Rotation programs are key components of any operation’s disease management strategy. Resistance remains a concern for growers, and multiple modes of action are needed at times to successfully prevent the spread of diseases.
“Pathogen resistance to fungicides is well known, and the performance of many fungicides has been affected to some degree by pathogens developing resistance,” says Dr. Fulya Baysal-Gurel, research assistant professor at the Otis L. Floyd Nursery Research Center at Tennessee State University. “Using different mode of action fungicides in a rotation program is important due to the risk of resistance development.”
Preventing powdery mildew
Powdery mildew is a common disease that can affect a wide range of herbaceous and woody ornamental crops. Several different fungi cause powdery mildew and each prefers different plants, so preventing the disease early is crucial to prevent it from spreading.
“Powdery mildew may cause cosmetic damage that results in reddish-brown patches, reduce growth by attacking tender shoots and leaf surfaces, and cause premature defoliation,” says Baysal-Gurel. “The disease spreads very quickly, with masses of conidia produced from each new infection. Preventive fungicide applications in a rotation are critical to control powdery mildew.”
Baysal-Gurel conducted several trials evaluating rotations of Mural®, Palladium® and Concert® II fungicides at 14- or 21-day spray application intervals for control of powdery mildew on dogwood and crape myrtle.
The initial fungicide application of Mural (7 oz./100 gal.) was made after observing the first symptoms of powdery mildew disease. Then, Palladium (6 oz./100 gal.) and Concert® II (35 fl. oz./100 gal.) fungicides were alternated on 14- and 21-day application intervals.
“The fungicide rotation program and both spray intervals significantly reduced powdery mildew severity and disease progress compared to the non-treated control in flowering dogwood, as well as on both cultivars of crape myrtle,” says Baysal-Gurel. “This study also showed that the rotation program significantly increased plant height compared to the non-treated control.”
“Securing reliable disease protection with a robust rotation that can be applied on an extended spray interval provides many advantages for the nursery,” says Nancy Rechcigl, technical services manager for ornamentals at Syngenta. “Selecting the proper fungicides to use in a rotation program is very important. Mural, Palladium and Concert II are not only proven to control powdery mildew, but can be used to control leaf spots and rusts as well.”
Rechcigl notes that Mural offers contact, translaminar and systemic activity, which allows it to have both protective and curative properties.
“As a foliar spray, Mural provides protection by inhibiting spore germination and penetration on the plant surface,” says Rechcigl. “Its translaminar and systemic properties allow it to penetrate and move throughout the tissue, stopping the growth of mycelium internally within the plant. This provides an advantage when battling difficult diseases like rust or powdery mildew.”
Powdery mildew is a disease that is most likely to develop in the early summer or fall, when temperatures are warm, relative humidity levels are 70 percent or greater and rains are less frequent. It is important to be aware of these conducive environmental conditions, as they can signal the ideal time to begin a preventive strategy.
“By incorporating products with both translaminar and systemic activity, like Mural, in your rotation with other protectant fungicides, you can likely extend your treatment interval while maintaining good protection,” says Baysal-Gurel.
Taking down downy mildew
Systemic fungicides can also be beneficial when combatting a disease like downy mildew.
“Downy mildew is hard to control for so many reasons – it is very often a systemic disease, not just a matter of leaf spots,” says Margery Daughtrey, senior extension associate at Long Island Horticultural Research and Extension Center at Cornell University. “It can establish during propagation and be passed on through the production chain. It can be carried on seed sometimes and cuttings all the time. Some downy mildew oospores linger in the soil for years and produce new epidemics when healthy plants are placed into contaminated flower beds.”
Dr. Mary Hausbeck, university distinguished professor and extension specialist at Michigan State University, recommends fungicide programs for a disease like downy mildew, particularly on impatiens.
“The fungicides available today can be very helpful in keeping impatiens healthy,” says Hausbeck. “It is important for greenhouse growers to adhere to the fungicide program so if the impatiens are planted into a landscape site contaminated with the downy mildew pathogen, the plant will remain protected.”
Rechcigl recommends a “systemic sandwich” approach to controlling downy mildew. This entails applying systemic fungicides as drenches at transplant and prior to shipping, and other fungicides with different modes of action and contact or translaminar activity as sprays in between to prevent resistance.
“The use of systemic products as a drench prior to shipping provides extended protection in the landscape for the consumer,” says Rechcigl. “This is critically important for the ornamental industry as a whole. If impatiens do not perform well in the landscape for the season, consumers will not want to purchase them in the future. We saw this occur several years ago when the disease was becoming more prevalent across the U.S.”
“Systemic fungicides often protect a plant very thoroughly because they are redistributed within the plant and are not as subject to variations in spray coverage,” says Daughtrey. “Using some systemics and some contact materials in a rotation is often wise. With impatiens downy mildew, in particular, it seems wise to use a thoroughly systemic, long-lasting fungicide as the last treatment before the plant material goes out for retail sale.”
To help provide a solution, Syngenta launched Segovis® fungicide in 2016. It provides a unique mode of action with systemic activity for long-lasting control of downy mildew. Segovis can help growers diversify their rotation programs while also mitigating resistance concerns.
“I’ve tested the active ingredient in Segovis across different downy mildews and it is a highly effective fungicide,” says Hausbeck. “I would certainly call it a ‘stand-out’ for downy mildew control. It’s a wonderful tool for greenhouse growers and provides a unique mode of action against downy mildews.”
Differentiated products can be crucial for greenhouse and nursery growers. With continued innovation occurring in the industry, there are newer, unique control options available in the fight against tough diseases.
“Others’ experiences have taught me that it is essential to use rotation programs with downy mildew management in greenhouses,” says Daughtrey. “Once a population is resistant to a previously valuable fungicide, the utility of that active ingredient is lost. Rotation preserves the effectiveness of the tools we have to use against downy mildews.”
“When highly effective products are chosen and used in a program that alternates among different modes of action, downy mildew doesn’t stand a chance,” says Hausbeck.
Syngenta has created greenhouse and nursery rotation programs for a wide variety of diseases. They can be downloaded at www.GreenCastOnline.com/Solutions