Meyer Lemon tree leaves yellowing and dropping
I planted a largish Meyer lemon tree in my yard last fall. It did well through the winter and is now covered in buds, but the leaves are turning yellow and dropping off rapidly. All the leaves now look unhealthy, even the ones that are still green. I've treated it for scale, and the last few days it looks like it's developing greasy spot, so I'll have to treat that, but the leaf situation has predated this problem. The store where I got the tree says it's either overwatered (definitely isn't- is more likely to be underwatered as I only water a minute or so once a week and it hasn't rained much this month) or underfertilized (I fertilized according to directions just a month ago). I would appreciate any tips anyone could give. I am worried it might be a lack of sunlight that's to blame, as now that the nearby trees are in leaf, it's getting a lot less light than it did.
Here is a link that might be useful: Lemon Tree leaves