What is impatiens downy mildew? 

Impatiens downy mildew is a serious threat wherever impatiens are grown, including Wisconsin.  Impatiens downy mildew has been so destructive in the past that it has made impatiens unusable as a garden ornamental.  The disease affects garden impatiens (Impatiens walleriana and I. balsamina), as well as native jewelweeds (I. pallida and I. capensis).  New Guinea impatiens (I. hawkerii) and its hybrids appear to be either resistant to or tolerant of the disease.  Other common garden ornamentals are immune to impatiens downy mildew and thus not affected by the disease.

What does impatiens downy mildew look like? 

Symptoms of impatiens downy mildew often first occur on leaves near the tips of branches.  Initial symptoms include an irregular yellow-green discoloration of leaves that can be confused with spider mite feeding injury.  Affected leaves often curl downwards.  Stunting and reduced flowering are other common symptoms.  As the disease progresses, leaves and flowers drop off, leaving a bare stem.  Eventual death of affected plants can occur.  The most distinctive characteristic of impatiens downy mildew is the presence of a fuzzy white material (actually the organism that causes the disease) that develops on stems, buds and particularly the under sides of leaves.

Where does impatiens downy mildew come from? 

Impatiens downy mildew is caused by the fungus-like water mold Plasmopara obducens.  This organism is commonly first introduced into a garden on infected impatiens transplants.  It can also be introduced by windborne spore-like structures called sporangia.  Once established in a garden, P. obducens can spread from plant to plant by wind or splashing water (e.g., rain, overhead sprinkling).  Cool, wet/humid weather favors disease development.  P. obducens can potentially overwinter in a garden in the form of specialized spores called oospores.  These spores can be found in soil and in infested plant debris.  Whether P. obducens can be introduced via impatiens seed is unclear.

How can I save a plant with impatiens downy mildew? 

Plants with impatiens downy mildew are unlikely to recover and can be a source of sporangia that can infect other impatiens plants, as well as a source of oospores that can allow P. obducens to overwinter in a garden.  If you see impatiens downy mildew, remove symptomatic plants (roots and all), place them in sealed plastic bags and throw them away in the garbage;  DO NOT compost these plants.  Also consider removing all impatiens within a three-foot radius of symptomatic plants.  These plants are likely infected but not yet showing downy mildew symptoms.  DO NOT use fungicides on plants that are showing symptoms as such treatments will not be effective.

How can I avoid problems with impatiens downy mildew in the future? 

Use a wide variety of bedding plants in your garden.  A diverse plant selection can limit the spread of disease-causing organisms (like P. obducens) and limit the impact of diseases when they occur.  Plants such as alternanthera, begonia, coleus, iresine and torenia are possible alternatives to impatiens.

If you decide to plant impatiens in your garden, consider using New Guinea impatiens or one its hybrids (e.g., ‘Sunpatiens’), which appear to be resistant or at least tolerant to downy mildew.  Newer varieties of garden impatiens (the Beacon and Imara XDR series) that have been bred for downy mildew resistance are also now available.  Keep in mind however that even resistant varieties can potentially develop downy mildew, and the severity of the disease will depend on environmental conditions.  Inspect impatiens plants carefully for symptoms of downy mildew prior to purchase.  DO NOT buy infected plants.  Once you have purchased your impatiens, DO NOT plant them right away.  Keep them in a holding area and watch them for symptom development.  Obvious symptoms of impatiens downy mildew may not appear for five to 14 days.  Keep plants from different sources (e.g., different greenhouses) as far apart as possible.  That way, if impatiens plants from one source are infected, you can limit spread to other plants.

When planting your beds, DO NOT use impatiens in the same areas as you did last year; remember that P. obducens can potentially overwinter as oospores in soil and old impatiens debris.  Space impatiens plants as far apart as possible.  This will promote good air flow and promote drier conditions that are less favorable for downy mildew development.  For the same reason, avoid overhead watering (e.g., watering with a sprinkler).  Instead, use a soaker or drip hose to apply water gently to the soil without splashing it onto leaves.

As a last resort, consider applying fungicide treatments for control.  Use a fungicide that contains the active ingredient mancozeb and that is labeled for use on impatiens.  Start applications before symptoms are present, and be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.  Apply the fungicide per label directions as long as weather conditions (i.e., wet or humid weather) are favorable for disease development.

For more information on impatiens downy mildew: 

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

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What is Hosta virus X? 

Hosta virus X (HVX) is a viral disease that causes serious problems on many hosta cultivars, including, but not limited to, ‘Gold Standard’, ‘Guacamole’, ‘June’, ‘Patriot’, ‘Paul’s Glory’, ‘Regal Splendor’, ‘Sagae’, Striptease’, ‘So Sweet’, and ‘Sum and Substance’.  Since its identification in 1996, nursery owners and gardeners in the United States have voluntarily destroyed large numbers of hostas to help prevent the spread of HVX.

What does Hosta virus X look like? 

All cultivars of hosta can become infected with HVX, but the type and severity of symptoms that eventually develop depend on the cultivar.  Not all hosta cultivars show symptoms and in some cultivars, symptoms do not develop until after the first year of growth.  When symptoms are present, they can include light or dark green discolorations along leaf veins, green and yellow mottling of leaf blades, puckering, circular discolored areas (called ringspots), and in some cases, wilting and tissue death (necrosis).  Symptoms are easier to distinguish in gold-colored hosta cultivars than in green cultivars.  When symptoms are subtle, holding an infected leaf up to the light and comparing it to a healthy leaf can make symptom identification easier.

Where does Hosta virus X come from? 

