Nothing, really. The bottom yellowing leaves are those which are about to be shed. The other spots are caused by a number of fungal infections. Unless they grow a great deal worse, there is no need to worry about it or treat.

What is a problem is the use of landscape fabric as shown in the picture. Rhododendrons need a coarse organic mulch, not a sheet of fabric with only a small hole aound the plant. In general no plants should be surrounded by fabric like this. It's bad for the plants and even though it seems to be a labor saving idea it winds up being just the opposite.

Thank you for responding. I am new to the garden and I thought laying the weed fabric before mulch was a good thing. Thank you for bringing this to my attention. Do you know if there is a way to slow weed growth before I lay mulch? I've also heard of pouring coffee or laying coffee grounds to make the soil more acidic.

Lots of gardening books show using landscape fabric just as you have done. They never mention the inevitable problems of poor water absorbtion, impeded gas exchange or the fact that weeds will penetrate from below and root into the fabric itself from above. Clean out any perennial weeds before putting down the mulch. Pull out any that appear after. Gradually it will get easier. Coffee grounds are good for improving soil structure and adding some nutrients. They are not at all likely to make the soil more acid. Pouring coffee itself on soil will not do this either. If a simple soil test shows a need for a lower pH, garden sulfer is the best additive, if rather slow acting.

Thank you again. I noticed yesterday we had heavy rain and it was puddling around this rhododendron. I recently read that they need well drained soil but I haven't found a way to help an established bed provide better drainage. Also on another post someone suggested my rhodo has root rot. From pictures I've seen this doesn't look like the case to me. Can you tell from the photo if I planted too deep?

The chief symptom of root rot diseases is a wilted appearance of one or many branches. There is no evidence of this in the photo. The best rule of thumb for planting rhododendrons is to keep the top 10% or so of the rootball above the soil line. The photo does suggest that yours may be planted a little too deep. Good drainage is important for two reasons. Waterlogged soil can encourage the fungal diseases that cause root rot. Equally important, rhododendrons need a lot of oxygen around their roots. Air cannot penetrate fine, compacted soil. The best planting mix contains 25 to 50% coarse organic matter such as conifer bark. In planting rhododendrons, it's best to think more of planting on the existing soil than in a hole dug into it. A wide, shallow basin is fine. The material surrounding the rootball should be highly organic. Kept mulched, you will have solved the drainage issue, acidity issues and fertilization all in one. There's no reason why you can't dig up the rhododendron and redo the planting. Just keep it watered after replanting.