I splashed some ph down on one of my 3 plants accidentally a few weeks ago. It sort of stopped growing in the last week and looks like the pictures now. The pH is 5.6-5.8 in each bucket.
Ph down is an acid, so yes it can harm the plant if applied directly. If it was washed off immediately, it shouldn't cause any lasting problems. From your pictures, your plant looks to have nute burn. An acid burn would cause more drastic damage. Try flushing the plant out and once it dries out for a couple of days, resume your watering and fertilizing. Cut back on the amount of nutes, and you should see good results within a couple of weeks.
Its not nute burn, they have only been on veg strength lucas formula with 24/7 600w MH lighting. It was barely 600ppm nutes. And it would have affected the other 3 plants since they all share the same water. I ripped it out and trashed it today. moved my 3 plants closer together and switched to the HPS bulb. Put 12 gallons (3 buckets and 1 outside res bucket now) of flower strength GH nutes (lucas). pH 5.75. Bout to go to the renasaince (sp) festival, ill post a pic when I get back.
