http://revach.net/article.php?id=3402

Section: Moadim   Category: Around The Year
Tu B'Shvat: Bnei Yisaschar - The Day Of The Esrog Tree
The Mishna in Rosh HaShana says there are four Rosh HaShanas during the course of the year. The Rosh HaShana for the trees is on Tu B'Shvat according to Bais Hillel. Unlike all the other Rosh HaShanas counted in the Mishna, where it says the plural form of the category that is having Rosh HaShana (e.g. Kings, Years, Plants, etc.) by Shvat it says the singular Rosh HaShana for "the tree".

The Bnei Yisaschar says that on Tu B'Shvat although it is Rosh Hashana for all trees there is one tree that we focus our attention on this special day. We have a kabala to daven for our esrog for the upcoming Succos. We daven to Hashem that he should provide us with a kosher and beautiful esrog. This we see in the Mishna, which also only mentions that Tu B'Shvat is the Rosh Hashana for "the tree" and not all the trees.

When you are scrambling for a good esrog next year remember how much you davened on Tu B'Shvat and you'll know what kind of esrog you'll end up with.