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Managing Soil Borne Diseases with Anaerobic Soil Disinfestation

Author: Michelle Grabowski. Anaerobic soil disinfestation (ASD) is an alternative strategy to reduce soil borne plant diseases in agricultural fields. This article discusses how it works and its potential effect on diseases in vegetables.

Plots treated with ASD under plastic and untreated controls 
under straw mulch. M. Grabowski, UMN Extension

What is ASD?

Anaerobic soil disinfestation (ASD) is a disease management strategy used to reduce or eliminate soil borne plant pathogens from agricultural soils. Despite its complicated name, ASD is a relatively simple process.

By adding carbon (a food source) and water (to fill pores in the soil), ASD harnesses the power of naturally occurring soil microorganisms to temporarily create conditions within the soil that are toxic to plant pathogens. The procedure was developed to provide an alternative management strategy to soil fumigation.

Here is a quick summary of the procedure:
  • A carbon source (i.e. molasses or wheat bran purchased from a feed store) is worked into the soil to a depth of 6 to 8 inches. 
  • The soil is irrigated until it is completely saturated to the same depth. Ponding of water should occur. This will take several hours. 
  • Soil is covered with plastic. The plastic covering needs to be completely sealed at the edges and must remain in place for 3 to 5 weeks.
  • The soil needs to be consistently 85 F or greater while the plastic is in place to be effective. In Minnesota this means that the procedure needs to take place during the growing season. Most growers choose spring or fall to allow time for a crop during the primary part of the growing season.   

Ohio State University has a great publication describing the steps for anaerobic soil disinfestation in detail.

What types of diseases is ASD a useful management tool for?

In both fruit and vegetable production, a number of different soil borne plant pathogens can result in root rot, poor growth, plant wilt, and death. Soil borne pathogens include fungi, bacteria, and nematodes. These types of pathogens often build up in the soil over multiple years of production, especially if crop rotation is not used. Research in other states shows that ASD is successful in eliminating the fungi Verticillium dahliae (wilt) and Fusarium oxysporum (wilt, root and crown rot), the water molds Pythium spp. (root rot) and Phytophthora capsici (root rot and wilt), bacteria like Agrobacterium tumefaciens (crown gall), and root knot nematode (Meloidogyne incognita) from soil.

How does ASD work? 

There appear to be multiple factors at work in ASD. Agricultural soils naturally have small pores filled with oxygen. When these are filled with water, the soil turns anaerobic (no oxygen available) and the microorganisms in the soil change dramatically. Most plant pathogens are not well adapted to these low oxygen conditions and die or have reduced growth. There is an increase in the number of bacteria that thrive in anaerobic conditions. As these bacteria consume the carbon source without oxygen, they release volatile organic compounds, including alcohols, organic acids, and organic sulfides all of which contribute to the suppression of plant pathogens. Some of these new microorganism are believed to be antagonistic to plant pathogens as well.

Testing ASD in Minnesota

In 2018, we tested ASD on two sites with known disease problems.
One site had a history of Verticillium wilt, a soilborne fungus that infects a number of fruit and vegetable crops, resulting in wilt and death of the plant.  The other site had clubroot, caused by a long lived pathogen (related to slime molds) that causes the roots of cole crops to become swollen and distorted.
Diluting molasses prior to field application.
M. Grabowski, UMN Extension

Sprinkler irrigation used to saturate soils in ASD trials.
M.Grabowski, UMN Extension

  • We used molasses as our carbon source, purchased as a liquid from a local feed store. We added molasses to soil at a rate of 0.41 lbs/sqft. The molasses was diluted 3 to 1 in water and applied evenly over the soil with watering cans.
  • At each site we left half the plot untreated for comparison purposes. 
  • The plots were watered with sprinkler irrigation until the soil was saturated and beginning to pond on the surface. 
  • We then covered the plots in 4 mil clear plastic sheeting, sealing the ends completely by covering them with soil. The plastic was left in place for 5 weeks in late May and early June. After removing the plastic we allowed the plots to air out for one week before planting (plant roots prefer oxygen in soil pores and will not do well planted into an anaerobic soil). 
  • We monitored disease and measured yield at both sites.

What did we learn?

Soil temperatures in the ASD plots were 10 to 15 degrees Fahrenheit greater than untreated soils during the 5 week treatment period, reaching a max of 98F.

A sour, swampy smell was present in ASD treated soils when the plastic was lifted, indicating that the soils had indeed gone anaerobic.

Clubroot trial

In the clubroot trial, only one plot developed disease. We planted broccoli, Napa cabbage, cabbage, and bok choy. Although this test needs to be repeated and tested on a larger scale, the initial results were promising.

Bok choy plant from ASD treated plot.
M. Grabowski, UMN Extension 

Bok choy from untreated plots with severe clubroot infection.
M. Grabowski, UMN Extension

  • Bok choy – heads in ASD were 5 times heavier than control plants and had no clubbed roots. Plants in untreated plots had bulbous distorted roots and small heads.
  • Cabbage– Roots in untreated plots were extremely distorted with no fibrous root hairs. Plants were stunted and had no harvestable heads. Plants in ASD plots had healthy roots with many fibrous hairs. One plant had a few bulbous galled roots. It was unclear if the ASD treatment did not completely kill the clubroot pathogen or if it had been reintroduced on contaminated tools or shoes. Tekila, a clubroot resistant variety yielded well and had healthy roots in both plots. 
  • Broccoli – Plants in the untreated plots had extremely distorted roots and stunted plants. In the ASD plots, plants were 1 to 4 times bigger and although healthy roots with many root hairs were present, several clubbed and distorted roots were also present on the plants. 
  • Napa Cabbage – The susceptible variety (Blues) had extremely clubbed roots, stunted, and wilted plants with no harvestable yield in untreated plots. In the ASD treated plots no clubbing was found on the roots and yield was an average 3.1lbs/head. Clubroot resistant varieties (Emiko, Yuki, and China Gold) did not have clubbed roots in either plot but produced significantly larger heads in the ASD plots. 

Verticillium wilt trial

It was difficult to keep the well-drained soils in the Verticillium wilt plots wet throughout the 5 week period. Soils in this test were completely dry when the plastic was removed. As a result, it is unclear how long the soil was actually anaerobic.

There was no difference in disease or yield between the ASD treated and the untreated plots in this test. Other research tells us that the soils got hot enough to reduce the survival of Verticillium in the soil but we are unsure if the soil stayed wet long enough for the treatment to be effective.

We recommend that if you try ASD on your fields, consider laying drip tape below the plastic. If necessary, water can be added to the plots without removing the plastic during the 5 week period.

Author: M. Grabowski, UMN Extension Educator

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