I had such infections before, I used a lot of natural (neem and chrysanthemums extract) and chemical pesticides as a spray none of them worked against those russet mites even tho I really used a lot of solution to fully cover every spot on the plants. What helped in the end to get rid of of them was a mixture of Kali soap 10ml per quart/liter, cinnamon oil 2ml per liter and spiritous 5ml per liter of water. I chopped the top of a 5 liter container filled it up with the above mentioned mixture and gave the plants a headover bath in it. I moved the plants vigorously in every direction in the solution until no more bubbles of air arized. When taking the plants out I had a towel on the substrate to avoid solution getting into the substrate. That killed the whole colonie, I did a spray with the same solution 4 days later to be sure. Problem solved.