We have this medium sized magnolia in a pot since a bit more than a year. It survived the winter with us ocassionally taking it inside the house to prevent it from frosting.
When spring came it started to bloom, and in a matter of a week the flower buds that it so slowly grew opened and fell to the ground.
Then the leafs started to appear. In the beginning they had a healthy tone of green and it was leafy. But over the course of the last month, the color of the leafs have changed, they show ocasional black spots, and they tend to dry very quickly (although I think it is not due to underwatering). It has slowly, but constantly, been loosing more and more leafs.
In this period I found green plant louse and later on cobwebs. In both ocassions I have fumigated it, and no insect has ever appeared again.
I am a bit clueless. I'm not sure what it has or how to help it. I appreciate any feedback.
UPDATE: A month later
Following Stephie's suggestions, during the last month I've been watering this magnolia just with distilled water and fertilising once a week with a iron and magnesium rich fertiliser.
The results are wonderful (see pics below). New leafs grew healthy and green, no more burnt/dried points or yellowed. The majority of old, already yellowed leafs, have recovered a healthy green color. The also feel better to touch, more soft and silkier, and less carton like. Overall the tree has increased its foliage drastically.
Here are some pictures 5 weeks after I started treating it.