Compost tea (or manure tea) greatly boosts growth and suppresses plant disease.
Compost tea isn't the tonic or disease cure it's sold as. The effects in the field are patchy, and aerated teas have no documented disease protection. Additives like molasses can grow E. coli and Salmonella, so keep it off food crops unless you're sure of the method. Spreading the compost itself as mulch is the better bet.
What is actually known
Compost tea, made by steeping mature compost in water, is a source of nutrients, but its disease claims are weak. Batches vary widely; results that look good in the laboratory are more variable in the field; aerated teas have no scientifically documented effect as pathogen suppressors, while non-aerated teas show a trend to reduce how often foliar diseases appear, though not how severe they are. There is a real food-safety downside too. Sugars and other additives commonly put into aerated teas can feed E. coli and Salmonella, so unless the brewing method keeps human pathogens down, extension advice is to save the tea for plants you will not eat. Using the compost itself as mulch is simpler, and composted mulch has documented disease suppression of its own.
The evidence
Evidence: checked against 3 sources in the research-based literature.
The 3 sources
The old-fashioned compost tea is made by adding mature compost to unheated water and allowing the mixture to steep or brew. This serves as a source of nutrients.
Compost teas are highly variable in their microbial and nutrient content from batch to batch.
Aerated compost teas have no scientifically documented effect as pathogen suppressors
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Milpa Gardens. “Compost tea (or manure tea) greatly boosts growth and suppresses plant disease.” Evidence grade: Contested. https://milpa.garden/beliefs/compost-tea-manure-tea/
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