Some of the leaves on my mango tree are turning brown and falling off (even as it grows new ones on other branches). The browning starts from the tip and works its way upward until the whole leaf is brown, then the leaf falls off. The leaves are crispy, not soggy, and I don't see any evidence of insects. Too little water? Too much? Not enough fertilizer? Too much? This tree made it though our cold winter and I'd really like to keep it. I planted it a year ago.