VT Grand Daddy Purple (First Grow Ever)

This is caused by overwatering. Let the top inch of the soil dry out completely in between waterings. Otherwise the roots do not have acces to oxygen and it will cause other problems like mold and rotting too.

Since you're growing such a huge plant make sure you're giving it enough nutrients. Yellowing leaves is usually a sign of Nitrogen deficiency but in this case it might also be the fact that your plant is so massive that its dropping some unnecessary leaves that are producing less than they are using Good luck eith the rest of your grow. That's an epic one you're growing

if not a stain, could be. wpm on stem isn't where it usually starts, but an open wound can make it possible for initial infection for sure. if it grows / gets worse you know it's alive and definitely mold. ther's no ridding the plant of it, if it has it. it will spread through air. the mycellium network underneath layers of plant will spread inside. if it grows, kill it. if it looks like powder fall soff when you gently jostle the area, it's wpm. just make sure it isn't discoloration. milk, baking soda, neem will not kill an infection. They can help prevent one andmaybe slow progress down. Even real fungicides used on commercial farms that you cannot buy over the counter in most places in america is 30% effective? lol, it's a joke calling them "fungicides" because they don't do shit. UVB i beleive has been effective fro vineyards - you can google for an article about it. they build a contraption that overlaps the trellis for the vines and slowly push it along shining UVB at it 15minutes after sundown (an important tidbit if you use UVB). That's been very effective. anyway... at minimum isolate it from all other plants. treat all other plants with some neem or horticulrual oil -- this adds some resistance to a colony digging in. Throw it out at first verification it is wpm. Based on how lorge teh colony is, it was already producing spores and spreading. They do this within 3-5 days of original infection. https://extension.psu.edu/powdery-mildew trust info like that and not some anecdotal nonsense in forums. it's beyond the resolution of human senses to accurately resolve.

cut it out with a wide margin around it. better to cut extra off than to leave some behind. if oyu see dead sugar leaves along that stem, good chance it is creeping along the stem, unseen. inspect, gently - without damaging, spread a bud out so you can see the stem underneath. any grey or white fuzz is bad, obviously. the dead sugar leaves are a sign to check. that will happen before the buds are discolored. that's easier to find, obviously. when to harvest? depends on how fast it keeps spreading and how much time you have left etc etc.. different choice for every person. if you don't see it elsewhere, and it's one plant of many, i'd cover (prevent more spores spreading from agitation) the plant and remove it. even that won't guarantee anything... it spreads before you see visible signs. "washing" buds does not disinfect them - whether rot or mildew.