Help identify white spots on leaves

I would not think it to be a nutrient deficiency. I relly don't encourage spraying the foliage on pot plants once they are almost out of the vegetative state. As soon as you start seeing some elongation. It looks very similar to leaf damage that I get on my melon crops if I foliar feed when it is too hot or too sunny.
Remeber you should never spray if the ambient air temp is 80 or if it is 80 and on the rise. Also on warm mornings you must also take into account the surface temp on the leaf surface. The air temp may be 80 but if the leaf is getting hit by strong increasing morning sun it could actually be much warmer than that. Have you noticed any spots on new growth? The last time it happened did it progress or negetively impact harvest?Just some things to think about.

uptheholler...Thanks for the response. The last time....No spots during the vegi stage. Leaves appears healthy green. It only appears at the onset of the flowering stage (usually mid-part). About 3 weeks before harverst the white spots started to spread followed by yellowing of the leaves. I kept my fingers crossed and luckily buds ripened and harvested. Now it's seems that the powdery spots are back again but still early to say if it will spread like what happed before. Shown in the pics are 3 varieties of sativa (no idea on what strain they are) and I have observed that its affects the same sativa(same as before) but not the other variety, yet (I based the variety on the shape, color, and size of the leaves and stem). Should I continue giving nutes (not foliar) or just wait. Do you think it has to do with the quality of seeds, soil (I use coconut fibre dust +charcoaled rice hulls+pearlite good drainage) ? Anyway again thanks a lot for the comment ............. from the tropical side of the globe.

It is inded possible that it is a seed borne virus but I wouldn't get too freak out by it. During the plantas senescense it will begin to shut down those leaves. Does it appear on new growth? If you remove the effected leaves is it continually showing up on new growth? Best of luck you gotta love old timey sativas!