Research advances for downy mildew control on rose

Roses in the United States have been plagued by downy mildew since the 1800s. Downy mildew is especially destructive to both greenhouse-grown and outdoor-grown potted roses. This disease may occur sporadically in some areas of the country, but is a predictable pest in other regions.

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The downy mildew pathogen that infects rose is specialized and does not cause disease on other ornamentals. There are many different pathogens that are responsible for downy mildew diseases, and most are restricted to only one or a few plant types. If you’ve been struggling with downy mildew on coleus, snapdragon or impatiens, for example, the rose downy mildew pathogen is not the culprit.

Downy mildew symptoms

On roses, downy mildew occurs on all aboveground plant parts, blighting leaves and canes. Sometimes the first symptoms of downy mildew are confused with a nutrient deficiency or spray injury. On leaves, spots may be purplish or brown and appear square since they may be limited by the larger veins. The downy mildew that infects roses doesn’t always produce a fuzzy mat on the underside of the leaves that is noticeable without magnification.

Infected leaves turn yellow and drop from the plant. A downy mildew infection may leave the plant nearly devoid of foliage. Once diseased leaves start dropping, the downy mildew is advanced and stopping it becomes very difficult.

When the canes become infected, purple spots occur and expand into a blight that affects large sections of the canes.

Disease-favorable weather

Downy mildew is very responsive to weather cues. When the weather favors downy mildew, symptoms on rose can explode almost overnight. Wet weather, high relative humidity and overcast conditions are triggers for downy mildew disease. Fog provides near-perfect conditions for an outbreak.

During wet weather, a fine weft of fungal threads can coat the underside of the rose leaves. This is where the downy mildew pathogen reproduces via a spore type called sporangia.

The lemon-shaped sporangia are produced on spore stalks. The sporangia develop and ripen during the night as long as there is darkness and at least six hours of continuous moisture. When the environment begins to dry in the early to mid-morning hours, the air currents pluck the sporangia from their spore stalks and carry them to nearby healthy rose foliage.

Downy mildew is most favored at about 64.4°F. Temperatures that are too high (80.2°F and above) or too low (39.9°F and below) stop the disease. If the weather becomes hot and dry, the downy mildew pathogen will be halted, at least for awhile. It is possible, however, for the downy mildew pathogen to lay quiet in infected rose tissue and wait for cooler weather.

Since the downy mildew pathogen can lay quiet in rose tissue without noticeable blighting, it is possible to receive rose plants that appear healthy only to have downy mildew symptoms develop later. It is also possible for the downy mildew pathogen to persist in a production facility, causing disease from one season to the next.

Fungal threads of downy mildew can survive in diseased rose canes. It is also possible that a specialized spore (oospore) can remain dormant in infected rose debris and soil, surviving harsh weather conditions and allowing the downy mildew pathogen to survive between rose crops.

Testing chemical controls

Newer fungicides are available to prevent and limit downy mildew on roses. When fungicide studies were conducted on site with a commercial nursery in the southeastern United States , several products were effective. Double Knock Out roses appeared to have mild downy mildew symptoms when they were received by the nursery.

Fungicide sprays in two studies were applied on a weekly schedule. An exception to this was Subdue MAXX EC that was applied as a drench every 30 days. The weather during the trials allowed downy mildew to progress but the disease never exploded as it can when conditions are especially wet and humid.

Rose plants were rated for downy mildew by counting the total number of infected leaves per plant. Plants that were not treated had nearly 20 leaves per plant infected with downy mildew.

In the first study, all fungicides that were tested limited the number of infected leaves compared with plants that were not treated. Products that limited downy mildew infection to five or fewer leaves included: Daconil WeatherStik (1 pint/100 gallons), Heritage WDG (4 ounces/100 gallons), Stature DM (12.8 ounces/100 gallons) and Junction DF (3.5 pounds/100 gallons). Some phytotoxicity was seen with applications of Daconil WeatherStik (1 pint/100 gallons).

In the second study, all fungicides were applied as sprays. Mandipropamid, a new product that is not yet registered, has targeted activity against downy mildews and other related pathogens. Mandipropamid was compared alone and in combination with other fungicides for activity against downy mildew on rose.

Plants that were not treated with any fungicide had approximately 25 infected leaves per plant. Products that limited disease to eight or fewer leaves included Mandipropamid (8.2 fluid ounces) alone or mixed with Heritage WG (1 ounce) or Subdue MAXX (1 fluid ounce). Subdue MAXX EC (2 fluid ounces) applied alone as a spray was also effective. Subdue MAXX EC is currently labeled only as a drench.

Downy mildew control tactics

* Use a combination of techniques to prevent and control downy mildew. Keep the environment dry, practice good sanitation and use effective fungicides. Early detection helps in the timing of fungicide applications.

* Look for dark, purplish spotting on leaves and canes as early symptoms of downy mildew.

* Keep air moving around plants whenever possible so that relative humidity is kept low. In outdoor production facilities, arrange plants in rows that take advantage of the prevailing winds to dry the foliage.

* Choose effective fungicides and reapply frequently, especially when the weather favors the disease. In some instances, a five- to seven-day spray fungicide spray schedule may be needed.

* Alternate fungicides so that label restrictions regarding the application interval are not violated. Help delay resistance to fungicides by alternating products with each application.

* Fungicides must be sprayed so that roses are thoroughly covered with the spray. Even though some products offer systemic movement in the plant, the entire plant must be covered with the fungicide spray for the needed protection.

* All plants with downy mildew should be disposed of or destroyed at a location far from the production facility. Cull piles must be avoided because these would be a likely place for the downy mildew pathogen to survive and cause problems for new crops.

The author thanks Michigan State University Department of Plant Pathology research assistant Blair Harlan and plant pathologist Amanda Gevens for technical assistance. This research was funded in part by the USDA-Agricultural Research Service through a cooperative agreement as part of the Floriculture and Nursery Research Initiative and by American Floral Endowment.

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- Mary Hausbeck