Dead spots on leaves- what’s wrong?

[GoldenGrass]

Hey folks! I’m growing indoors under a viparspectrum 300 watt LED light about 3 feet above the plant in coco coir, amended with Kind Hot Soil compost. The plant is about a month old and a foot tall, in a 4x3x3 grow tent. Temps have been pretty stable between 70-75 F and 50% humidity. I’ve grown this way several times before now, no added nutrients, and had fantastic results. However, with this newest plant, I’m having weird symptoms I’ve never seen before, and I’d appreciate any help you have to give. My plant has started having spots that look like bits where the leaf just up and died- usually on the lower to middle leaves, and they see to be clustered near the central vein of the leaf. I’ve inspected them closely under magnification and I see no sign of fungus or mold. In some places the underside of the leaf appears normal, in others, it goes all the way through. It seems to be getting worse every day. No signs of insects either- plus as an indoor plant I’d be very surprised if insects got in. Google has not been helpful, I can’t seem to find anything that looks quite like this. Aside from this issue this plant has been growing strongly and vigorously.

Help! It’s an autoflower that’s going to start flowering any day now so I’d like to correct this before it’s too late. What’s wrong with my plant?

 

[GoldenGrass]

Hey again- now it seems the tips of some of my leaves, the ones up top, are turning very yellow. Looks like light burn but my plant is still like a good two feet away from the light. I still have no idea what’s going on and the spots are getting worse. :(

 

[mancorn]

Take your pictures in natural light, otherwise impossible to see what's happening.

 

[GoldenGrass]

Take your pictures in natural light, otherwise impossible to see what's happening.

Haha oops, that’s fair. Here are some natural light pictures of both the yellow tips on the upper leaves and the spots on the lower ones.