Dealing with root rots? Consider anaerobic soil disinfestation

Author: Marissa Schuh

Growers have been reporting soilborne issues in tunnels from rhizoctonia in lettuces to tomato pathogens like Verticillium. One way of dealing with these pathogens is to starve them of oxygen, killing the microbes -- be they fungi, oomycetes, nematodes or bacteria.

Bottom rot in lettuce caused by the soil borne pathogen Rhizoctonia solani. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The process of killing pathogens this way is called Anaerobic Soil Disinfestation (I know, I hadn’t thought of anaerobic vs aerobic since gym class).  It has a big name, but it's a relatively simple process. There are three steps in anaerobic soil disinfestation...

1. Add a carbon source

This will feed the beneficial microbes who are going to take the oxygen out of the system. The most common and most effective carbon source is molasses, but researchers have been exploring alternative carbon sources like wheat middlings and cover crops.

Recent studies on cover crops as the carbon source for this process have had mixed results. Research in Pennsylvania has suggested that of the cover crops, buckwheat is the best option. Work in Ohio didn’t find that cover crops varied much in how well they performed, however they all performed worse than wheat middlings.

The carbon source needs to be relatively easy for the microbes to break down. The C: N ratio matters a lot: cover crop residue with soft green leaves will work much better for this purpose than tough straw. This resource from Tennessee that lays of the ratios of different carbon sources. 

Whatever you choose, you will need a lot of it. Rates range from 4.5 to 9 tons of dry matter per acre (this is ~.4 lbs per square foot). It will then need to be worked into the soil, aiming to get the materials 6-8 inches deep.

2. Add water

To get the microbes working and start getting oxygen out of the soil, add enough water to saturate the soil as deeply as you’ve integrated your carbon source.

3. Seal everything up

A plastic tarp then is used to seal everything up, keeping oxygen from entering the system. This can be clear or black (carbon source seems to have a greater impact than tarp color), but should be applied to the area as quickly as possible. Also, make sure to keep air out by burying the edges.  This layer needs to be airtight -- consider than when choosing your plastic and preparing the space.

How long does the tarp need to stay on? As long as temperatures under the tarp stay about 85 degrees Fahrenheit, the process should take 3-4 weeks. If the area starts to smell bad, that is a sign that you have created the anaerobic conditions you need. Work in Pennsylvania has suggested that the right temperatures exist between April and the end of September to get this process to work.

Thinking about trying this out? These resources from Ohio and Tennessee clearly lay out the process and things to think about.