HVX is caused by a plant virus of the same name (i.e., Hosta virus X).  This virus only affects hostas.  HVX is typically introduced into a garden on new, often asymptomatic hosta transplants.  The virus can then be spread from hosta to hosta by use of garden tools (e.g., pruners, shovels, knives, string edgers) and other items that have become contaminated with the sap of infected plants.  Many growers are unaware that their hostas are infected and because of this, unknowingly move HVX from hosta to hosta in their garden or nursery.

How do I save a plant with Hosta virus X? 

Once a plant has become infected with HVX, it is infected for the remainder of its life.  There are no chemical treatments to eliminate the virus.  Although a grower’s first instinct is to prune diseased leaves from a symptomatic plant, this can lead to spread of the virus if contaminated pruners are then used to trim other plants.  The only way to eliminate the disease is to destroy infected plants (and any associated plant debris) by burning (where allowed by local ordinance) or landfilling.  DO NOT compost hostas with HVX.

How do I avoid problems with Hosta virus X in the future?  

Carefully inspect hostas prior to purchase, and avoid buying any plants with symptoms of HVX.  Remember that even asymptomatic plants may carry HVX, and symptoms may (or may not) develop after you bring plants home.  When purchasing hostas (especially expensive ones), consider requesting that the seller provide proof that the plants are HVX-free.  Virus testing kits are available for HVX testing and are relatively inexpensive.  Note that several other plant viruses can affect hostas; therefore a negative HVX test result does not guarantee that your plants are totally virus-free.  Whether or not your new hostas are pretested for HVX, keep them isolated from other hostas in your garden for several weeks, and watch new plants for symptom development as they adapt to their new environment.  Once hostas are established in your garden, try to minimize any wounding that could release sap and provide an entry point for HVX.  After working with individual hosta plants, wash your hands thoroughly with soap and water.  Also, decontaminate any items (e.g., pruners, shovels, knives, pots, work surfaces) that may have come in contact with hosta plants by treating them for a minimum of one minute with a solution of one of the following:

• 2.75 tablespoons of Alconox^® (a type of lab detergent) plus 2.5 tablespoons of sodium lauryl sulfate (SLS), also known as sodium dodecyl sulfate (SDS), in one gallon of water, or
• 14 dry ounces of trisodium phosphate in one gallon of water.

These ingredients can be ordered on the internet.  If you opt to use SLS, be sure to wear gloves and safety goggles, and mix the solution in a well-ventilated area as SLS is a known skin and eye irritant.  Once treated, rinse items with sufficient water to remove any residues.  Proper sanitation is critical for preventing the spread of HVX, as well as other plant viruses.

For more information on Hosta virus X: 

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

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What is hollyhock rust? 

Hollyhock rust is the most common fungal leaf disease of hollyhocks (Alcea rosea).  Many other ornamentals (e.g., flowering maple, rose mallow) and weeds (e.g., common mallow) in the mallow family are also susceptible.  Hollyhock rust can quickly spread, causing stunting of plants and premature leaf drop.  In rare instances, hollyhock rust can result in plant death.

What does hollyhock rust look like? 

Initial symptoms of hollyhock rust are orange to yellow spots (roughly ⅛ to ¼ inches in diameter) on lower leaves.  Irregular brown areas may also develop on stems.  As the disease progresses, brown to dark-red raised “bumps” (fungal reproductive structures called pustules) develop on the undersides of leaves beneath the orange/yellow spots.  Severely affected leaves often develop holes that give them a lacy appearance.  These leaves may eventually shrivel and die.

Where does hollyhock rust come from?  

Hollyhock rust is caused by the fungus, Leptopuccinia malvacearum which can be introduced into a garden by windborne spores or on infected transplants.  Further localized spread of spores is possible by wind, by splashing rain, or by splashing water from a sprinkler.  Warm and humid temperatures favor the growth of the fungus.  L. malvacearum can survive the winter in hollyhock leaf and stem debris.

How do I save a plant with hollyhock rust? 

Once symptoms of hollyhock rust appear, control can be difficult.  If you observe the disease very early in its development, remove symptomatic leaves and dispose of them in your municipal garbage (where allowed) or by deep burying them.  Consider follow-up fungicide treatments, although such treatments will be most effective when applied before any symptoms appear.  If you decide to use fungicides for control, select products that are labeled for use on hollyhocks (or more generally on ornamental flowering plants) and contain the active ingredients chlorothalonil, mancozeb, myclobutanil, tebuconazole, or triticonazole.  When making more than one application, DO NOT use the same active ingredient for all treatments.  Instead, alternate the use of at least two active ingredients with different modes of action to help minimize problems with fungicide-resistant variants of the hollyhock rust fungus.  Myclobutanil, tebuconazole and triticonazole have similar modes of action and should NOT be alternated with one another.  Be sure to read and follow all label instructions of the fungicides that you select to ensure that you use products in the safest and most effective manner possible.

How do I avoid problems with hollyhock rust in the future?  

Remove weedy mallow plants (especially common mallow) from your garden; they can serve as a source of L. malvacearum spores.  Also, be sure to remove all hollyhock debris in the fall to eliminate another place where the fungus can overwinter.  Weeds and plant debris can be disposed of as described above.  DO NOT use seeds from infected plants, and inspect new hollyhock plants for rust symptoms prior to purchase.  Some hollyhock varieties are resistant to some, but not all, variants of L. malvacearum.  Thus use of resistant varieties may not be a reliable method for control of hollyhock rust in all situations.  Avoid planting hollyhocks densely to allow better air circulation that will promote more rapid drying of plants, as well as reduced humidity.  Water and fertilize properly to promote optimal growth of your hollyhocks.  DO NOT water with a sprinkler as this will wet leaves; use a soaker or drip hose that applies water directly to the soil.  Fertilize only when needed based on a soil fertility test.

For more information on hollyhock rust: 

Contact your county Extension agentContact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu. 

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