There are a number of fungi who can cause you problems. If you followed the planting instructions in my last post, these shouldn't give you problems because they thrive in poorly drained soil. If you have constant standing water and the organisms get started, water splashed on the stems can cause die-back. Most fungi are most active in warm summer conditions. If fungi are a problem in your area, it is probably best to remove any fallen dead leaves and add a mulch of another kind. Bark mulch seems to have a limiting effect on these organisms. The biggest problem with the fungal diseases is that often by the time you see the symptons it's too late to do anything, especially if they have caused root rot. If you find stem die-back, you can trim out any affected branches and treat with a fungicide. If you have very hot and muggy summers, be sure to leave good air circulation around your rhododendrons when you plant them. I think half of the pruning we do here is just keeping things from growing too much into each other and cutting down on the circulating air.
Some other fungal type things you might encounter in hot humid weather are powdery mildew, which looks just like what you find on zinnias or phlox in August here. Leaf spotting from botrytis or rust is also a possibility. Often these will be a problem on injured leaves. I'll repeat again, so as not to scare anyone off from growing these gorgeous plants, I've never seen any of these things in our 10 acres of gardens where we are growing at least 50 different cultivars, and large numbers of some of those like mucronulatum and poukenense.
Sunburn can be a problem if the rhododendron is in too much sun. You'll see yellow leaves. These are pretty much shade loving plants, and if you see sunburn, you'll know that you need to move your rhoddy to a shadier location. If just the edges of the leaves, especially if just on one side of the plant are affected, it is probably wind burn and you need to give the plant a little bit more protections. If the burned look is on the edges of the leaves on the whole plant, it might be an overdose of fertilizer. Light green leaves usually indicate the need for more nitrogen, as in most other plants.
Tomorrow we'll go on to insect pests. Just one more pretty picture for tonight.