Why don't you take that photo or a branch off the tree to your local extension office. They can diagnose the problem and advise you of any appropriate treatment. It does not look like citrus greening, which is devastating, to me but it's always best to have an expert check with citrus in Florida.
This is really important since you say this is a new tree. If there were more lemon trees in the nursery from the same grower it is likely this problem could be widespread.
