Yellow Rhoddy leaves
Magnesium in the form of Epsom salts is sometimes recommended for rhododendrons. Magnesium is an essential element and lack of it will cause yellowish areas between green leaf veins on older leaves. 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 green-colored 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 soil pH should be corrected for long term good growth.https://pnwhandbooks.org/plantdisease/host-disease/rhododendron-lime-induced-chlorosis is a link to this leaf pattern due to pH too high..