The presence of a white fungus on a tree is often, but not always, the sign of a sick plant. Fungal infections vary in the degree of harm they may inflict on the tree, so proper diagnosis and treatment is a must.
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Powdery mildew, a common infection that appears on almost every type of tree, appears as a white powder on leaves and may spread to other parts of the tree. Humidity and warm temperatures encourage fungal growth, while planting resistant cultivars and using proper sanitation practices fight the development and spread of the infection.
Black knot infects many types of fruit trees, but most seriously affects tart cherries and plums. According to Cornell University, the disease worsens with each passing year without intervention. Knotty growths form on branches, potentially girdling the wood and pink or white fungus may appear on older knots.
Lichens combine fungi and algae “growing together in a mutually beneficial, symbiotic, relationship,” according to the University of Minnesota Extension. The harmless patches of growth appear in a range of colors, including white.