Here is a copy of information regarding downy mildew on Impatiens plants (a serious disease that may affect plants in the Waukesha area this year). If you love to grow impatiens plants, please read this important information!
Impatiens Downy Mildew & the Home Gardener
By:
Brian Hudelson
For years, Impatiens have been an old standby
for reliably providing color in shady spots.
Impatiens are a staple for many gardeners because they are inexpensive
and thrive in shade and part sun, covering large areas in wide swaths of color.
Over the past two growing seasons, the
use of this common ornamental has been threatened in many parts of the US by impatiens
downy mildew. This disease is not a new
one; reports of the disease in Wisconsin date back to at least the 1940’s, with
other credible reports in the US dating back to the 1800’s.
Recent problems with impatiens downy mildew appear
to have had their origin in the UK in 2003.
Some greenhouses in the US reported issues in 2004, although these
outbreaks were relatively scattered and did not cause significant losses. By December, 2011 however, severe impatiens
downy mildew was reported in Florida, and many northern greenhouses reported
the disease in early 2012. The disease
showed up in many landscape settings across much of the US during the 2012
growing season. By year’s end, over 30
states (including Wisconsin) reported the disease.
Downy mildews are not uncommon
diseases on ornamentals. However, the
pathogens that cause these diseases tend to be very host specific. Impatiens downy mildew is caused by a fungus-like
microorganism (technically a water mold) called Plasmopara obducens.
The pathogen can infect both ornamental
impatiens (including Impatiens walleriana
and I. balsamina) as well as native
impatiens such as jewelweeds (I. capensis and I. pallida). New
Guinea impatiens (I. hawkerii) and its
hybids appear to be resistant/tolerant, and other ornamentals are immune (i.e.,
they are not affected at all).
Impatiens downy mildew typically
develops during cool, wet/humid weather.
Therefore spring and fall are periods when the disease is most likely to
develop. Initial symptoms of impatiens
downy mildew can include off-color, light green leaves with a stippling pattern
that resembles spider mite feeding injury. Infected leaves often curl down. Younger tissues are especially susceptible to
infection, so symptoms are often first observed on the terminal growth. Stunting and reduced flowering can result,
followed by leaf and flower drop, stem collapse and, in severe cases, plant
death. Plant collapse is more likely to
happen in a landscape setting where early symptoms of the disease are more
likely to go unnoticed.
If you see symptoms
characteristic of impatiens downy mildew, check the undersurfaces of leaves for
a white, downy growth. This growth is
reproductive structures of P. obducens. This pathogen reproduces prolifically,
particularly when weather conditions are cool and moist. Under drier, hotter conditions this downy
growth may be sparse and difficult to see.
P. obducens also produces thick-walled
survival spores that are formed in infected tissue and can be found in dead
impatiens debris, as well as in soil.
The long-term survivability of these spores (particularly in Wisconsin’s
climate) is not completely understood, although similar spores produced by
other downy mildew organisms can survive several years.
The best strategy for managing
impatiens downy mildew is to try to prevent the disease. Once symptoms are observed, management
options are much more limited. To help limit
the impact of impatiens downy mildew:
·
Use a
wide range of herbaceous ornamentals in your garden. Disease problems tend to be more severe
and have their biggest impact when people plant a lot of the same type of ornamental. By interspersing a variety of ornamentals,
you can help limit spread of pathogens.
In addition, if you do have a disease problem, only a few plants will be
affected and loss of these plants will have less of an aesthetic impact on your
landscape. Other good alternatives to
impatiens include alternanthera, begonias, coleus, iresine and torenia.
·
Consider
using New Guinea impatiens or varieties that are hybrids of New Guinea
impatiens. New Guinea impatiens and
their hybrids are resistant/tolerant to impatiens downy mildew and should
perform reasonably well even when the pathogen is present.
·
Carefully
inspect any impatiens that you buy (particularly susceptible varieties) for
downy mildew symptoms. If you see
symptoms, DO NOT buy the plants and inform the supplier that you suspect he/she
has a disease problem.
·
DO NOT
plant your impatiens right away. Oftentimes
obvious symptoms of impatiens downy mildew may not appear for five to 14
days. Keep impatiens in a holding area
before planting them in the landscape to be sure they are not infected. Keep plants from different suppliers as far
apart as possible. That way, if one set
of plants is infected, you can attempt to limit spread to other impatiens.
·
DO NOT
plant impatiens in the same location as last year. Because the impatiens downy mildew
organism produces (potentially) long-lived resting spores, you should keep your
new impatiens plants from a potential source of the pathogen (i.e., debris from
old impatiens plants and even the soil in the bed where they were planted last
year). Moving plants from place to place
each year (called “rotation” in agricultural circles) is something you should
consider doing for all annuals every year.
·
Space
plant as far apart as possible in the garden. Allow enough space so that plants will not
overlap when they have grown to their full size. Adequate spacing will allow for good air
movement that will promote rapid leaf drying.
Dry leaves are less favorable for the downy mildew organism to
infect.
·
Work in
potentially contaminated beds last. If
you are concerned that you have had an impatiens downy mildew problem in the
past (or suspect you have one during the current growing season), this pattern
of gardening will help prevent inadvertent spread of the impatiens downy mildew
pathogen as you move from bed to bed.
·
Clean and
decontaminate garden tools (and other items). The impatiens downy mildew pathogen could
potentially survive on bits of plant debris or even soil clinging to tools,
pots, shoes, tires, work surfaces, etc.
Rinse tools and other items with a strong stream of water to remove
these contaminants, then use either 10% bleach or 70% alcohol (for at least 30
seconds) to complete decontamination.
Note that alcohol is preferred for tools as bleach can be corrosive to
metals.
·
DO NOT
water overhead or with a sprinkler.
Such watering techniques wet leaf surfaces which can provide a more
favorable environment for infections to occur.
Consider using soaker or drip hoses instead. These hoses apply water directly to the soil
and minimize the amount of water that ends up on leaves.
·
Remove
infected plants immediately. This
will help limit spread of the pathogen to other plants and limit the number of
resting spores that end up in the soil.
Bag up and seal any symptomatic plants in a garbage bag. You may also want to remove asymptomatic
impatiens plants within a three foot radius of the infected plants as these
plants also likely will be infected. Put
the bagged plants in the garbage; DO NOT compost these plants.
If you are
concerned that you are having problems with impatiens downy mildew, you can
submit a sample to the Plant Disease Diagnostics Clinic to have the problem
diagnosed. You can find get details on
sample submission at the clinic website (pddc.wisc.edu), by calling (608)
262-2863 or by emailing
bdh@plantpath.wisc.edu.
Written
for Wisconsin Master Gardener Volunteers.
April,
2013
Root Candles has a great new line at Waukesha Floral, inspired by nature and the concept of using scents to enhance one's "mental state of mind". 
