Downy Mildews
Friday, 22 July 2005 11:44
Downy mildews 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.
See photos: Downy Mildew on Rudbeckia
Downy Mildew on Argyranthemum
See more photos (scroll down to "downy mildew")
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.
If you've had a history of problems with downy mildew on certain crops, start a preventive spray program during favorable environmental conditions. Other perennials that may be prone to downy mildew include Aster, Artemisia, Centaurea, Coreopsis, Geum, Lamium, Potentilla, Veronica and Viola. Some of the labeled fungicides include Aliette, Alude, Compass, Champion, Heritage, Junction, Kocide, Protect, Stature, Stature DM and Zyban.
Fact Sheet with more photos: Downy Mildew
Leanne
