The old time gardeners used both compost and manure tea as a liquid plant food and this is how I ve usually thought of the mixture. Compost teas are not only being used to feed plants, to restore or enhance the soil microflora, they are being sprayed onto the foliage to control foliar diseases. Let me summarize the research results here in gardening shorthand form. Apple scab was controlled with a spent mushroom compost extract in experiments by Cronin and Andrews in 1996. No, they have to be used in combination with other good organic gardening practices.
Compost tea is no more a silver bullet than is any other chemical or product available to gardeners.
It is however, one that is easily and readily available to every gardener and one that will not create health or environmental problems with its use. The research seems to indicate a wide range of things that get to work when you spray compost tea on your plant. Problem spores are inhibited from reproducing, compost actively kills some pathogens and out-competes others for the available food source. There is no one single way it works on all pathogen problems. In fact, if compost tea is analyzed, it contains yeasts and fungi that attack problems as well as control chemicals such as phenols and amino acids that work against plant diseases.