Its not that simple really!

Until we know how you have grown it e.g. in a pot or in the ground, and what type of soil it is in, it is difficult to say if it is a physiological problem or a disease.

Get back to us with some more detail then we might be able to take it from there!

A picture of both sides of an affected leaf would also be handy!

If lack of acid was the problem most of the leaves would be looking pale, rather than just a few, but you can buy a pH meter quite cheaply to check your soil if you are unsure - it needs to be 6 or less.

Being evergreen Rhododendrons tend to loose some leaves throughout the year rather than in Autumn/Fall like deciduous plants. I find in my garden that leaves (which are going to drop) will turn yellow then drop off.
As Steragram says, if the soil was not acidic enough then there would be signs on the 'whole' plant.