Question
Hi Melissa.Please tell is caustic soda helps in removing lichens and fungus from apple trees and when the apple be sprayed in winter or in summer

Answer
Dear Vivek, The main question would be whether the caustic soda is packaged and labeled for horticultural use.  While it is true that the soda, being base, will help kill and loosen the lichen, being acid, I would be very careful about the concentration.  If not sure, there are products available in most feed stores that would be effective.  Now would be a good time to spray.  I prefer to get plants cleaned up in early spring to avoid a lot of problems later.  Be careful not to damage the bark, and if in doubt, spray a test area to see how effective and safe it is.  Good luck, Melissa