There are many organic production techniques that are easy to recommend because they are proven to make ecological and economic sense. Crop rotation, cover cropping, and the use of beneficial insects to control greenhouse pests are a few that come to mind. Applying compost tea to suppress plant diseases is a practice that I would not yet put in the same category, but I hope to someday.

Why use compost tea? It’s long been recognized by organic growers and researchers that the use of compost can help prevent some plant diseases. So, it makes sense that liquids derived from compost could also have disease-suppressive characteristics. Besides stimulating growth, compost and compost ‘juice’ can also help fight off diseases by inoculating plants with beneficial organisms. Some of these good guys are bacteria and some are yeasts or fungi. These organisms are beneficial if they form a physical barrier against pathogens, or if they effectively compete with or attack the plant pathogens.

What is compost tea? Before going further, it’s important to explain what compost tea is, and what it isn’t. Compost tea is not the dark-colored solution that leaks out of the bottom of the compost pile. That’s called leachate, and although it may contain soluble nutrients it may also contain organisms that can cause illness so it isn’t suitable for spraying on food crops. Some people make compost tea to be the ‘extract’ of compost made by suspending compost in a barrel of water for a short period of time, usually in a burlap sack. The resulting liquid can then be applied as a soil or foliar fertilizer. To others, it’s not compost tea until the extract is fermented or ‘brewed’ usually with some type of microbial nutrient source such as molasses, kelp, fish byproducts, and/or humic acids.

Making the tea. Compost teas can be prepared using either an aerated or a non-aerated brewing process. Aerating means introducing oxygen into the tea, by injecting air or by showering recirculated tea through a porous bag of compost suspended over an open tank, for example. Non-aerated teas are made by simply mixing the compost, water, and other ingredients and allowing them to ferment with little or no disturbance. Both methods require a fermentation vessel, high-quality compost, water, incubation time, and filtration of the end product prior to foliar application through spray equipment.

It’s not clear whether it’s necessary to aerate during compost tea production. Aeration can speed up the production time, but it may also add to equipment cost and complexity. There are claims that aeration helps kill off the organisms that can cause illness. Very few studies have compared aerated and non-aerated compost teas.

Does it work? The answer is…it depends. It’s not easy to study how compost and compost teas affect plant pathogens. No two batches of compost are exactly alike, and teas can be made from compost using several different methods. This variability is one reason why research sometimes shows that compost tea suppresses disease, and sometimes it doesn’t.

Research results do suggest several factors that may enhance the effectiveness of compost tea for suppressing disease. One is the use of compost made from animal manures (horse, cattle and poultry) versus other kinds of compost feedstocks. The compost should not be more than 9 to 12 months old. With non-aerated compost tea, at least five to eight days fermentation time may be needed. For aerated compost tea, 24 to 48 hours of fermentation is considered sufficient. Once brewed, compost tea should be applied promptly. It has a short shelf life because available oxygen is used up and the tea turns anaerobic, killing the beneficial bacteria.

Keep it safe. There are food safety concerns about the use of compost tea in food crop production because it may be a source of illness-causing microbes like coliform bacteria or Salmonella. To reduce food safety risks, growers should follow the recommendations of the National Organic Standards Board’s compost tea task force (on the web at: www.ams.usda.gov/nosb/meetings/CompostTeaTaskForceFinalReport.pdf)
which include:

• Use only potable water to make compost tea or to dilute it.
• Sanitize all of the equipment used to prepare compost tea.
• Make compost tea only from compost that has maintained a temperature of at least131 degrees F. for 3 days and has been mixed so all of the pile or windrow has heated up.
• Avoid additives when fermenting compost tea, as these can promote the growth of harmful organisms. In particular, simple sugar sources, like molasses, should be avoided.
• Additives can be used if sample batches of compost tea are tested before using it to make sure it meets the EPA’s recreational water quality guidelines for coliform bacteria.
• If compost tea is made with additives but is not tested, or if it doesn’t meet water quality guidelines, then food crops may not be harvested until 90 to 120 days after the compost tea has been applied (as with raw manure use on organic farms).

Ask first, spray later. Since the National Organic Program of USDA has not officially adopted any compost tea guidelines as of this writing, organic farmers must check with their individual certification agency before making or using compost tea.

Some recent research. A 2-year study by the Rodale Institute and Pennsylvania State University evaluated the use of aerated compost tea for disease suppression and crop stimulation in grapes, potatoes, and pumpkins. They first prepared a compost tea by combining compost and water at a ratio of 9:1, and then adding other materials such as kelp, humic acid, and fish hydrolysate. The mixture was mechanically aerated to create aerobic conditions in the solution. The tea was analyzed and shown to contain an adequate population of beneficial organisms and a safe level of human pathogens.

Compost tea was applied in both years of the project. Pumpkin plots were evaluated for powdery mildew. Potato plots were evaluated for late blight. Grape plots were evaluated for powdery mildew, downy mildew, black rot and gray mold.

During year one, approximately 50% suppression of powdery mildew was observed in the compost tea treated grape plots. A slight reduction of gray mold, along with an increase in the level of downy mildew, was observed in the compost tea treated plot during the first year at one of the vineyards. Compost tea failed to suppress powdery mildew on Howden pumpkins in year one, but reduced the number and size of pathogen colonies in year two. Compost tea did not reduce severity of late blight on Superior potatoes when disease was present in year two.

The study found that compost tea offered more measurable benefits in stimulating crop growth, yield, and quality than in suppressing disease. The study’s conclusion makes sense: The efficacy, consistency, and practicality of using compost tea for disease management needs to evolve considerably before recommendations can be made to growers.

For more information on making and using compost tea, visit ATTRA’s web site: http://attra.ncat.org/attra-pub/compost-tea-notes.html or call 800-346-9140.