Leaves talk: reading yellowing, dry tips and spots
A leaf changing colour is a diagnosis, if you can read it. Overall yellow, brown tips, soft or dry spots: what each signal says and how to respond before it spreads.
A leaf that changes colour or shape is a diagnosis, if you can read it. Plants have no other way of telling you what is wrong, and the signals are surprisingly specific: the kind of yellowing, where it appears, whether the leaf is limp or dry, tell different stories. Learning to read them is the difference between acting in time and noticing too late.
Before you panic: an old leaf, the lowest one, yellowing slowly and dropping is normal. The plant is pulling resources out of the older leaves to push them into the new ones. If it happens to one or two low leaves now and then, while healthy leaves open at the top, that is life taking its course. Worry when the yellowing is widespread, fast, or reaches the young leaves.
Widespread yellow across several leaves, often the lower ones, with the plant a little limp: suspect number one is too much water. Roots suffocating in a soaked substrate can no longer carry nutrients up, and the leaf yellows even with the pot full of water. Check the substrate with a finger and, if it has been sodden for days, slide the plant out and look at the roots: soft and dark means rot under way. It is the commonest problem and the most misread, because it looks like thirst.
Brown, dry tips and edges with the rest of the leaf green: air too dry, or salt build-up. If your humidity is low (winter, radiators), it is almost always the humidity. If you have fed generously, it is salts burning the fine roots. The two causes often live together. Raise the humidity and, if in doubt about salts, flush the substrate with plenty of water.
Dry brown spots with a yellow halo
usually sunburn from direct sun, if the plant is exposed, or a fungal problem if the environment is damp and the air still. Sunburn is dry and confined to where the sun lands; fungus tends to spread, and often starts at the edges or from a wet leaf.
Soft dark spots that spread and "rot"
this is rot, bacterial or fungal, encouraged by water on the leaves and stagnant air. Here you act fast: take the affected leaf off with a clean blade, reduce moisture on the foliage, increase air exchange. On Begonias and other hairy species it is the reason you never wet the leaves.
Small pale new leaves, with the stem stretching
this is not a disease, it is not enough light. The plant is reaching for light and making undersized leaves. It is not a water problem nor a feeding one — it is the position. Move it closer to the light.
The method, whatever the symptom
do not change three things at once. If you water less, feed more and move the plant all on the same day, and it improves, you will never know what saved it — nor what to do again next time. Change one variable, the likeliest one given what the leaf has told you, and watch for a week. Plants respond slowly: patience, here, is a form of competence.