Its normal for some to fall off. But if its alot then probably you need some nitrogen .Usually its normal during the 4th week for your plants to need a boost of Nitrogen
If your in the usa .The amount of daylight hours now ,outside are not long enough for the veg stage.Im only getting 11 1/2 hours of day light in my state now. Thats fine for flowering .But veg needs at least 18 hours of light per 24 hours.
Usually it tends to cause you to get Bronchitus or Pneunomia..Try to clean off any bud rot you see then hit the spot with rubbing alcohol on a Q-Tip this kills any remaining mold spores and prevents it from spreading.
I just harvested some Parradise seeds - Opium fem. Outdoor grow. I am very impressed.Out of five strains I grew this season. Opium is the only one that did not get bud rot. The plant grows amazing the whole thing is covered in frosty buds. It loves to be topped. And the smell and taste is great...
I always use Serenade. It does work very well.I'm having problems with bud rot and it also helps that. I have no problems using it late in the grow. But myself I would not use it 1 week befor harvest. I like to give it time to wear off before I harvest just to be safe.