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Pyracantha

Grayish white fungus on branches, not leaves

The white growth on leaves and stems is the powdery mildew fungus growing on the surface of the plant tissue. Much of this growth consists of spores, which can be blown about to susceptible plants nearby. Small structures, called haustoria, grow within host cells, injuring them as they obtain food. Powdery mildew will not usually kill a plant but may weaken it and reduce winter hardiness. The unsightly fungal growth greatly reduces the aesthetic quality of the plant. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal growth. Some mildew fungi affect older leaves first, such as on lilacs. Others affect newer shoots, such as on roses or crabapples. When new shoots are affected, leaf curling and shoot stunting and twisting may be severe.

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