Any idea what would cause this? It's only on one plant, and all of the plants are watered and fed with the same water and food. Basically, I have no idea WTF is going on. The only thing different from this plant and the others, is that this plant is from bagseed, and the other 3 are premium genetics.
Go here http://www.icmag.com/ic/showthread.php?t=11688&page=1&pp=15 and read the section on Calcium deficiency.
Not a calcium problem. Based on the coloring in the spots and leaf stems, my best guess would be a phosphorus deficiency. But the bright pink showing on the base of the stem is a little suspicious, if its even real. Maybe its just a bad picture?
I adjusted the picture in photoshop so you could actually see it. the stems have been red on this plant for a long time, they aren't pink
If you havn't figured out your rust spot problem yet, I'd say it looks like salt buildup in your medium. Flush with a mild nute solution with 3x the amount of water as your pots hold medium. The rust spots won't ever go away but you shouldn't see any new ones.
i got the same prob with my two plants... i went from flushing the nutes out to adding untill they burned, 7.2-5.9 on the ph untill i decided to leave it at 6.6, killing little white bugs i found with termador after ciggerette/havenero tea didnt work, injecting superthrive into the main stem, foliar feeding, finding a light green mold beginning to grow in the bottom of the soil (clear containers and roots= fine), so i soaked the bottom 1/4th of the pot in a 50/50 solution of hydrogen peroxide... and still... the spots persist... calcium defficiences dont look the same because in this case the spots group around the viens of the leaves... contrast to the scattered spots of a calcium deff... the rust is also at the leaf tips and on some of the edges of the blades... and some turn a super dark green staying very light around the viens... when they dye they curl and cup and leave a very bumpy look to them where the spots used to be... -not to mention... it kept my blue dream from ever developing any wieght or potency... shit strait up sucks