Answer: I'm not familiar with scab disease on Magnolias so I wonder if the problem with your tree was properly diagnosed. Magnolia may be subject to leaf spots caused by the bacterium Pseudomonas syringae or a large number of fungi. Leaf spots rarely require chemical controls. Rake up and dispose of infected leaves.
Canker diseases will kill branches. Cankers on branches can be pruned out. Keep trees healthy with regular fertilization and by watering in dry weather.
Verticillium wilt may cause death of a few branches or may kill the tree. Prune out dead branches and keep the tree healthy.
For a positive diagnosis of the problem, take a leaf sample to your local Cooperative Extension office. Helpful folks there can identify the problem and make control recommendations.
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