Question
We had a $300 live oak planted and staked back in November.  It thrived and put on lots of growth but then suddenly all of the leaves turned brown.  The tree is alive.  I have been feeding it every two weeks and watering it with rain water almost every day.
What happened?
The nursery that planted it (I took photos in) said that it was pesticide poisoning.  Fields of corn, milo, etc. surround us.  If that is the case, why didn't the other trees (two Chinese pistachios were planted two days later) react the same way?
There is new growth but it is pitifully slow in coming back.
Should we get it replaced while we can?  Can you give an intelligent guess as to what happened?  It was not staked well and the root ball was loose for a while. I have added more soil.

Thanks for your help.

Answer
I agree with you IF it were herbicide from the spraying of the adjacent fields all the nearby vegetation would have been damaged or killed. May have been a bad planting job--could have been planted too deep. I would have the nursery replace it.
Plant the new tree by digging the hole twice the size of the root ball and as deep and fill with good top soil or potting spoil. Mulch around the tree with not more than 3 inches deep of organic mulch not piled up on the trunk. Do not plant any deeper than the top of the root ball. Water every third day for the first couple of months (if it does not rain) with 1 inch of water--place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop. After a couple of month water once a week if it does not rain.  

I would stake it for the first year then remove the stakes. Do not fertilize for the first year and do not add any soil around it after it is planted.