That's a very bad case of 'black sooty mold '. But that's not the problem! You'll need to investigate to find the insect that is creating the sweet sticky excrement, called honeydew, upon which the simple black mold feeds.

The insect might be on the rhody itself, or on a plant that grows overhead so that the honeydew drips from above.

The usual culprits are scale insects and aphids. Are you familiar with either of those? Are your plants situated under a tree of some kind?

By the way, the BSM will begin to flake away on its own, but only once the insects are discovered and controlled. In and of itself, the mold does not feed on plants, just the liquid excrement.

Thanks. We have some silver birch and poplar trees around us so it's possibly from them. I'll investigate further for aphids

Why r my leaves charred and its about to flower?

They appear to have a bad case of cercospora leaf spot, typically caused by getting the leaves wet. You should not get the leaves wet. Instead try watering the soil. If you use a sprinkler, consider using drip irrigation or water the plants close to sunrise (6-8am); that way, they will get wet of course but then the sun will rise and dry them. Leaves with leaf spots should be thrown in the trash when they fall.

There are also some areas where the leaf edges are browning. How did that start and progress? Problems with winter injury, lack of water and high salt content (from some high nitrogen liquid fertilizers) could also cause that. You should water when a finger inserted into the ground to a depth of 4" feels dry or almost dry. Make sure you fertilizers sparingly as rhododendrons are not heavy feeders like roses. A good choice is Holly-Tone for azaleas, camellias, etc at half strength only once in Spring (unless your soil is sandy or has mineral deficiencies or toxicities). But for right now, wait until the problem is corrected as the shrub is stressed and you do not want feed a stressed plant. Once the plant has become established in your garden (1-3 years), you can discontinue fertilizers and let them feed off the decomposing mulch. Obviously, maintain 2-4" of organic mulch year around.