The following query was posted to a citrus forum but the mystery remains unresolved. Hopefully someone here will recognize the symptoms. Sorry - I do not have pictures. "The leaves of my Meyer lemon seem to be infected by a fungus or virus. A newer leaf appears normal until it is put up to a light. The affected areas appear light green next to the healthy dark green portion. A thin pattern of brown lines can be seen in those areas on the underside of the leaf; the pigment is inside the leaf as it does not wipe off. Interestingly, on an older leaf nearing the end of its life (i.e. turning yellow), the affected areas remain green while the surrounding area yellows. Any idea what this is?"