Best way to determin nute burn. Answer these questions to barrow it down. Growing medium ? Type of feed ? Ph pen ? Ppm stick ? Temps in tent ? Time in cycle ?
Yes. I think nute burn. You also have a little too much nitro as evidenced by the dark green color and waxy appearance. You've probably been running a little too strong and have some buildup at this point. Run a couple of gallons of PH'd water through the medium and let it dry out.
A flush with a product like GH florakleen would get you back on track quick. It's $9 for a quart at my local hydro store. It's much better and faster then water at freeing up lockout and washing out the root zone.
If your growing coco ph pen an ec stick is essential. Get a blue lab ec stick for 65 quid an a cheap ph pen for 3 quid on ebay an replace when required. Thats all i do.