University of Vermont Extension
Department of Plant and Soil Science
PERENNIAL FLOWER DISEASES
Dr. Leonard Perry, Extension
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
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
to this include tickseed (Coreopsis), black-eyed Susan (Rudbeckia),
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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