If the leaves are a solid green the addition of Epsom salts would not be useful. Lack of iron causes much the same symptoms as lack of magnesium, but with the younger leaves showing yellowing between the veins. Iron deficiency is frequently caused by too high a soil pH, often the result of mortar or mortar building debris in the soil near the roots. A soil test should be performed to see whether high pH is a problem and if it is the soil should be acidified. For a quick but temporary solution, ferrous sulfate can be added to the soil or chelated iron can be sprayed on the foliage, but the pH should be corrected for long term good growth.
Calcium is also essential to good rhododendron growth.
Calcium can be obtained either from gypsum or from agricultural lime. Gypsum will not raise soil pH, while lime will, therefore, lime is not generally recommended in areas with naturally alkaline soil or water.