Q.What is the issue with my magnolias ?
I have 3 Magnolia trees in pots & all are showing signs of this ‘burn’.
I never water the leaves, only the roots.
This week I sprinkled & watered in some ericaceous feed.
Would anyone know what the issue is, & what should I do, if anything ?

Certified GKH Gardening Expert
A few things appear to be causing the issue. First, it is important to measure and follow instructions on feed packaging to avoid over, or underfeeding. Because this is an acid mix, overfeeding will lead to a severely low pH. this can cause nutrient lock out, or many symptoms of what you are seeing here.
Another thing that can cause these symptoms is overwatering. This is the most common issue. They do not tolerate being wet constantly without a period to dry out thoroughly down to about 3 or 4 inches.
This article will help you to care for these trees, but be advised that most will species of magnolia will NOT do well in container for extended periods: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm