Mold

40-50% Rh day or night? Respiration at night can release a lot of moisture from the underside of each leaf, if there is no airflow to take away the released moisture from under the leaf, then it shuts down that leaf until the wind takes it away, possibly bacteria or mold would have a chance to multiply if moisture was left all night unattended repeatedly, as the plant grows it may get worse as it moves more water, it grows, the canopy thickens, morning sunlight unable to evaporate deep into a canopy for leftover dew from failed respiration. Once enough "time" has passed mold and bacteria will form, this process will x10 at 77F and above. Why it's important to always have a gentle breeze at night. Even what can seem like good RH at 50% can easily be 70+ all night given size of a plant, poor ventilation along with poor water practices. Need diary for more information.

good advice below. always take pics so others can understand what your talking about and trouble shoot any miscomunciation.

Happens sometimes in late flower. Especially outdoor when it rains a lot. You could feed them Silica to strengthen the cells membranes. That makes it more difficult for mold to infect the plants. I put Wollastonit a cheap stone powder into soil at the beginning of the grow. You can also geht liquid silica but stuff is more expensive

Forgot to say that it most likely happens AFTEr the canopy is fully or nearly fully grown -- that's when the plants are pumping out a lot of moisture and your lights (photoperiod grows) are amped up to 100% for 12 hour operation, causing higher light-cycle temperatures... absolute humidity is much higher in warmer temps given same RH% and when temps drop, relative humidity spikes very quickly. Not happening with a bunch of tiny plants in early vege. My wpm showed up 2-3 weeks into flower like clockwork. The size of the canopy was the determining factor and why it was so consistent as to when it showed up. I had a beefy dehumidifier too. When i downsized my crop, all my problems went away. You may just need a stronger dehum. I was trying to do a 10'x10' grow inside and that would have needed 2000 watts of dehumidifier power, lol.

pictures would help to verify mold. If this is an indoor grow, usually it is caused by condensation occurring or constantly spraying plant with foliar sprays... Avoid foliar sprays without a specific reason. Foliar feeding should not be necessary if properly feeding through roots. Pre-emptive sprays, like neem oil, is overrated and mostly useless. When temperatures drop after lights turn off, RH% can skyrocket - that whole "relative" thing is at work. So, make sure you are not hitting the dew point after lights go out -- a wireless temp/rh probe is well worth the 10 USD. They'll be +/- 3-5% but that's good enough accuracy. Even if your RH is steady when you look at it, when the lights turn off temperatures can drop significantly. The warmer your light-cycle is with same RH%, the more likely to hit dewpoint - again "relative" part of the term matters. Download a dewpoint table for reference. Get a probe, if you grow photoperiod plants, in there so you can see what happens after lights out -- if growing autoflowers, you can just peak inside and manually take note of the temp/RH% and no probe is needed. The first 30-60mins after lights out will be the key. FYI, to save you some wasted effort -- none of the anecdotal remedies work on mold/fungus. It's just a bunch of people lying to themselves. Once infected, there is not 'cleaning' the plant. There is no effective remedy. Even the stuff commercial farmers have access to but the common consumer does not is only 25-33% effective, lol. That's the shit too dangerous to trust average people with. So, milk won't work, sodium or potassium bicarbonate won't work, the "cornell" formula won't work... None of it works. All you'll do is spread it to the stems and whatever that foliar spray drips down on, lol. It'll make it worse even if it looks better for 24-48 hours. Your eyes are not sufficient to resolve whether it is gone or not.. the fact it regrows in a fwe days shows how useless your eyes are in this context. Don't smoke infected buds. You'd just be lying to yourself if the surface looks clean. The mycelium network under the top layers is still there. You'll still be smoking mold even if the surface looks clean. Amputate infected bits. Even that won't fix it... The spores are already spreading by the time you can see a colony, so you likely have spores all over any plant in the same tent/area. You won't die smoking it, but probably cough a lot more and it won't taste so great, lol. I had wpm issues for several cycle in a row. Once i figured out that i was hitting dewpoint after lights out, the wpm stopped. I was so pissed off at that point i didn't even clean my tent between grow cycles. That shows that mold is almost 100% caused by a bad environement. There were defintely spores in that tent but that 2nd cycle didn't grow one mold spot because the envrionment didn't allow it to happen. No matter how confident you are, if you don't have temp/rh readings after lights out, it's a very good chance that is when the mold has an opportunity to grow.. when RH spikes and you don't realize it.

It could be many reasons. From you touching buds to bringing infection from outdoors to your grow area . Its natural in nature , life cycle. Humidity is essential , but goes in hand with good air circulation and many prevention ....

Hi bud. A few pictures would help. A diary would be more helpful. Causes of mold can be many things. Improper ventilation, bugs/pests, genetics, humidity, manually transmitted from another plant aka didn't wash your hands or tools...

Previously cut branches between buds and ventilation can be some of the others triggers.