My lemon tree appears healthy, has quite a few lemons on it, but the underside of some of the leaves are becoming almost completely brown. I think it is only on one branch. It does not start as spots, it just starts at an edge and takes over half the leaf. Wherever the brown patches are, the leaf on top is more yellow than green. I cannot find any pictures online that look like this. They are not crispy and they don’t fall off. I think more and more leaves are starting to get like this, on that one branch. Lemon tree lives in a very sunny window during the winter and I bring it out on the very hot and sunny deck in the spring. I have a mister that runs 10 minutes every hour during the day, on the opposite side of the deck, so it does not get directly sprayed. I have had this tree for about three years, bought when it was about a foot tall, now is about 5 feet. Last winter it got aphids I think, I treated it with neem oil and they were gone before I brought it outside. I think that happened two years in a row, but this brown leaf thing only started this year. Lemons ripen slowly, probably as usual, and they taste great. You can see that some of the leaves have a type of yellow mottling, too. I thought that might be normal or from the neem oil. The leaves are growing in a normal shape, and are not curling. I feed it monthly with Espoma Citrus Tone. Do you know what this could be and how I can fix it? Could it spread to my key lime tree 20 feet away or my herb and lettuce garden 5 feet away? Thank you!