My Magnolia Tree Has Leaf Blight

Identifying the Blight

• Bacterial blight is not difficult to diagnose on the magnolia tree. Black or brown spots show up on the leaves or the veins may turn these colors. Twigs often turn black and buds fail to open. Cankers are evident on stems and the tips of new shoots die.

Causes

• The pathogen Pseudomonas syringae causes the blight problem in magnolia trees, according to the University of Illinois. It afflicts trees that are unhealthy or that have wounds. Damage to trees during pruning gives the pathogen an entry point. Exposure to frost when early shoots are developing is also a factor. Unusually wet weather or rapid growth during spring or fall also cause susceptibility to this disease.

Controlling the Pathogen

• Treat the blight with an application of copper hydroxide during the autumn. Copper sulfate is another type of chemical control that is effective against the disease. These chemicals kill the bacteria during the winter. Another application just before the new buds start to sprout in the late winter is recommended. If you wait until the spring, the chemicals may burn the leaves.

Prevention

• Protect the young magnolia during the spring to prevent bacterial blight from affecting its leaves and other areas. Place it under a plastic shelter to shield it from unusually wet weather or frost. Fertilizers that are high in nitrogen also must be avoided. They cause rapid growth of the vegetation, making the tree more susceptible to the blight. Prune the tree in January or February instead of the fall to prevent recurrence of the blight.