Home owners generally know what this looks like when it affects processed wood. However, sometimes it's difficult to determine if a live plant has rot. Look for mushrooms growing on the tree. That's an indicator of excessive moisture in that area. Watch for softening areas that may turn brown or black.
Some fungus can cause white spots.
For stem, root, and fruit rots, control by allowing for good air circulation and good drainage for plants. Add vermiculite or perlite to clay-like soils. Adding organic humus and compost will also aid in drainage by loosening hard and clay soils. Use sulfur sprays several times in the season. Destroy infected parts.