Half the leaf is turning yellow and curling

I can't see anything to be worried about, the yellow margin on the leaf is called "variegation" and occurs due to constricted vascular bundles in that area of the leaf being unable to support the requirements for healthy chlorophyll, hence the yellow colour. Mosaic virus, which is very rare, or any type of minor/major nutrient deficiency usually presents as either blotches or irregular shapes, not with straight lines like variegation. It is quite common in cannabis and not something to be worried about. It can not be fixed or stopped, it is just a small genetic "flaw" present in some cannabis seeds. It will not affect growth, yield or potency. Sometimes it will be just the odd leaf here and there, other times it will be a single leaf and other times it can affect 10-20% of the leaves all over the entire plant and from birth right through to harvest. If you google "variegation in cannabis" this will show you pictures that look exactly like the leaf in question. Hope this helps, Organoman.