It's hard to say for certain from just a picture. I will say that frequent shallow watering will cause the absorbing roots to stay near the surface leaving them susceptible to drying out when the watering schedule is changed or temps increase. Based on the fact that Mesquites, in general, can survive on the natural amount of rainfall that occurs in the desert these trees were given more water than they 'needed'. Is root rot possible? Yes. Could the roots have been stressed when you changed the irrigation? Possibly. The trees could also be planted too deeply. The link I included is a fantastic website for irrigation advice. Here is a link that might be useful: Irrigation guidelines

Prune out some of the branches to lighten the load up top. Basically remove any that are crossing each other, any shooting straight up. Do NOT clip the tips, remove an entire branch. Stake it from both sides - what it had was a shipping splint that should have been removed at planting to let the tree stand on its own. Most small trees should not be staked at all. http://www.daytonnursery.com/tips/Tree_Staking.htm I use nylon stockings as the ties because they flex, and the tree's trunk is stimulated to thicken. Pound the stakes in on opposite sides of the tree. Hold the tree up and figure out how low you can put the ties before it starts wobbling. 2 ties is all it needs - one at the place where it starts to wobble and one halfway between there and the ground. They should hold it lightly, so it can sway a bit - that's how trees are stimulated to develop strong trunks. Check it every month by loosening the ties more and see if it's strong enough to hold itself up.

Chilean Mesquites make great shade trees. Yours should grow at a moderate rate and mature to a size of 30 ft. x 30 ft. Mesquites can tolerate growing in a lawn situation although it isn't ideal. The water needs of your lawn are drastically different than the trees. While lawns usually need water twice a week in the summer, an established mesquite only needs water once every three weeks or so. The amount of water applied is different also. Grass only needs water to moisten the top 6 - 8 inches, while trees need water down to three feet. To compromise, be sure to deep water your trees periodically either with a hose or drip system. Eventually the shade from the trees will cause the grass to become sparce. Even shade-tolerant grasses will struggle under the trees. When this happens you may want to cap off the sprinklers under the tree and use a shredded bark mulch to cover the bare soil. Don't prune newly planted trees, except to remove broken or dead branches. Wait for at least one year to do any pruning - early June is the best time, but minor trimming can be done at other times. It is normal for mesquite branches to droop to the ground. On the branch you mention, try working from the tip of the branch back and remove a shoot where it intersects with another branch or shoot. This will lighten the weight on the end of the branch. As far as staking goes, be sure to remove the original nursery stake that is next to the trunk and replace it with two wooden stakes placed outside the rootball. Allow the trunk to move a bit within the ties. This creates strong wood in the trunk. The ties should not cut or bruise the trunk - old pieces of hose with a wire inside are great or you can purchase ties. Your goal will be to remove the stakes in about a year. More information on pruning and staking can be found in the book Pruning, Planting and Care by Eric Johnson, Ironwood Press. It's available in local bookstores or on-line. Another great book is Desert Landscaping for Beginners by the U of A Master Gardener Press. It has a chapter on pruning, one on watering, one on turf care, citrus, roses, cactus, landscaping for wildlife, etc. I've included the link below. I hope this helps. aztreelvr certified arborist Here is a link that might be useful: Desert Landscaping for Beginners

It can take several years of staking and some pruning to get the heavy top growth under control. Do not tie the trunk snugly to the stake like a splint. It will never get stronger. Support the trunk between two stakes, with a bit of slack so the trunk can have some movement. I use old pantyhose because they flex and stretch a bit. The stakes should be on opposite sides of the tree, about a foot away from the trunk. Place the ties high enough in the tree so the top doesn't flop over, and also one or two ties further down to help support it. Carefully shorten and prune out the top branches to make a good top structure and remove the excess growth as needed. Mesquites have rampant top growth that is too much for the trunk. I've removed as much as 2/3 the top of a young mesquite that was flopping under the weight. Every few months, check by loosening the ties and see if it's supporting itself ... eventually it will be able to do so and you can loosen the ties more, and then remove them when it's standing on its own.