Bud Rot only occurs during the flowering stage and essentially it's a mold that is growing inside your buds. Eventually it will completely consume the buds making them turn a dark gray and sometimes even white with spores. Early signs of Bud Rot would be sugar leaves curling in and wilting near the buds. If you notice this happening, be sure to peek around and see if you can find any moldy or browning parts of the bud. The affected bud will also feel mushy not firm like healthy bud.
How to Cure:
Bud Rot is generally caused by too much moisture as well as a lack of airflow in the grow space. This is why it is best to always lower the humidity to around 55% during flower, and to never spray your plants during flower either. Having plenty of fans and defoliating dense plants will also help prevent bud rot. Defoliating the bushy plants helps create airflow through the denser parts making sure mold does not grow in there either.