Why do people Pray for a Esrog on Tubishvat what the source and what is the connection between Tubishvat between them?
has a source from the Ben Ish Chai and the Bnei Yissachar with Tefilos
I believe it may appear in the Netiei Gavriel?
Anways, some say that on Tu B'shvat, you should pray for a good esrog, some 7--8 months from now.
The reason is straightforward enough, as I understand it: Tu B'shvat is the new year (Rosh Hashanah) for fruit trees, so you pray now that the fruit tree that will produce your mitzva fruit (i.e. esrog) later this year will do so well.