What would cause this?

So just on one stem of the plant and only on 1 half of each leaf...weird??

mosaic virus?

looks like a light bleach....what kind of light are you using, how far from plant? maybe a lil more info will help...good luck

That color variation is caused by a couple of things. The usual culprit is a sharp Ph change causing lockout. I have had this happen a couple times. If I get lazy and stop adjusting ph levels, I will sometimes get a Ph lockout for a few days or more and when I finally get the Ph corrected, the plant starts using the nutrients that were locked out and then you get that variation in color. Most times it is Magnesium getting locked out. If the Ph curve is more severe then it can lead to more serious lockout like nitrogen... Just check Ph and get it in the correct range.. Soil: 6.6 to 6.8 Hydro: 5.8 to 6.2 The picture you posted is most likely caused by lockout of some kind.

The other thing that may cause this is soil hotspots.. If you have mixed your own blend of soil, you must mix your ingredients thoroughly. If you have soil with splotches of blood meal, or lime or bone meal etc. you will have some roots growing directly into the hot spots and causing some parts of the plants to discolor or even die.

Too much light is one of the RAREST events in growing history... I have over 100 watts per square foot and 12,000 lumens per square foot. Unless you have a 1000 watt hps in a 2ft x 2ft. closet, light aint your problem.

Im not a pro by any means... I just know this has happened to me and it was ALWAYS a Ph issue..

good for you on all those lights, in my case it was a 1000 watter AND it was the light not ph so dont say you cant put too much light on a plant. that was MY problem so it could possibly be his as well. like i said...not enough info on his op to make a more accurate diagnosis. bottom line is light bleaching is a common problem with some growers.............