Compost Teas Benefit Growers

Catherine Crosby is currently a Ph.D. student at Washington State University. She obtained her B.S. in molecular biology at Western Washington University in 2004, and her M.S. in soil microbiology at WSU in 2008. Catherine enjoys whitewater kayaking, playing the flute and practicing Shudokan karate when she is not in the laboratory.

Organic farming practices are becoming more widespread as more consumers demand organic produce, meats and fibers. Organic food sales have increased around 20 percent each year for more than the last decade. This surge in consumer demand has fueled organics in all areas, from the small-scale local CSA (Community Supported Agriculture) farm to the corporate giants planting large-scale organic fields.

One of the biggest challenges in growing organic crops is managing pests and diseases without most industrial pesticides. Conventional agriculture often approaches disease management with the mindset of applying a curative-treatment when a problem presents itself. But organic farming, lacking the quick-fix chemicals, must focus more on prevention of disease by careful attention to plant, soil and farm health.

Cultural practices for disease prevention are the cornerstone of successful organic farming. One key technique called crop rotation is easiest for growers with diversified crops. Crop rotation is a first line of disease prevention. Each crop type is grown in a different field each year. Crops are moved around the farm allowing one to five years between crops susceptible to the same insects and diseases to prevent buildup of disease causing organisms in the soil.

Maintaining clean zones around crops that are free of closely related weeds will eliminate habitat for both insects and microbial pathogens. Careful cleaning of field equipment helps prevent the spread of any organisms that might enter the field.

After a rain or watering, fieldwork can be delayed because many organisms are easily spread in water droplets. Unfortunately, preventing disease by careful management is labor and cost intensive, and does not always prevent every problem.

The demand for new methods to suppress disease in organic systems has sparked many new and innovative research projects. One promising area of research is in biological control of plant pests and disease.

Biological control involves using other organisms to control the disease-causing agent. In both conventional and organic agriculture, biological control of insect pests is becoming common.

Hormone signals can be used to trap or confuse insect pests, disrupting their reproductive cycles. Also, harboring large populations of beneficial insects, such as ladybugs, will help to control the populations of damaging pests, such as aphids. In some cases, viruses and microbes that cause disease in insect pests can be applied to fields if the pest population reaches a critically high “threshold” level.

The principles of biological control are the same for microbe-caused diseases of crops. Growers attempt to apply organisms to the field that will out-compete or interfere with the disease cycle. Most biological control agents are very specific; that is, they act on a single pathogenic organism. Due to this specificity, traditional biological control agents must be developed separately for each disease organism.

Even if a successful biological control practice is developed, it may not be transferable to other fields, even if the disease organism is the same. Development of a widely effective, generic biological control agent would be extremely valuable.

Compost teas, an aerated mixture of compost, clean water and various nutrient additives, contain complex microbial communities, and have shown potential as a biological control agent. A good compost tea can provide nutrients and microbes from a safe composted source.

Farmers have observed disease reduction when using compost teas on their crops, but until recently there has been little science to back up such claims. Most scientific studies of compost tea show disease suppression under greenhouse conditions, but show poor transfer of such results to the field.

What are the characteristics of effective disease suppressors? Compost teas contain a diverse microbial community, but not all of those community members will survive field conditions. Successful biological control agents must be present when pathogen pressure is high. They must be able to survive on the leaf surface or in the soil, and be active against the target pathogen. Adding sticking agents to the compost tea can help keep the microbial organisms on the leaf surface, and help them survive. Effectiveness against the target pathogen then becomes the research focus.

The microbial community in compost tea varies widely depending on compost source, additives and brewing conditions. Traditionally, compost tea is brewed by hanging a bag of compost in a barrel of water and allowing the compost tea to steep, anywhere from a few hours to a few weeks. The longer the mixture is allowed to ferment, the more nutrients will be released. This type of compost tea is not aerated, and tends to favor microbes that can live without oxygen (anaerobic) and favor slow-growing fungi.

Much of the current research focuses instead on aerated compost teas, which in addition to compost may contain nutrient additives such as rock dust, seaweed extract, whey powder, and/or sugar sources such as molasses or glucose. Aerated teas are brewed under oxygenated conditions, created by bubbling air through the mixture. These aerated teas have shorter brew times, ranging from 24 hours to two weeks.

Compost teas are easy to produce on the farm site, and can be made using on-farm compost, which helps recycle farm waste into a nutrient source, and provides native organisms for the microbial community in the tea.

Non-aerated teas have been shown to have disease control properties against some fungal pathogens that cause damping off, and powdery mildew in wine grape production.

Studies using aerated teas show more variability in disease control abilities. The variation in the microbial communities between batches of aerated teas likely accounts for a lot of this variation.

Researchers at Washington State University (WSU) are investigating the properties of effective compost teas. This is a component of the Biologically Intensive & Organic Agriculture Program, the first in the nation.

To better understand the variation of aerated compost tea microbial communities, WSU researchers developed four distinct recipes that favor different types of organisms. A minimally aerated tea, brewed for 10 days, supported larger fungal communities. Another tea with minimal nutrients and aeration supported protozoa.

The other two tea recipes, one with added nutrients and one without, were brewed under full aeration and grew large populations of bacteria.

Three batches of each tea were brewed and the chemical and microbial properties were tested for consistency between batches, and for differences between the recipes. The three batches of each recipe were very consistent in pH, dissolved salts, soluble carbon and nitrogen, and microbial populations.

The four recipes showed significant differences in the chemical properties, and in microbial community members. This indicates that with careful attention to starting materials and brewing conditions, aerated teas can be brewed with good consistency. The differences between recipes also underline how brewing conditions affect which microbes are amplified from the same starting inoculum.

The same four tea recipes were tested in the laboratory for activity against the cabbage pathogen Xanthomonas campestris, which causes blackrot. When Xcc was mixed with the compost teas, Xcc failed to produce the hormones and slime layer necessary to infect the plant

However when the same experiment was repeated with filter-sterilized tea (the same chemically, but without the compost tea microbes), the Xcc does produce the normal hormones and slime layer.

This indicates that compost tea microorganisms can inhibit the infective stage of Xcc, though the mechanism is still unclear.

In the next phase of this research, the same four tea recipes will be tested on healthy cabbage seedlings inoculated with Xcc. Until further testing is done in the greenhouse and in the field, conclusions cannot be drawn about the use of compost teas to suppress blackrot in cabbage.

Although early scientific evidence and grower accounts offer evidence of disease reduction by compost tea, growers must note that compost teas are not currently allowed to be used for disease control and are classified as an experimental pesticide.

While they can be applied using the same rules as compost for plant nutrition, compost teas applied to reduce or prevent disease is not permitted by EPA regulations. That said, compost teas are used widely in agriculture as a plant and soil fertility supplement.

Although the potential for compost tea as a disease control agent is still unclear, compost teas do offer nutritional benefits to plants by supplying soluble nutrients.

In Whatcom County, growers are using compost teas in greenhouse production of ornamentals and vegetable starts, on hazelnuts, blueberries and other food crops. Sound Horticulture, Inc. (alison@soundhorticulture.com; 739-9095), a local compost tea consulting firm, can help to develop compost tea formulations for specific sites as well as provide more information on compost tea use.

The Biologically Intensive & Organic Agriculture program (BioAg program) at Washington State University provided funding for this research as well as for many other projects in agricultural sustainability. For more information visit the Center for Sustaining Agriculture and Natural Resources at: http://csanr.wsu.edu. §