|What causes mold on bottom of onions causing rot before
maturity? Onions have not been over watered.
|You're probably dealing with Basal Rot, a fungal disease. Fusarium is a soilborne fungus that is often associated with maggots or other insect injury. The fungus causes rot where the roots attach to the basal plate of both yellow and white onions.
Try crop rotation, protect plants from insect or other injury, or grow cultivars that are resistant to the disease. "Golden Cascade", Valient", "North Star", or "Sentinel" are reported to be resistant to the fungus.