Courtesy of James K., Virtually Green, San Francisco, CA
I’d be cautious about using a compost tea on your crops, and doubly so spraying it on any edible foliage, unless you were sure it was produced using an ingredients formulation and preparation process that prevented unsafe levels of E. coli.
Compost tea, as you can well imagine, can have very high levels of bacteria and other microorganisms.
Depending on the ingredients formulation and the preparation process used by you or by a commercial producer of compost teas, the likelihood of E. coli may range from nil to substantial. And, as you know, an outbreak of E. coli (Salmonella, etc.) amongst your farm produce customers can ruin your whole day not to mention your whole farm business.
Compost tea formulations and preparation can safely handle the E. coli risks but you must be do it properly.
Depending on the compost tea formulation and preparation process, a particular compost tea may help one type of plant resist pathogens and grow better but have a negligible or even negative impact on other types of plants.
Here’s a link to a Wikipedia article that goes into detail about formulation, preparation processes, benefits, and the E. coli issue, with good list of references.
Here’s a good discussion of compost tea by CalRecycle, a program of the State of California. It’s worth noting that they cite the Soil Foodweb folks as a good resource for information on compost tea.
Soil Foodweb: http://www.soilfoodweb.com/
And lastly, here’s a link to Lynn Chalker-Scott’s research site, where she writes regularly (with tons of references) about various horticultural “myths”, debunking some and endorsing others. Scroll down the page a bit and you’ll see an article she did about Compost Tea (and other useful topics as well).