David:

If you`re absolutely sure it`s a fungus and it`s not due to
overwatering, clayey soil, etc. - I have to say it sounds like a fungus
that I`m especially familiar with, especially due to the conditions you
describe. The `every now and then a plant dies over a period of a week
or two` statement kind of clinches it for me.

It sounds a lot like Pythium root rot, although it could also be
rhizoctonia, but Pythium is more likely since it`s usually confined to
one or two plants in soil-based systems such as gardens.

A solution? There are several. 1) You can make sure your plants
aren`t unduly stressed.

2) Don`t overwater, especially if you have a lot of clay in your soil
(in a soil environment, the pathogen spreads quite easily in
water-logged soils which create surface water conditions, especially in
poorly drained soils containing clay, which is why I pointed out clayey
soils... natch).

3) Look for the ingredients metalaxyl or agral or any kind of
surfactant (any chemical which breaks down the surface tension of water
- even a bit of soapy water (made with biodegradable soap, of course!))
in any pesticide from Canadian Tire or Home Hardware or whatever
gardening store is close by - this item sprayed onto the soil surface
around the affected area (NOT everywhere in your garden, and always on
the soil surface, as the ingredients could potentially stress your
plants, making them even more susceptible to infection... plus, I`m
sure you`d like to ensure you`re not munching down on a healthy dose of
pesticides!) every two days (or according to directions on the label)
could do the trick in preventing it from spreading to other plants.

Please feel free to go to my website at
http://www.GCILifeSciences.blogspot.com - my other discussion groups
are listed there, and every now and then, you might find information
relavant to your gardens!