Powdery white coating on my guava leaves and shoots in cool weather (amrood)

A white to greyish powdery coating on the young leaves, shoots and sometimes the small fruit, which you can rub off, is most likely powdery mildew. It tends to appear in cool, dry weather with morning humidity and on shaded crowded trees, and badly hit shoots can stop growing, while affected fruit can stay small and rough. Confirm with your KVK, since the dusty powder on top of the leaf that wipes off is the clue and other leaf problems look different. Manage by opening up the canopy: prune to let light and air through, since dense shady trees hold the disease, remove and destroy badly affected shoots, and avoid pushing too much soft growth with heavy nitrogen. Where control is needed, sulphur-based sprays and other KVK-recommended fungicides applied early when the first patches appear and repeated as advised, and rotated by mode of action, are used, but take care with sulphur in very hot conditions as it can scorch. Always follow the label, the pre-harvest interval, and residue and MRL limits. Get a diagnosis first so you do not confuse it with a different leaf disease or a pest, and treat at the early stage when it is easiest to stop.