I'm no disease expert but my guess would be environmental conditions may have stressed the plant. If it's been very dry, first plenty of water then I'd try either sequestered iron or a good drench with seaweed extract
Billericay - Essex
Knowledge is knowing that a tomato is a fruit. Wisdom is not putting it in a fruit salad.
Shade and water from the bottom. Stand in a shallow dish and keep the saucer topped up until the plant has stopped taking it up. This could take 3-4 days. Rhodedendrons like moist growing conditions in shade or semi shade so the heat wave has probably caused the discolouration. It may take a few weeks to recover.
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If it's been very dry, first plenty of water then I'd try either sequestered iron or a good drench with seaweed extract
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.