Compost tea: principles and prospects for disease control¹

By Craig Coker, Council President

An increasing body of experimental evidence indicates that plant disease can be suppressed by treating plant surfaces with a variety of water-based compost preparations, known as compost extracts or compost teas.  Compost teas can be either non-aerated (NCT) or aerated (ACT).  Both methods intentionally ferment well-characterized compost in water for a defined period of time.  ACT refers to any method in which water is actively aerated during the fermentation process.  NCT refers to methods that do not disturb or only minimally disturb the fermentation after initial mixing.  Both methods of compost tea production require a fermentation vessel, compost, water, incubation and filtration.  Nutrients may be added before or after fermentation and various spray amendments can be added prior to application of undiluted or diluted tea.

There is debate over the necessity to aerate during compost tea production.  Aerated production methods are associated with reduced production times.  Nonaerated production is associated with low cost, low energy input and many documented reports of plant disease control. Common designs for ACT production include showering recirculated water through a porous bag of compost that is suspended over an open tank, recirculating water through a vortex nozzle mounted above a tank,  or by injecting air through a hollow propeller shaft, venturi nozzles, aquarium stones, or fine bubble diffuser mats.

NCT has traditionally been made by mixing one volume compost with four to ten volumes of water in an open container, initially stirring the mixture, and then leaving it undisturbed at 15-25^oC. (59-77^oF) for at least three days.  Regardless of the preparation method, compost teas are typically applied with conventional pesticide spray equipment after filtering out material that would clog the nozzles.

Very little data directly compares nonaerated and aerated production methodologies for plant disease control.  A variety of foliar diseases have been suppressed by applications of NCT, including: powdery mildew on roses and apples; gray mold on green beans, strawberries, grapes and geraniums; root rot on potatoes, tomatoes and grapes; Fusarium wilt of peppers and cucumbers, and damping-off of pea seedlings.  The literature review cites 43 studies supporting the use of NCT for pathogen suppression.  Research on the use of ACT to control foliar and fruit diseases is more limited; the review cites 16 studies that have been conducted.  These studies report mixed results, with one showing reduced incidence of brown rot blossom blight on sweet cherry, but several reporting no reduced incidence of powdery mildew or other foliar diseases.  Impacts on plant health and yield can be crop specific and general inferences about disease suppression or yield cannot be made.

Multiple modes of activity are involved in suppressing plant disease with NCT; yet to date no studies have determined the mechanisms involved with ACT.  Induced resistance, antibiosis, and competition have been used to explain suppression of foliar pathogens by NCT.

Considering the diverse microbial community in compost tea, it is likely that multiple modes of activity associated with microbial antagonists are involved in disease suppression. Several studies indicate that applying the microbial component of compost tea is necessary for disease suppression.  However, it is not clear whether pathogen inhibition is due to parasitism, competition for nutrients and colonization sites, or if applied organisms produce antibiotics in situ once established on plant surfaces. Regardless of the mode of action or source/type of microorganisms, preventative application before pathogen infection appears necessary for optimal control through all known modes of action.

Several of the processing steps used to make NCT can impact the suppressive properties of compost tea.  Besides aeration, these include choice of compost feedstocks, compost age, water ratio, fermentation time, added nutrients, temperature and pH.  Choices in application technology include the dilution ratio, application equipment, timing, rates, spray amendments, and adding specific microbial antagonists.

As the body of published research expands, it becomes obvious that there is no one ideal management level across all host-pathogen systems for the compost tea production and application factors.

Compost feedstocks that appear to offer the best control with the use of NCT are those derived from animal manures (horse, cattle and poultry).  Other feedstocks reported in the literature include grape marc, spent mushroom substrate and yard trimmings.  Due to the potential for transferring detrimental effects, compost for compost tea should be certified free of human pathogens and herbicides.  Herbicide contamination of compost tea is becoming a potential issue with the increasing occurrence of clopyralid and picloram contaminated compost.

There is increasing knowledge on how old compost can be before it is no longer useful for making suppressive NCT.  Several studies report that compost cannot be more than 9 to 12 months old.  It is still unclear how the compost-to-water ratio of NCT affects disease suppression, but limiting the ratio of 1:10 is apparently effective.  Several studies of NCT have indicated that disease suppression varies widely in relation to the fermentation time.  Usually, a five to eight day (and up to 16 days) fermentation time is needed for any level of disease control.  Much less is known about the effect of fermentation time on the efficacy of ACT. One study suggests as short as 18-24 hours.  Fermentation nutrients are often sold with compost tea brewers.  These nutrients typically contain molasses, soluble kelp, humic materials and lesser amounts of organic materials and minerals.  It is not presently known what affect these added nutrients have on disease suppression.

There are a number of developments unrelated to plant disease control that will potentially impact how compost tea is made and used. Two of the more important issues are the lack of compost tea standards and the potential for human pathogen growth during fermentation.  An increasing number of businesses are selling compost tea to gardeners and growers.  It is difficult for buyers to be assured of the product contents or functions because there are no standards for determining the suitability of compost tea for a particular use.  Several studies have indicated the transfer of human pathogens from naturally contaminated compost to food surfaces with NCT.  Pathogen growth appears linked to the use of fermentation nutrients; pathogen growth does not appear to be supported if ACT or NCT is made without nutrients.  The Compost Task Force of the National Organic Standards Board recommended that compost tea be made only from compost that had met criteria for pathogen destruction (131^o F. for 3 days) or contain less than 3 MPN salmonella per 4 g dry weight compost and less than 1000 MPN fecal coliforms.  In addition, the Task Force recommended that readily available carbon sources such as sugars and molasses not be used as fermentation nutrients for compost tea production.

[1] Scheuerell, S. and W. Mahaffee, “Compost Tea: Principles and Prospects For Plant Disease Control”, Compost Science & Utilization, Vol. 10, No. 4, 2002, p. 313-338