Could be genetics – Following on the last point, it’s possible the symptoms are sometimes the result of genetic variation. For example, I grew a few plants of a strain called “Purple Sunset” and all the seedlings displayed the mottling and stripes. Yet the symptoms never spread to the several other plants in the grow tent. About a year later, I germinated more Purple Sunset seeds and saw the symptoms again. Just like the first set, the seedlings grew fast and healthy, but some of the leaves showed the odd stripes. I contacted the breeder about it, and he told me that he’s noticed some seedlings of this strain do that when grown under LEDs. To me, this is evidence the symptoms may be triggered by genetic factors as opposed to a virus.