Hi. I’m a first time grower. Got a mixed autoflower pack and I’m trying to grow one of each Northern lights, Amnesia, and blueberry. The seeds all germinated in 2 days and are now 25 days old. I have them in 5 gallon buckets growing in a mix of Miracle Grow potting soil amended with coco coir and some home grown worm castings (another new hobby). I have all my plants outdoors in a greenhouse. I’ve noticed some fungus gnats and mealybugs on some plants so I decided to treat with neem oil and dr. Bronner’s sal suds. I mixed one and a half teaspoons each in a quart of water and applied to all of my plants including tomatoes, cucumbers etc. Sprayed in the evening after sun was down. The next day my northern lights plant experienced some significant leaf burn and this was my best plant of the three. The other plants are fine so I think what happened is I didn’t shake the neem oil/sal suds mixture enough and one plant must have gotten a heavier dose. My question is do I leave it alone or cut away damaged fan leaves?
Several things: miracle gro is terrible soil for cannabis. For future reference you would be much better served using a medium designed and buffered for cannabis.
Over watering; fungus gnats are a foolproof sign of this. Exacerbated by the MG soil.
Neem oil is a product I hate. Peroxide is a much better alternative and should be an integral part of your grow.
Pests are best controlled with products that are safe for food. I use Captain Jack’s Deadbug Brew or Safer Caterpillar Killer.
Add more drainage to buckets or transplant into fabric pots.
Finally; if you haven’t yet you should invest in a decent PH and TDS meter along with PH up and DOWN and use them.
Damaged leaves are going to remain that way; once damaged they will not recover. Look to new growth for improvement.
Now that’s an answer 
Thanks for the reply. Despite my mistake of using a partial miracle grow mix, my plants did appear to be thriving right up until my neem oil application. Unfortunately there is a lot of conflicting information on the internet. Many sites swear by neem oil as an organic safe product if used as directed. I think my Northern lights plant will recover as it still has many healthy leaves and stems. Just wanted to know if I should leave it alone or prune away burnt leaves.
diatomaceous earth Is a good thing to mix into the soil to treat for insects.
The only time I prune is if the leaf looks like it’s impacting the plant. I figure if the leaf is still green then it has usable nutrients. If it has turned completely brown then I have it off there as it has served its purpose
I typically use the 1.5tsp neem, and 1/2 tsp soap for a quart. Mix really well. You can even use less neem if you want.
BTi or mosquito bits are best for fungus gnats. Better yet is to try not to overwater. (It happens to me too).
Thanks for your reply. Lost in the whole discussion was the answer to my original question of what to do about the burnt leaves…trim away or leave alone…no pun intended. And if trimming away a leaf how much of the stem do I leave?
The burnt leaves will not recover.
You can leave them until they fully turn brown, then a gentle tug will pull off the leaf and stem.
You can cut them now. Right about where the leaf stem hits the main stalk.
It’s up to you.