help! sudden tree death.

I have what I would consider to be a bit of a problem. My trees are capitulating, one by one, I have already lost 4 and it seems one more is on it's way. I have them in a fairly well draining mixture of about 60% turfface and the rest potting soil. It has been hot here in Philadelphia lately but the problem started before the heat wave when it was still in the 70's and 80's, I believe we are a zone 7. The problem isn't due to a lack of water as none of the plants have dried out. I have sprayed the plants liberally with a fungicide and this seems to slow the problem but hasn't stopped it entirely. The leaves simply all wilt starting from the top and then by the end of the week a once heathy looking plant is dead. What can I do to stop this?

Have you any pics? Have you pulled any of the dead ones out of their pots to inspect the root? Sounds like a root problem to me but would like to see some pics first.

A mixture of Turface & potting soil is unsuitable for most bonsai in the Philadelphia area. What kind of trees do you have left? If they are hardy in Zone 7, get them out of the pots & into the ground immediately, without disturbing the roots. Otherwise, be very careful watering. Let them dry out between waterings. Are they in the sun?
Join the nearest bonsai club (I think it's Brandywine).
Iris

Thanks for the replies. Yes they are in the sun, there is a Hickory above them, but it is young and only really affords 1 or 2 hours of shade on the early side of midday. I don't water too much, only when the top inch or so of soil drys out. I work though the day so sometime I have to water in the morning if I know the tree will not make it 8 hours without water. And oddly enough the trees in the smallest pots have gone first. One of the small potted trees is still left but all of my other small ones are gone. I have a hornbeam, apple, cherry, forsythia, and what is left of an azalea left. I will try and get pictures this weekend.

ps- I am unable to join the Bonsai club in my area, the only meetings are too early and too far out to make. A few of my friends in the city are trying to form something but it is loose and unorganized at the moment and nobody has any real experince with bonsai. I would like to join a club, but it is just not an option.