Plant Pathology Update:
Dr. Larry W. Barnes, Professor and Extension Specialist
Department of Plant Pathology
Texas A&M University
College Station, TX
Mildews:
Downy Mildew and Powdery Mildew
Mildew is sometimes used generically to indicate certain fungal diseases of plant foliage. The actual mildew diseases that can cause plant losses actually fall into two specific categories: downy mildews and powdery mildews. These two different mildew diseases can each cause plant loss and specific control strategies are required.
Symptoms of infection:
Powdery mildew usually develops as an obvious white powdery growth on the upper surface of the leaf. The white powdery substance that occurs on infected leaves consists of powdery mildew spores and mycelium. Early, severe infection of young leaf tissue can cause twisting and distortion , with powdery mildew occuring on both the upper and lower leaf surfaces.
Downy mildew infection is usually characterized by a greyish-purple growth on the under surfaces of infected leaves, which represents a combination of the sporangia of the fungus (spore producing structures of the fungus) together with the mycelium of the fungus. Downy mildew infection commonly results in the development of yellow "blotches" on the upper leaf surface of leaves. These yellow blotches usually occur on leaves in the lower plant canopy first; always examine leaves with these patterns of leaf yellowing for early detection of downy mildew.
Conditions that favor mildew diseases
Moderate temperatures (cool to warm, depending on the particular powdery mildew) and high relative humidity favors powdery mildew development. Poor air circulation that results in air stagnation also favors powdery mildew development because such conditions favor the accumulation of pockets of high humidity around the leaf canopy. Strange as it may seem, wet leaves actually deters powdery mildew infection, a fact that some greenhouse rose growers have used to help manage powdery mildew in cut rose crops.
Cool, wet, high humidity weather favors downy mildew development. Dense canopy growth and tight plant spacing encourage downy mildew spore production on the lower leaf surface. Splashing water from overhead irrigation or from rainfall not only splash downy mildew spores from leaf to leaf and from plant to plant, but also provide the wet leaf surface required for downy mildew spore germination.
Both powdery and downy mildew pathogens are spread primarily by wind-blown, air-borne spores. Both of these mildew pathogens produce spores on the leaf surface (powdery on the upper or lower leaf surface, downy usually on the lower lea surface), that can be readily blown and splashed from plant to plant. In addition, some of the downy mildew diseases are thought to be introduced into greenhouse producion systems on infested or infected seed.
A word about host susceptibility:
Most commonly grown greenhouse crops are susceptible to both powdery mildew and downy mildew. However, it is important to realize that there is high host specificity between the mildews and their individual hosts. The powdery mildew that infects and damages gerbera daisy is not the same powdery mildew that causes problems in begonia; the powdery mildew that infects zinnia is different than the powdery mildew that infects kalanchoe. This same host specificity exists with the downy mildews also; rose downy mildew will not go to pansy; downy mildew of snadragon will not go to allysum. The powdery and downy mildews are very specific to their individual hosts. If mildews are occuring on several different hosts simultaneously, it simply indicates that spores of the specific pathogens were present and the hosts were susceptable.
These mildews can usually be effecively managed by the timely use of effective fungicides, together with efforts to manage the greenhouse environment to disfavor disease development. Fungicides for mildew contol include:
Selected Fungicides for Disease Control:
PATHOGEN SYSTEMIC CONTACT
powdery mildew
Banner Daconil 2787
Bayleton Phyton 27
Triforine Triact
Terraguard
Systhane
Pipron
Rubigan
downy mildew
Aliette Manzate
Pace (rose only)
Fore
Daconil 2787
Triact
Always be sure to check the fungicide label for specific clearance using fungicides. Apply all fungicides in strict accordance with fungicide labeling.
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