My turnips are ready for harvest and rotten in the middle. The outside is perfect and no evidence of pest infestation. My soil could be on the dry side
Both can be affected by internal rot, due to malfunction of the cells, in trun due to either calcium and/or boron deficiency, exacerbated on light dry soil.
Do a soil test to check pH. If it is 6.5 or above, it is okay for vegetables, Below pH 6.5 it needs lime. If that does not help, apply a borinated ferilizer from the local farm coop, or apply a complete liquid feed to the growing plants when small, as these contain trace elements including boron, or seaweed products. Also watering during dry spells will help on light soils and the incorporation of farmyard manure well rotted.