Chlorosis (nutrient/pH-related)
How to recognise it
Yellowing of leaves, often most pronounced between the veins which stay green; most common on acid-loving plants grown in alkaline soil.
Life cycle and conditions
Not a pest or pathogen — a physiological response to unavailable nutrients (usually iron or manganese) at the roots, typically caused by soil pH being wrong for that plant.
Prevention
Check a plant's soil pH preference before planting; grow acid-loving genera in ericaceous, peat-free compost in a pot on alkaline soil rather than in open ground.
How to treat it
Move the plant to more suitable soil, or into a pot with peat-free ericaceous compost — this addresses the actual cause rather than masking the symptom.
A chelated iron tonic greens up leaves quickly, but it's a temporary patch, not a cure — the yellowing returns unless the underlying soil pH mismatch is fixed.