The Learning Curve of a Lemon Tree

My lemon tree appears to be dying. The leaves are yellowing with spots of green that turn brown as the leaf falls off. It is budding and getting ready to flower, but this spotted-leaf thing has me concerned. I mail-ordered a sulfur-based fungicide (good for powdery mildew in roses and squash and cucumber, so I'll be using it A LOT this summer) with fatty acids, which are an "organic" contact poison for mites and aphids. I made a VERY DILUTE solution of the stuff and applied to the leaves. Here are the things I know I did wrong this year:

1. I did not fertilize the tree at all after adding some soil to the pot last spring. I've read that potted trees should be fertilized monthly. It produced three very good lemons, but it needed some nutrients during that time and afterward. I gave it a belated dose last week.

2. I was watering too much for cold weather. The soil needs to really dry out between waterings, and our enclosed sun porch gets surprisingly humid. The humidity actually encourages fungus growth.

3. I needed to order the fungicide sooner. With the tree on the front sun porch and my winter hangout in the kitchen at the back of the house, I wasn't paying attention. I tended to get out there to water at night, so I did not see how sick the plant was.

If it does not recover, I will use the pot for something else, but I will be very sad.