Can anyone offer advice as to why my Comfrey plant has yellow spots all over some leaves, mainly the older ones. The new ones seem fine.

It is located in the SE corner of our yard and receives a fair bit of sun which was burning it, so I have put a large potted Spruce on its Western side to filter some of the harsh sun, which helped.

The soil is a nice black free draining mix with a bit of Mushroom Compost thrown in for good measure.

I was going to dose it with some Phostrogen(Acid loving plant food), as that mention's on the box that it can help with yellowing of leaves, but I will wait until I hear back from GardenWeb....