Powdery Mildew on Ornamentals

The presence of a white, dusty mildew on infected plant partsmakes powdery mildew easy to diagnose. Late in the season, tinyblack fungal fruiting bodies may appear on the mildewed surface. Powdery mildew can occur on leaves, stems, buds, and flowers. Ornamental plants that commonly show signs of powdery mildewinclude lilac, viburnum, crabapple, phlox, rose, zinnia, begonia,and turf.

Powdery mildew is favored by cool nights followed by warmdays. The disease is most common in shaded areas and in siteswhere plants are crowded and air circulation is poor.

    A number of measures can be taken to control powdery mildew:
  • Plant mildew resistant cultivars when available.
  • Keep plants properly spaced and pruned to promote good aircirculation.
  • Plant where sunlight will be adequate.

Remove and discard diseased plant tissue at the end of theseason to prevent overwintering of the fungus. Fungicides such asFunginex, sulfer, Bayleton, Rubigan, Zyban, Domain, and Daconil2787 can be applied at the first signs of disease. Read thefungicide label to be sure the product is labeled for the plant inquestion. The use of fungicides is generally not warranted forwoody plants that show powdery mildew late in the season. By thattime the leaves have served their purpose for the season and willsoon be dropping.

This article originally appeared in the September 16, 1994 issue, p. 138.

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