Can I just use regular fertilizer to fix the yellowing?

No, regular fertilizer might make it worse if the pH is wrong. You must fix the acidity first so the plant can actually use the nutrients.

Will these leaves turn green again?

Usually not. Once a leaf has turned yellow from chlorosis, it will likely stay that way, but new growth should come in healthy once the pH is corrected.

Why are my rhododendron leaves turning yellow?

Check these first

Check the soil pH with a home testing kit.
Inspect the undersides of the leaves for tiny insects.
Look for green veins against yellow leaf tissue.

Likely causes

Chlorosis
Most likely
How to tell: The leaf veins stay green while the rest of the leaf turns a pale yellow color. This happens when the soil pH is too high for the plant to absorb iron.
What to do: Test your soil pH and aim for a range of 4.5 to 5.5. Apply an iron chelate supplement or sulfur to lower the pH levels around the root zone.
Lace bugs
Possible
How to tell: You will see tiny yellow or white stippled spots on the top of the leaves. Check the undersides for small black spots of excrement and the insects themselves.
What to do: Spray the undersides of the leaves with insecticidal soap or neem oil. Repeat the application every 7 to 10 days until the bugs are gone.

When to worry

If the yellowing spreads rapidly across the whole shrub, it is time to act. A few yellow leaves deep inside the plant are normal as they age and drop off.

Stopping it coming back

Keep your soil acidic by adding pine needles or peat moss to the mulch around the base of the plant.

Not sure which one it is? Photograph your rhododendron.

This page lists the usual causes. Pocket Botanist looks at your actual leaf and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.

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Questions people ask

Can I just use regular fertilizer to fix the yellowing?: No, regular fertilizer might make it worse if the pH is wrong. You must fix the acidity first so the plant can actually use the nutrients.
Will these leaves turn green again?: Usually not. Once a leaf has turned yellow from chlorosis, it will likely stay that way, but new growth should come in healthy once the pH is corrected.