On a page which Jim L steered us to, I found the following brief note:

Moo the Dew!
Experiments conducted in Brazil have shown that a solution of fresh cow's
milk in water can be as effective as conventional fungicides for controlling
powdery mildew on greenhouse grown plants. In most cases, solutions of ten
percent or more milk applied twice weekly performed at least as well as
benomyl or fenarimol fungicides applied once a week. This should please
growers looking for alternatives to synthetics - you'd be hard pressed to
find a more "organic" fungicide than milk! (Note: Sprays containing thirty
percent or more milk caused an innocuous mold to grow on the leaves.)

As for why fresh cow's milk has fungicidal properties, researcher Wagner
Bettiol points out that milk contains phosphates and potassium salts, both
of which are known to help control powdery mildew. He also cites milk's
broad anti-microbial properties as a factor, saying that various
constituents of milk has been shown to inhibit the growth of particular
fungi, and that others may even induce systemic disease resistance in
plants.

Reprinted from Bonsai News, Lake Charles Bonsai Society, September, 2000.

Any comments from our plant pathologist, or is this another "old cow's
tale"?

Regards,

Bart