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Downy Mildew Update

June 14, 2013

Over the past two growing seasons, downy mildew has been the focal point of discussion amongst growers, landscapers and gardeners. Although downy mildew is by no means a new disease for growers, it recently has wreaked havoc on impatiens and continues to be a concern to the ornamentals industry.  

Downy mildew diseases (Bremia, Basidiophora, Pereonospora, and Plasmopara) infect a wide range of annuals, cut flowers, perennials, and woody ornamentals.  Besides impatiens, there have been numerous reports of downy mildew infections on alyssum, basil, buddleia, coleus, coreopsis, geum, lamium, pansy, rose, rudbeckia, and viola to name a few.  These pathogens are specialized,  with each one attacking a range of closely related host plants and usually not affecting more than one genus of plants.  For example, the downy mildew species which attacks impatiens is different from the mildew that attacks coleus.

Identifying Downy Mildew

Downy mildew is closely related to the water mold diseases Pythium and Phytopthora.  Unlike water molds which attack the root systems, downy mildew diseases attack the above-ground parts of the plant.  The reproductive structures (sporangia) of downy mildew are the most distinguishing characteristic of this disease, and most commonly appear on the undersides of leaves as white, gray or slightly purple velvet-like fuzz.

Quite often the symptoms of mildew infections are visible before the sporangia develop.  These early symptoms appear slightly different on various ornamentals, but generally yellowish, tan, or reddish purple leaf splotches will form on the upper leaf surface.  In some plants, these spots will appear more angular and are bound between the veins of the leaf.  Other symptoms of downy mildew include chlorotic foliage, downward curled leaves, stunted plants, and flower buds that do not open properly.  These symptoms are similar to those often observed with chemical or foliar nematode injury.  In most cases where downy mildew is present, the ‘fuzzy’ sporangia on the leaf undersides are directly opposite the discolored patches found on the upper leaf surfaces.

Downy mildew grows inside the vascular system of the plant, down through the water vessels.  Once inside, the disease becomes ‘systemic’ and can be very destructive.  It can be difficult to detect downy mildew while the infestation is just beginning to occur and may take a few days or weeks for it to become well-established.  But once the environment turns favorable for the disease, an epidemic can occur very quickly.  

Environmental Conditions Necessary for Infections

Infections most commonly begin during periods of cool temperatures, moist conditions, and high humidity.  Being a water mold, downy mildew does require free water on the leaves for the spores to germinate and enter into the plant.  A film of water on the leaves for more than six hours is sufficient for germination and entry.  Temperatures of 50 to 75º F combined with free moisture; provide ideal conditions for this disease to explode.

To reduce the likelihood of downy mildew infestations, maintain plants with dry leaves going into the night, reduce the humidity to less than 85%, prevent condensation from forming on the leaves, increase the air circulation around the crop, and irrigate early in the day to allow the leaves to dry.

Preventing and Controlling Downy Mildew

Downy mildew can be prevented using several of effective fungicides listed below, but none of these products will clean up an established infestation.  Preventative fungicide applications act as a barrier, not allowing the disease to infect the plant.

The best strategy is to rotate products containing the active ingredients boscalid + pyraclostrobin, cyazofamid, dimethomorph + ametoctradin, fenamidone, fosetyl-aluminum, imazalil,  mancozeb, mandipropamid, mefenoxam, mono-and di-potassium salts of phosphorous acid, and pyraclostrobin. Many growers find it necessary to apply one or more applications per week (depending on environmental conditions) to crops highly susceptible to this disease.  

It is particularly important to rotate between the chemical classes to prevent the development of the downy mildew fungi from building up resistance (tolerance) to these fungicides. Using tank mixes can be used as a strategy to prevent disease resistance to these products.

It is helpful for growers to know which plants they produce are susceptible to downy mildew diseases and to scout these crops at least once per week, looking for the presence of symptoms or the disease. If necessary, the appropriate control strategies should begin when the conditions are conducive for disease development or in the early stages of infection (before an outbreak occurs).  

Remember, none of these products are effective curatively against downy mildew diseases; therefore, growers must take a preventative approach.  When implemented properly, preventative programs have been proven effective at preventing downy mildew from infecting susceptible crops.

The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned. ALWAYS READ PRODUCT LABELS AND USE THEM AS DIRECTED ON THE LABEL.

Paul Pilon
Perennial Solutions Consulting


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