White fungus on magnolia trees is caused by a disease called powdery mildew. According to the Morton Arboretum, there are more than 1,000 types of fungi that cause powdery mildew. The type of fungus varies with the host.
Powdery mildew is spread by spores that blow in the wind. It overwinters on infected parts of the plant and in fallen leaves on the ground and infects trees and plants in late summer. Magnolia is particularly susceptible to this disease.
The disease is characterised by a greyish-white, powdery growth that covers the top surface of the leaves as well as the stems. Later in the season the leaves may yellow or brown or show signs of deformity, or fall prematurely from the tree. Powdery mildew usually appears late in the growing season and is a cosmetic problem rather than a serious threat.
Begin fungicide treatments as soon as the first white patches are noticed. Remove and destroy fallen leaves to prevent powdery mildew spores from overwintering on the ground and reinfecting trees the following year.
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