Did you spray any chemical? I think it could be phytotoxicity. Generally, chemical like diuron (an algicide and herbicide) uptake causes this kind of symptoms.
According to University of California, "Leaf veins can turn yellow or white (vein clearing) after herbicide application due to citrus root uptake of bromacil or diuron. After simazine exposure, leaves yellow between veins with the chlorosis increasing in severity in proportion to the amount of herbicide exposure."
When citrus roots take up too much diuron, major leaf veins turn yellow.
Posted by: Dr. Ravishankar Narayana
Posted: October 17, 2017