As this post is EXTREMELY old, I'm sure you've already either figured out how to get fruit from your lychee, or else chopped it down in a fit of rage because it never fruited... But for people with the same problem who are coming to look for the answer in the comments, it looks like your tree is out in the open (which is a bad thing). If it flowers and then the fruit never sets, it's likely because it is too windy for the tree. Lychees are notorious for fruit not setting unless you block almost ALL wind from it. If you look at pictures of lychee farms during fruiting season, every tree is covered in giant tents. You may have to put a similar tent over your tree when the flowers start to come out as well if you want the tree to produce fruit.

Did you mean that you use an iron drench on your trees? If not, you might want to try that, as the soils in some sections of Florida are so alkaline that they might not be helped much by foliar sprays. The University of the Florida suggests the following. "In high-pH, calcareous soils, chelated iron in a soil drench should be applied. For young trees, mix 0.5 to 2.0 ounces (14 to 57 g) of iron chelate in sufficient water to move the material into the soil around the base of the tree. For mature trees mix 0.5 to 4.0 ounces (14 to 113 g) of iron chelate per tree per application. Iron should be applied 2 to 4 times during the growing season." Here is a link that might be useful: Lychee Growing in the Florida Home Landscape

You most definitely need to keep the soil moist. I'm assuming they are in pots? The general logic of keeping them cool and dry in winter is for when they are planted in the ground. In other words, they shouldn't be irrigated in winter, and less humidity is desirable. That doesn't apply if you have them in pots. I'd imagine that temperature isn't a problem, as long as it's above 40 degrees...which it must be if all of those other plants are doing well. The only other thing I can think of is that the leaves do have a life expectancy, and when new vegetative growth starts, the older leaves start to brown and drop off. Generally, the vegetative growth should show up in time to replace the older leaves, but if the temps in your greenhouse are too low, that new growth may be delayed a bit. Vegetative growth initiates around 70 degree temps. I would strongly recommend that you maintain a good watering schedule. The hardest part for me is keeping my lychees moist but not drowned. I try to water just to the point of absorption, but not saturation. You may want to give them a good drink for the next week, then start a regular schedule. I hope that helps.

I'm so sorry, but, it doesn't look completely dead yet. Your potting mix looks very wet or is it just very black? Young lychee plant don't need the extreme moistness of larger trees does. Carefully pulled out the tree and look at the root ball/system. If they are still whitish and not dark brown. Then the lychee still has a fighting chance. All those leaves will eventually drop or you can pull them off now. It will be a slower process to regrow new leaves, but, if the root system is dark brown, then it is most likely dead. But, whitish roots, still have hope for recovering. Will be slower, but will recovered. I have some young plants like that too, all the leaves died and turned brown. But their root system is still alive, just took longer to recover, after such a stressful period of leaf drops. Hang on to the little guy, it will recover, eventually. Just don't give it water unless you notice the soil is getting on the dry side.