The leaf undersides suggest you got something fungal wind blown going on there, regardless of tree type, good sanitation - clean up leaf litter and all old diseased leaves on ground etc., remulch, pruning, and more air is needed. then copper spray.

The fruit pic looks like some kind of mechanical damage as noted and could also be from bird pecks or thorn punctures if your tree has any... which could lead to the necrotic areas around the holes.

Thank you all for the feedback.

I'm located in Austin Texas. Unfortunately I find it difficult to clean under the tree because many of the branches are so low and they have thorns. I want to do some serious pruning but am concerned that doing so now will lead to to new growth with frosts arriving in the next few weeks. The trees will look a lot more attractive once I've cut off the lower branches. As it happens, the lower branches are also where most of the fruit is.

I've ordered some copper spray which I'll apply after trying to rake out some of the fallen leaves.

I can see when one of the lemons could have been damaged by animals but what about the lemon with the brown spots? Are they edible or should I just thrown them out?

Thanks again.

I do not know if the fruits are safe to eat, but I wouldn't take the chance. I would trim the lower branches anyway and clean up the area to prevent the bacteria from spreading from wind or water splashing back up onto the tree from rain. If you are going to get frosts, the leaves and fruit may become damaged.