Magnesium is an essential element that helps roses maintain beautiful green foliage while preventing discolouration and premature leaf fall.
The most significant sign of a Magnesium shortage in the soil will be the brown blotches that appear on the leaves, while a yellowing on the inside of the leaves between the veins will also be evident. The oldest leaves will be worst affected.
Apply Epsom Salts or Magnesium Sulphate.
Alternatively you can buy a pack of Trace Elements from your local garden supply shop. It is chrystals that you can mix into water and pour onto the ground around the affected plant. This is to be used once only in most gardens but may need repeating in well used soils.
Any treatment will take a couple of weeks for you to see signs of improvement in the new growth.
Important: signs and symptoms will vary significantly between varieties, even within similar categories. The information provided here is a basic summery of the most common affects and will not always be applicable to all rose varieties.