rose leaf discolouration
 

Hi
The leaves on my "City of London" rose bushes are turning yellow and
dropping. there are no black spots on the leaves and I have sprayed
regularly with systhane. They are well watered. The "deep secret"
bushes in the same bed are unaffected.
I have posted a pic of the leaves if anyone can help. I am wondering if
it is an iron or magnesium deficiency?
Thanks

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