why do my ripening white nectarines rot at least half the fruit?
the problem of peaches that rot when they are still on the tree is quite common and widespread, it is called brown rot and in essence it is the exact same disease that affects fruit even after harvesting, when it becomes moldy in the refrigerator drawer after the we bought perfect at the supermarket. This disease is caused by a fungus, called monilia, which in addition to attacking the fruits can also attack the flowers, which dry up and fall off, or even the branches, which are attacked by rameal cancers, which lead them to death. This fungus is quite difficult to detect, because it hibernates on the plants, often holed up between the cracks of the bark, or on the leaves of the previous year fallen to the ground. The defense, once the fruits are ripening, is practically impossible, and it is generally good to intervene cautiously before ripening, on the small fruits. So, now that your peaches have been attacked by the monilia, you can't do anything about it except to prevent the fungus from remaining in your garden; then try to remove all the affected fruits, and destroy them, or at least take them away from your peach tree; when autumn arrives, also collect all the fallen leaves and, if necessary, at the end of winter, practice a pruning that goes to ventilate all the parts of the foliage well.
Next year, try to act preventively, treating the fruit with special fungicide products against the monilia. In organic agriculture sulfur-based products are generally used.