Not enough info, Anne - don't know which fir trees you have, a photo would help, could it be drought? As for black spots on plants, depends which plants - roses are prone to black spot, other plants get bacterial or fungal leaf spots.

I used to plant garlic for black spot, seemed to work but then these things tend to be a bit random. Fir trees breathe through their bark so make sure the foliage isnt too dense if its a hedge try taking out the odd branch.

thank you Ill try your suggestions