Compost tea for disease control

The claim

Compost tea, especially actively aerated with a pump and molasses, suppresses plant disease and inoculates soil with beneficial microbes.

What the evidence actually shows

The distinction between non-aerated and aerated tea is the whole story, and it usually goes unmentioned.

The foundational review in Compost Science & Utilization distinguishes the two and finds a range of foliar and soil-borne pathogens suppressed by non-aerated compost tea, while noting that very few controlled studies existed for aerated tea.

Washington State University Extension then tabulated university trials from 2003 to 2005 — Cornell, Iowa State, Ohio State, Michigan State, Penn State, Rutgers and Minnesota — and found aerated compost tea ineffective in 16 of 18 crop and disease combinations tested.

So the version with some support is the simple steeped one, and the version that attracts equipment, additives and enthusiasm is the one that failed.

The part that is a genuine safety issue

Two points from the WSU review deserve prominence. Aerated compost tea is not registered as a pesticide, so disease-control claims for it are not permissible. And some batches harbour E. coli — brewing with molasses at ambient temperature is an effective way to multiply whatever was in the compost, including human pathogens. On food crops, that is not a theoretical concern.

What to do instead

Use the compost. Applying compost directly delivers the organic matter, nutrients and microbial diversity that tea is supposed to extract, without the pathogen risk or the equipment. Where a specific biological control is wanted, use a characterised registered product with a known organism and a known count.