Q.About Planting Of Mango Seedlings?
I planted the seedling of Mango, which I propagated from seed. It is 5 monthes old and I have following questions: 1. Should I remove the old cotyledons from the seedling in case if seedling is 5 month old? Othervise when I have to remove it? 2. How deep should be the pot for 5 month old seedling? 3. What king of soil it should to be in the pots and should I use the fertilasers? 4. On the leaves appeared the black-brown spots(look on pictures). What kind of mango diseases it is and how to manage it?

Certified GKH Gardening Expert
The spots look like they are caused by mealy bugs, hence the cottony growths. I would remove the old cotyledons.
This article tells you about the container, size and soil. The next article discusses mealy bugs.
https://www.gardeningknowhow.com/edible/fruits/mango/mango-trees-in-pots.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm