One side of my apple fruit turning brown and sunburnt on the tree (seb)

A brown, tan or bleached patch on the sun-exposed side of the fruit, often becoming sunken and rough, is most likely sunscald or sunburn, caused by intense direct sunlight and heat on the fruit skin, not by a pest or disease. It is worse on fruit suddenly exposed after heavy pruning or leaf loss, on the south and west sides of the tree, and in heatwaves. Confirm with your KVK to rule out a disease patch, but the position on the sun-facing side and the timing with hot bright days are the clues. Prevention is the main tool. Keep enough leaf cover over the fruit by not over-pruning or stripping leaves near the fruit, train the canopy to shade developing fruit, and maintain steady irrigation since water-stressed trees scald more easily. In high-risk orchards, anti-hail and shade netting reduces both sunburn and hail, and some growers use approved protective sprays such as kaolin-based or other reflective particle films on the fruit, applied per label and KVK advice, to lower skin temperature. Avoid sudden heavy summer pruning that exposes fruit. Get it checked so it is not confused with a disease, then focus on shade and steady water.