I'm no expert but it looks like the tree is suffocating. It must have been planted too deeply to begin with then the mulch dressing is really finishing it off. If it were me I would remove the mulch and rake away some of the top soil almost to the point were the feeder roots are visible and as far out as the drip line. Then I would put a light top coating of wood chips (not shredded which could compact) over the top. This might seem drastic but you have nothing to lose since it looks like if you don't do something you'll probably lose the tree anyway. The tree looks like some kind of Poplar. Very pretty. I love the conical shape. Perfect for the space. It would be a shame to have to get rid of a tree like that.

What state are you in? Die back can be from a number of things including moisture problems, citrus pest and diseases. My first impression is that it looks like the rootball is staying too wet. The tree should always be higher than the surrounding yard so the water has a place to go. The retaining wall might be causing moisture problems by not letting water flow away from the tree. The dirt you piled up behind the retaining wall on the right of the photo looks higher than the dirt the tree is planted in. When it rains there is nowhere for the water to go and so it sits in a pool. I hope this helps.

Yup. Your tree is thirsty :-) Leaf cupping is a sign of a dehydrated tree. This is actually what our commercial citrus growers in California use to determine watering requirements for their citrus trees. And yes, you want to plant your tree so that the well or ring for the water is lower than the trunk. Here's the idea - you want the water to run AWAY from the trunk, and not pool at the trunk. You want your watering well to be at the drip line (edge of the leaf canopy) or a little beyond because that's where the feeder roots are. Now, don't mound up around the trunk - you don't want to bury the tree, better to have the root flare exposed a bit. Make the well lower. Mulch under the tree well. Here in California we lose a lot water due to evaporation with our hot, dry summers. Just keep the mulch away from the trunk so the trunk doesn't stay damp. That can cause foot rot. You want to water at least once a week, or more often if it gets really hot. Test your soil to make sure it's damp about 18" down or so. You want to water fairly deep, but not excessively so. We're now finding out that our "deep watering for citrus" philosophy was a bit off. Since citrus as shallow rooted trees, as long as you have watered and soaked the soil down to about 18", you're watering in well. Forget misting your tree, but an occasional washing off, especially in the hot, dry summer is always nice for the tree. It removes dust as well as red spider mites, which we can get here in California during our warm, dry summers. Forget trying to measure your water, just fill your well and then check after a few hours to make sure the water is soaking down at least 18". If your soil becomes dry 6 to 12" below the surface, it's time to water again. Fertilize every 2 to 3 months with a fertilizer formulated for citrus, which will be higher in Nitrogen, and will also have all the micronutrients included. Several good choices out there, GreenAll as Tim has mentioned, GroPower Citrus & Avocado Food which I prefer due to its very high humic acid content, and even Vigoro Citrus Fertilizers are all good choices. Patty S.

Cedar- apple rust? Just a guess since I have never had it (luckily). Spray with myclobutanil (Rally, Nova, or Immunox fungicide) early in the season. For me that means 2 applications in May and I never have the problem. Do you lots of eastern redcedars growing nearby?