University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
One of the attractions of perennial flowers is their ability to tolerate a wide range of problems, usually without serious or lasting damage.  There are a few diseases to watch for that can make perennials unsightly, or cause more serious damage.

Powdery mildew, like many diseases, is aptly named as it resembles a powdery white mildew on leaf surfaces.  Unlike many diseases, this one doesn’t need leaves to be wet in order to spread and become a problem.  It is commonly found on some tall garden phlox (Phlox paniculata) and lungworts (Pulmonaria).  There are fungicides, both synthetic and organic, for powdery mildew.  Good air circulation from proper spacing may help as well.

Keep in mind that cultivars (cultivated varieties) often have differing resistance to diseases such as this one.  ‘David’ is a white phlox mostly resistant, while ‘Mt. Fuji’ is a white phlox very susceptible.  Another interesting point about this disease is that it is not the same powdery mildew strain that gets on lilacs and annual flowers such as zinnias.  If you have this disease on these plants, however, conditions are likely ripe for it on perennials. 

Downy mildew is quite a different disease, and is rather difficult to control.  This appears as gray downy growth on undersides of leaves under cool and humid conditions.  Perennials
susceptible to this include tickseed (Coreopsis), black-eyed Susan (Rudbeckia), speedwell (Veronica), asters (Aster), bluets (Centaurea), dead nettle (Lamium), and roses.  There are fungicides you can use to prevent this from spreading as it is beginning.  Keep in mind that fungicides are preventative—they prevent more disease but usually don’t get rid of that already present.  Avoid watering plants with this disease in early morning, and discard severely infected plants.

Rust diseases cause rust-colored spots on leaf surfaces, particularly undersides of leaves.  Most commonly it is seen on hollyhocks, but it can occur on many others such as asters and hibiscus.  The latest perennial to show a new rust strain to our country, only still in a few areas, is the speedwell.  Daylily rust on this popular perennial has become quite prevalent in the last few years in over half the states.  It was first introduced into our country in the southeastern states in 2000.  Watch for this on purchased plants, especially from southern sources, removing infested leaves.  There are several fungicides that can be used, but check first for resistant cultivars.

If you have hostas, beware of the relatively new Hosta Virus X.   Often symptoms resemble natural leaf variegation making diagnosis difficult.  Most distinctive is an irregular color feathering along leaf veins.  A hosta nursery specialist, or Extension diagnostician, can help identify infected plants.  ‘Gold Standard’, ‘Striptease’, and ‘Sum and Substance’ cultivars are often infected.  ‘Blue Angel’, ‘Frances Williams’, ‘Bressingham Blue’, ‘Love Pat’, ‘Great Expectations’, and ‘Sagae’ are some of the cultivars immune or resistant.

Remember, if using any pesticides, follow all label directions.  Make sure you know what you are treating, as fungicides for instance only work on fungi and not bacteria or virus.

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