@kristincarol: They are planted in our backyard. The backyard soil has a lot of stones/rocks since we purchased the property, and it looks like a construction site. I was told by the nursery that native soil is sufficient for planting avocado, as they have sensitive root system and don't like compost and manure. I told the nursery the soil in my backyard has a lot of stone and the quality is poor, but they told me it's fine, and not to water the trees often. When we were planting the trees, we filtered away the rocks/stones, so the soil was like thin powder form. We mulched the trees on top of the soil. Since I haven't been watering the young trees much, the leaves started to fall off. The nursery told me that it's lacking of water, so I started to irrigate more often. Then, the new leaves, buds, and branches are starting to die back. I contacted them again today, and they said the tree has root rot now, and the most I can do at this point is to reduce the irrigation amount and keep my finger crossed. I did research online, and i'm not sure if it's really root rot or not. If it is, I doubt the trees can survive without taking any treatment.

I am admittedly not an avocado expert, but I do have two fully mature trees and live in a huge avocado farming area. Avocados need a LOT of water, but they don't like to sit in the water, which is why most of them here are planted on hillsides. We water twice a week (on irrigation), but our trees are on a somewhat steep incline/hill, so that the water that isn't absorbed can run away. kristincarol's advice about wanting a deep leaf litter (6" or so, I mean deep) around their roots is also spot on - if you move the leaf litter, you'll even notice little feeder roots growing in the bottom layer of the leaves. They do not like to be in bare soil that is exposed to the sun to turn into cement, as they take up most of the water/nutrients very close to the surface. You also want to be fertilizing every three months or so if they are putting out fruit (follow the directions carefully since those are such little guys you have there), in case the nursery didn't mention it.

Anyway, I personally would suggest frequent shallow/short watering so that they're not sitting in water as it looks like you don't have them on mounds/hills, but enough so that the feeder roots can uptake the water. Don't water if the soil is soggy. I'd also try to get some more leaf litter in there - if you have other trees to steal dead leaves from it should work until your guys build up their own, although I would imagine you could substitute something like wood chips or deep straw, but I would avoid anything that would mat down like grass clippings. The mulch should help somewhat with keeping the surface soil moist and you should be able to avoid watering as often. I bet you can save them.

I think I'd find another expert to help in your area outside of that nursery, what they're telling you is so all over the map that it's kind of scary.

Cut off the fruit it is not helping. One of my transplanted trees took a full 60 days to get over transplant shock. Loosen up the soil around the tree.

dont overwater, it will wilt and look dry.

Mulch, mulch, mulch and more mulch. I go down to the feed store and buy straw and place maybe 5-8 inches on top of the soil. Also don't rake up any leaves, let them stay there. Eventually this will break down into the soil, which the avocado will love. Avocadoes have a lot of roots near the top, so they need water, but I do let mine dry out before I water the next time.

I assume those are raised planters and not pots. I'm sorry but that is not going to work long term even if you can get them going. Maybe if they are 6' square and less than a foot above the soil but what would be the point?

Avos can benefit from a bit of a raised mound as they don't like wet feet. If you soil is poor, you can build up a shallow mound of better soil. A mound that is 1 foot tall will need to be around 8-10 feet in diameter. Avo roots spread out and virtually none will be deeper than 18" for a decade. In ten years you will want to leave all the leaves sitting on the ground a foot deep and 10 feet in diameter. Avo roots do not have root hairs and rely on a fungus that grows to get the water into the roots. The leaves help this along. Trying to keep one going in a pot requires a lot of care and knowledge.