Could the white patches be woolly aphid? These can affect citrus, try gently rubbing them off or with a toothbrush, they are like furry greenfly.
we find they lose leaves inside, probably thd fluctuating temperatures,
we stand ours outside after the dangers of frost, not for another month yet, be sure to keep them moist and fed with citrus food.

It sounds like powdery mildew which is a fungal growth and can causes the leaves to curl, turn yellow and eventually drop off. Powdery mildew usually comes alive during spring when conditions become warmer and more humid. It's spread from pathogens which drop from fallen plant debris and dormant buds overwinter and they also spread through spores carried in the air

Powdery mildew is not too serious and rarely threatens the life of the lemon tree and a fungicide spray should get rid of it.

The other thing that this could be is silver leaf miner. If they are more of a trail than spots then this could be the problem and an insecticide should be used.