some plots were biofumigated with mustards, then solarized with plastic. The box in the bottom right corner holds a datalogger that recorded soil temperature every hour. Mike Bomford photo.|
Monitoring Survival of Sclerotia
We collected one bag from each depth and location in each plot two, four and six weeks after treatment. After each harvest, we washed, counted, and plated all of the sclerotia from each bag on sterile soil. We incubated the plates of soil and sclerotia for eight weeks at 16 ºC to stimulate germination, then recorded the number of sclerotia that successfully germinated from each combination of treatment, depth, and location.
|Plates of sclerotia collected from experimental plots were incubated at 16° C with a 12:12 photo period. After germination was complete, counts were done to determine how many sclerotia survived different treatment conditions. Brian Geier photo.|
|Sclerotia sprouting apothecia, the mushroom-like structures that can spread spores, leading to plant infection. Ed Dixon photo.|
|Number of Germinating Sclerotia at Different Soil Depths and Treatments|
|The graph clearly shows that solarization was almost completely lethal to S. sclerotiorum to 15 cm, and that biofumigation, by our method, had no effect on the fungus.|
Daily Soil Temperature Fluctuation (°C) in Solarized and Unsolarized (Control) Plots
|This chart shows the daily fluctuation in soil temperature in solarized (purple) and unsolarized (blue) plots. Soil depth, in centimeters, is shown above each treatment label. Fluctuations were less pronounced deeper in the soil than at the soil surface. Soil temperatures in solarized plots consistently stayed above 30 °C each night.|
This study suggests that a month of summer soil solarization can control populations of S. sclerotiorum to a depth of 15 cm in Kentucky high tunnels. The effect was seen at both at the middle and edge of solarized plots. Biofumigation, by our methods of incorporating a mixture of Brassicae juncea leaves and stems at a rate of 900 g/m2, did not reduce germination of S. sclerotiorum sclerotia.
Looking Ahead: 2009 and Beyond
We began this project with 40 accessions of mustards from around the world, and ran several trials to evaluate which grew best in our climate and had a high glucosinolate content. "Pacific Gold", a variety developed specifically for biofumigation, was a clear winner in all studies, and we are now using this variety in all of our biofumigation studies.
In 2009, our final year of SARE-funded biofumigation research, we will conduct another year of high-tunnel research on biofumigation of S. sclerotiorum. The study will be conducted at our research farm, and will be very similar to the on-farm studies we conducted at Au Naturel Farm. One major change we are enacting is the use of mustard biomass from an existing cover crop that we will plant into the high tunnel in the summer.
In past studies, we assumed that a cover crop of mustards would not grow well inside a high tunnel in the middle of summer. We were also concerned that growers would not want to take high-value high tunnel space out of production to plant a cover crop. However, Paul and Allison Wiediger, our farmer collaborators, reported success with biofumigation after incorporating a cover crop of Pacific Gold mustard grown in their high tunnel in mid-summer.
Taking these observations into account, our 2009 study will look at biofumigation with an on-site stand of Pacific Gold mustard grown inside the high tunnel.
With further study, we may develop specific recommendations for how biofumigation techniques can be reliably and effectively used.
Last updated April 17, 2009