It's still a bit early for the fall change, but wouldn't give up on it too quickly. These symptoms can be caused by over-watering, which would be my first guess as you mentioned it's still being watered several times a week after 18 months in the ground; most red oak plantings should be able to make it on a weekly deep watering at most by this point.

Here is a link that might be useful: Over-watering of oaks (vs oak wilt)

I'm in the Austin area, and there is a 70-foot red oak next door. No sign of early fall in that guy. You're bubbling a "good sized" red oak several times a week? Overwatering is a well-understood problem for red oaks. See, for example,

http://www.oakwilt.com/over_watering.html

I agree that, in the worst of the summer heat, a weekly light watering (or, just being on a lawn that is watered weekly) would be fine.

Oh, my apologies. I didn't notice that bostedo posted exactly the same link I had. Good stuff.

Thanks for the input. The tree has been getting watered with the lawn, twice a week for twenty minutes. I will adjust downward and hope it is not too late.

There was a post on one of my Facebook groups discussing signs of early fall. In the parking lot at work (Plano) there are three cedar elms that are losing leaves already. I noticed the color change 3 weeks ago. I've also been seeing geese flying south for 4 weeks. Something to think about.

Twice a week for 20 minutes (unless you are getting 1 inch in 20 minutes) is not too much water, IMO.

Red oaks are noted for root rot. You get root rot by having the soil they are rooted in saturated with water. See URL below, and scroll to the bottom of the page. Do you by any chance have mushrooms growing around your tree?

I guess if the tree was just planted 18 months ago, it can't be all that big. I would water deeply, but much less often.

Here is a link that might be useful: root rot

20 minutes of lawn watering doesn't sound much to me. Often times you are supposed to hand water directly into the rootball. YOu didn't say what size so it's had to say how many gallons of water to pour directly into the rootball so it stays moist (rather than sopping wet).
You'd be surprised how long it takes for the sprinklers to put out at one inch of water. It is much better to water deeply once a week with one inch of water.

Here is a link that might be useful: watering practice

I water my small trees (figs and ornamentals) with a slow trickle all night long. That way, nothing is lost to evaporation and the water sinks slowly to the bottom of the root system.

I've planted several oaks, and watched them grow successfully for a decade or two. In the fierce summer heat, I put a sprinkler over their drip-line for an hour. Once every two weeks in the first year or two, and once a month thereafter. Don't put a lot of water in a puddle a the base of the trunk. That's a guarantee for root rot. Spread it over the drip line, and ideally even wider.

jgross57: Are you saying that a bubbler near the base is running 20 minutes twice a week with the lawn sprinkler circuit? If that is the case, you may want to turn the bubbler off for the reasons Daninthedirt has raised. The tree should be getting pretty close to being able to survive on just the lawn irrigation in its root zone and, as others have mentioned, 2x20 min/week of just the lawn irrigation should probably not cause a rot problem for a properly planted tree. I'm no expert, but strongly agree with lou_midlothian_tx and the rest on the fewer deep waterings to promote deeper root growth in both the tree and lawn - especially during the initial years. Our Shumard red oak is happy with the deep watering as needed to keep the St. Augustine beneath it alive - only time this is more than once a week is when temps get stuck in triple digits for many days. The dripping hose in the root zone mentioned by carrie751 works well for the dogwood that occasionally needs a bit more.

pkponder: We're starting to see a hint of yellow in the sumacs a few miles south of Plano, but our cedar elms are still green. With triple digits back in this week's forecast, would certainly welcome an early fall arrival.

Thanks for the input, sorry, I was away for a few days and couldn't get on line. The red oak is about ten feet tall and eight feet wide. It has a bubbler on it which waters for twenty minutes twice a week as part of the sprinkler cycle. No mushrooms, but tree is now fully brown. Will turn the bubbler off and hope for the best.

Yes, bostedo makes the point better than I did. Twice a week, 20 minute lawn watering is OK. That won't soak the root ball. What's good for the lawn should be fine for the tree. But twice a week bubbling at the base of the tree will soak the root ball. For a small tree, most of the roots are still near that root ball.

I have a young Chinquapin that is about 15 feet tall, and it has a nice patch of green grass around it (out to about ten feet away from it) where the lawn gets regular water.