White substance on stem around plant "wound". Is this powdery mildew?

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.