Yew with brown discoloration of the needles.

[Javaman4373]

I have a yew over wintering in a cool basement under a grow light, on for about 12 hours per day. Some of the needles are getting brown discoloration on their upper surface and are still bright green on the undersides. My diagnosis is that the light is scalding them. I placed a white plastic bag over the tree to shade it some. Do you think my interpretation is correct?

 

[Wires_Guy_wires]

Yes, I think your interpretation is correct.

Growlight LED can put out more usable light than the sun sometimes, because they produce just a very specific wavelength.
In some plants this can lead to LED bleeching; xanthophylls and other cyano-pigments that usually act like a sunscreen by diverting and scattering light, aren't able to divert the excess energy away from the chlorophyll A and B, and those chlorophylls then oxidize and break down. This makes them turn brown, or even white in thinner leaved plants.

That sounds very technical but imagine you found the ideal wavelength of light that solar panels work on, and blast those solarpanels with so much light that their electrical circuits fry. Essentially, that's whats happening in the foliage.

Increasing the distance between the light source and the foliage by half a foot should stop it from spreading, if not, increase the distance again. However, what's broken is broken.