Diseases
Downy mildew - Veronica
Downy mildew is a relative of Phytophthora and Pythium. Symptoms appear as a light gray colored, fluffy growth on the underside of leaves as seen here. Downy mildew symptoms appear very rapidly and tend to be much more difficult to control than powdery mildew. Conditions that promote leaf wetness, such as high relative humidity, overhead irrigation and close spacing, favor this disease. With a hand lens, closely examine the bloom of sporangia (microscopic stalks bearing spores) on the underside of the oldest leaves. (They will look like many tiny branched trees, each bearing tiny lemons). This can help you distinguish downy mildew from powdery mildew. Downy mildew sporulation also tends to occur on the underside of the leaves, whereas powdery mildew can occur on either the upper or lower surface.
Other perennials that may be prone to downy mildew include Aster, Artemisia, Centaurea, Coreopsis, Geum, Lamium, Potentilla, Rudbeckia and Viola .
It is important to distinguish between downy and powdery mildew because they are managed differently. Many of the fungicides labeled for powdery mildew will not control downy mildew.
Fact Sheets: Downy Mildews of Ornamental Plants UMass
Downy Mildew UConn
More information see message posted July 22, 2005.
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