|Type:||Oomycete (Water Mold)|
|Conditions:||Moist, shaded environments, most active during wet or very humid weather.|
|Parasitism type:||Obligate parasites|
Downy Mildews are plant diseases caused by organisms in several genera of the water mold group, closely related to the mildews that grow in moist environments such as bath tiles and walls. They can be devastating to certain plants if left uncontrolled: a downy mildew of grapes decimated European vineyards during the nineteenth century.
They are easily controlled once discovered, so careful monitoring of susceptible hosts is key, because if left unaddressed, the organism can quickly spread and completely overwhelm the host species.
Symptoms and Signs
The mildews are easily visible on the undersides of leaves, fruits, and branches as a whitish or gray mass of hyphae. The upper surface of leaves are often chlorotic, with white or gray patches that can easily be confused with Powdery Mildew.
The causal organisms overwinter on field trash or on the buds and stems of woody plants. When conditions are conducive, the mildews grow rapidly. They go dormant during warm, dry periods.
The various species and genera grow specifically on certain groups of plants.
- Bremia spp. -- Lettuce (Lactuca)
- Peronospora spp. -- Alfalfa (Medicago),
- Exclusion: Carefully inspect vulnerable species when purchasing for any signs of mildew. Promtly treat infections, and remove infectious wastes.
- Cultural Controls: Keep plants pruned and well spaced to allow good airflow and light, which will help keep plant surfaces dry. Susceptible species should not be irrigated by sprinkler.
- Physical Removal: Badly infected parts can be pruned out.
- Chemical Controls (organic): Bordeaux mix
- Chemical Controls (synthetic): Copper-based sprays
- Disposal: Spores may survive in compost, so burning is often recommended.
- Otis C. Maloy, Timothy D. Murray, et. al. Encyclopedia of Plant Pathology, John Wiley & Sons, Inc., 2001. ISBN 0-471-29817-4, vol. 1, page 379