Karen Rane, Plant Disease Diagnostician
While powdery mildew can develop throughout the growing season, this disease is most common in late summer and fall. The disease gets its name from the gray to white, powdery fungal growth (Figs 1 and 2) that develops on the surfaces of leaves and stems. In severe infections, entire leaves will be killed.
Powdery mildew is really a group of diseases caused by several closely related fungi. Many powdery mildew fungi are host specific – that is, the powdery mildew fungus infecting lilacs is not the same as the fungus on phlox. Powdery mildew fungi spread by airborne spores, and disease development is favored by warm days and cool nights. For more information on these diseases and their management, refer to BP-5, Powdery Mildew of Ornamentals.
Some zinnias show excellect resistance to powdery mildew. Fig. 3 shows a cultivar of Zinnia haagaeva located in the same garden as the severely infected Z. elegans shown in the first two photos. Note the lack of any powdery mildew growth on these leaves.
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Figure 1. Powdery mildew on
|Figure 2. Close-up of white fungal growth of powdery mildew||Figure 3. Zinnia haagaeva, resistant to this powdery mildew|
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