Defying downy mildews

It used to be that powdery mildew was the mildew that held our attention. At home, we saw it on our lilacs and zinnias. Within greenhouses, it was notorious on roses and African violets, and then it cropped up on poinsettias. But the new century belongs to downy mildew.

It’s not that this category of diseases is new, but it has new importance for growers of ornamentals. Downy mildew is nothing like powdery mildew, it just sounds similar. Downy mildews are not even fungi—they are water molds, more closely related to algae than to fungi. Three of the most important downy mildews are discussed here.

Downy mildew of rose has been a problem on roses grown under glass for many generations. When humidity soars and temperatures are cool (66 degrees F is ideal for spore germination), downy mildew can blight leaves and defoliate plants. Knowledge of environmental conditions that facilitate disease outbreaks and of effective new fungicides has made management of downy mildew easier in greenhouses. However, there are still big challenges from downy mildew for garden roses. Whether you are producing rugged landscape roses or hybrid teas, downy mildew can be hard to manage. Resistance to downy mildew needs to be added to resistance to black spot as the Holy Grail for rose breeders—roses deserve protection from both.

Growers should watch for brown or purple patches in rose leaves, with the sides of the spots bounded by veins so they appear more squared than round. Reddish patches may appear on the young canes as well. The downy mildew sporulation on these patches is very sparse (this is where the pathogen gets its name of Peronospora sparsa), but with luck and a hand lens you may be able to make out the whitish spore stalks present during humid weather. Remember that downy mildew can look very much like phytotoxicity from a chemical treatment, and that defoliation is a typical symptom.

Downy mildew of impatiens is both new and old. Curiously, there are herbarium specimens of Plasmopara obducens, the impatiens downy mildew, on wild impatiens from North America from the 1800s. Yet, the disease on our beloved bedding plant, Impatiens walleriana, did not shake the industry until 2011-2013. The downy mildew was not noted on I. walleriana until a report in the UK in 2003, and one from the U.S. in 2004. Both I. walleriana and I. balsamina, the balsam impatiens, are susceptible to this downy mildew, along with wildflowers such as I. capensis and I. pallida, known as jewelweed and touch-me-not. Pathologists suspect that oospores produced by the downy mildew in impatiens stems will allow the pathogen to overwinter in flowerbeds, then attack plantings in subsequent years.

Research is underway to better understand the role of oospores. The sporangia of this downy mildew are easily spread on air currents. The disease can easily survive on impatiens growing in frost-free areas, providing inoculum to producers and gardeners in those areas year after year. The level of disease seen in any locality will vary from year to year depending upon the amount of inoculum available and the weather. The disease was not apparent on impatiens bedding plants in the landscape until August or September in the Northeast, Midwest, and mid-Atlantic U.S. in 2013, although it appeared much earlier in 2012. This may have been partly due to skillful use of fungicides by greenhouse growers, using long-lasting systemic materials at the end of production. Downy mildew has also been seen on self-seeded young balsam impatiens in the spring in a few locations.

Scouting for the first signs of downy mildew will not give you a successful management program. Seeing downy mildew on impatiens in the greenhouse means that you shouldn’t try to sell any impatiens that year. Once you have seen sporulation, the disease will have spread far beyond where you think it is. Fungicides can protect, but they won’t cure. So if you note yellowing or drooping leaves, and white downy mildew sporulation on the undersurface of the leaf, it’s all over for that year. Even northern U.S. growers producing impatiens should treat preventively with fungicides, unless they are absolutely sure that any impatiens they bring onto the property are disease-free. Prevention with fungicides is very effective; responding after symptoms appear is not.
 

Coleus downy mildew, caused by a Peronospora species, is more important now that coleus has increased status as a potential impatiens replacement. It can affect coleus during production and in the garden, with the most severe symptoms in wet or highly humid weather. This disease goes only to coleus (seed and vegetative), perilla, and agastache.

Growers should examine leaves for vein-bounded yellow patches, dark brown dead spots or flecks, or twisting. The most severely affected cultivars also show leaf drop. Last year, we trialed coleus cultivars for their susceptibility to downy mildew, and identified a number that performed very well, with no or negligible symptoms. Those that showed no sporulation of downy mildew in our 2013 trial were Dark Star, Twist of Lime, Henna, Under the Sea Bone Fish, Under the Sea Electric Coral, Under the Sea Lime Shrimp, Burgundy Lace, Cranberry Bog, and The Whirlpool, as well as two experimental lines from the University of Florida. Symptoms occurred, but were not disfiguring, on Dipt in Wine, Keystone Kopper, Marooned, Royal Glissade, Velvet Mocha, Wizard Scarlet and Wizard Jade, Indian Summer, Redhead, Trusty Rusty, Under the Sea Gold Anemone and Under the Sea King Crab, Gator Glory, Golden Dream, Ruby Dream, and Spumoni. Choosing any of these cultivars to grow would simplify downy mildew management in greenhouse and landscape.

Management of downy mildews is achieved by having a good idea of which crops are likely to develop the disease and what environmental conditions trigger an outbreak. Keeping conducive environmental conditions to a minimum and using protective sprays when appropriate will help prevent losses.

Plant breeding efforts and comparative trials will help the industry to identify which plants can be grown without the need for protection against downy mildew. It can be as simple as growing New Guinea impatiens instead of impatiens—the New Guinea impatiens does not have a problem with downy mildew.

Whenever possible, growers should choose cultivars or plants with less downy mildew sensitivity—this will reduce their frustration during production and their customers’ frustrations during garden cultivation.

Margery Daughtrey is senior extension associate in the Department of Plant Pathology and Plant-Microbe Biology at Cornell.