The notorious O-O-MY-CETE

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On the road to healthy plants, successful management of downy mildew is possible with attentive scouting, sanitation and preventive fungicide applications.

April 20, 2021

Symptoms of downy mildew on Coleus spp.
Photos courtesy of Margery Daughtrey

"Downy” mildew gets its name from the white to grey fuzzy-looking sporulation produced on the underside of leaves. Making rank beside Pythium and Phytophthora, downy mildew pathogens belong to a group of fungal-like organisms called oomycetes. Notoriously difficult to control, these pathogens thrive in cool, moist conditions. Unlike true fungi, oomycetes (also called water molds) produce motile swimming spores, called zoospores, that swim in surface water until they find suitable tissue to infect.

For downy mildews in particular, managing humidity and leaf wetness is key to limiting disease.

Downy mildews cause disease on a wide range of host species, but individual pathogens are fairly host specific. This means the downy mildew species that cause disease on roses are not the same species that cause disease on impatiens. Unfortunately, there are plenty of downy mildew species to make the rounds. In addition to roses and impatiens, downy mildew can be a predictable pest on numerous other crops including veronica, coreopsis, phlox, alyssum, snapdragon, basil, coleus, and viburnums.

The primary means of pathogen spread is through infected propagative material and airborne sporangia, although spread for some of these diseases is by seed. Spores can travel great distances in air currents – think tropical storm events and hurricanes. Once inside a greenhouse, spores can circulate and spread rapidly through vent systems. Some species also produce thick-walled, survival spores, called oospores. These spores can persist or overwinter in infected plant material, organic debris, cull piles, or landscape plantings.

Downy mildew diseases can progress rapidly, so early detection and intervention is critical to minimize losses. The downy sporulation that gives this disease its namesake is not always present and since this sign is produced on the underside of leaf tissue, it may easily be overlooked. Early symptoms of disease may be subtle and can go undetected for some time. Be on the lookout for plants exhibiting stunted growth, cupped, twisted or distorted foliage, or yellow to brown angular leafspots (often vein-delineated). These symptoms are often mistaken for other issues including spray damage, nutritional deficiencies, or insect infestations.

If the leaves start dropping, there’s no stopping

Downy mildew doesn’t discriminate by age as young seedlings are susceptible along with mature plants. Downy mildew can cause severe stunting when plants are infected young whereas older plants may only exhibit leaf lesions. Since all downy mildews are different, generalizations in symptomology across host species, or even cultivars, is not possible. For example, the now infamous impatiens downy mildew causes complete defoliation and can wipe out entire plantings of Impatiens walleriana if conditions are favorable for disease.

Proper sanitation is important to reduce inoculum, limit spread, and manage disease. Once leaves have started dropping from the plant, the disease has most likely progressed to the point of no return. Sometimes it can be hard to accept defeat but recognize when it is time to bust out the trash bags and see those plants to the door. Severely affected plants should be bagged up in the production area and removed off-site. Pay careful attention to not scatter any spores on your way out. After diseased plants are discarded, spray and protect the remaining plants with fungicides.

Check ID at the door

Do not confuse downy mildew with powdery mildew. Although both sound like something sweet, they are not even second cousins genetically speaking or in the context of control options. For downy mildew, we need oomycete-specific or specialist fungicides in the mix for best control. When in doubt, send symptomatic plant samples to your local cooperative extension office or a diagnostic laboratory for a definitive diagnosis. The chance of successful management increases exponentially when you start out with the right diagnosis and the right fungicides.

Fungicides to the main stage

Downy mildew pathogens are prolific sporulators and at high risk for fungicide resistance development when fungicides with the same mode of action are used repeatedly. Most of our modern fungicides have single-site modes of action and are medium to high risk for resistance development. In an effort to increase the longevity of these products in the marketplace and reduce the risk for resistance, rotation programs that include two or more modes of action, or FRAC groups, are recommended. Alternating or tank mixing single-site fungicides with contact fungicides that have multi-site activity, like mancozeb, also helps prevent or delay the onset of resistance.

Downy mildew pathogens sporulate on the underside of leaf tissue and obtaining adequate coverage with contact fungicides can be challenging. As such, we recommend using products with locally systemic and translaminar (across plant tissue) activity as the foundational fungicides in your rotation program to ensure maximum distribution across susceptible plant tissue.

Stature® SC fungicide (dimethomorph, group 40) and Orvego® fungicide (dimethomorph, group 40 + ametoctradin, group 45) are two oomycete-specific solutions offered by BASF. These products provide translaminar, locally systemic activity and when used preventively, have been shown to significantly reduce sporulation of downy mildew and Phytophthora pathogens. Ametoctradin, the active ingredient in Orvego fungicide, provides a unique mode of action for resistance management. It delivers highly effective inhibition of zoospore (remember those are infectious propagules) formation and release and even at low concentrations, zoospores burst within seconds following exposure. Exceptional plant safety combined with anti-sporulant activity to stop pathogen spread, make Stature SC fungicide or Orvego fungicide clear choices for downy mildew control.

Conditions that favor downy mildew are also conducive to diseases like Botrytis blight and leaf spots. Use Pageant® Intrinsic® brand fungicide and Orkestra® Intrinsic® brand fungicide between sprays for continued control of downy mildew and protection against other diseases that may pop up.

On the road to healthy plants, successful management of downy mildew is possible with attentive scouting, sanitation and preventive fungicide applications. Don’t wait to spray!

Dr. Lookabaugh is a BASF horticulture professional based in Raleigh, North Carolina.

*Always read and follow label directions. Intrinsic, Orkestra, Orvego, Pageant and Stature are registered trademarks of BASF.