That is def. nute burn.....had it myself. Best recommendation is to flush with water right now. DO NOT add nutes with a free hand. Also, a problem of mine. Nutes are a funny thing....they can be your friend or enemy. For gnats, I got a venus flytrap plant...worked like magic!! Less artificial interference with nutes on plants will make you a very happy man. I was not able to reverse my nute burn so had to chop her pre-maturely. Best of luck!

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'Nuff Said!