Category Archives: Disease – Herbaceous Ornamental

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Fact Sheet, Misc

Wood Mulch and Tree Health

What are the benefits of wood mulch?  Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways.  When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature.  Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees?  Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies.  Sour mulches can also produce gases like methane and ammonia that can be toxic to plants.  Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off.  If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch and should be removed.  Replace the sour mulch with a high quality, composted mulch and consult with your local Extension agent about testing the soil for nutrient deficiencies.  Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems.  Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet.  This wetness can contribute to bark decay.  In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see UW Plant Disease Facts D0094, Root and Crown Rot).  To avoid these problems, make sure mulch is applied at least four inches away from the trunk of a tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects?  Insects such as earwigs [see UW Bulletin A3640, Controlling Earwigs (available at https://learningstore.extension.wisc.edu/)], centipedes (see University of Wisconsin Garden Facts XHT1113, Centipedes), millipedes (see University of Wisconsin Garden Facts XHT1108, Millipedes) and sowbugs (see University of Wisconsin Garden Facts XHT1110, Sowbugs) can feed on decaying organic matter in mulches.  While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants.  If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.  Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches.  Typically, termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants [see UW Bulletin A3641, Controlling Carpenter Ants (available at https://learningstore.extension.wisc.edu/)] and powderpost beetles (see University of Wisconsin Garden Facts XHT1053, Powderpost Beetles) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch.  Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young.  Since wood mulch is composed of small wooden pieces, it would not serve as a home.  To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices that insects might use as entry points.  Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens?  Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases.  To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree.  There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see UW Plant Disease Facts D0045, Dutch Elm Disease), can serve as breeding areas for native and European elm bark beetles.  Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree.  Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see UW Plant Disease Facts D0075, Oak Wilt) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Bretziella fagacearum) and move it in the landscape.  This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs) can serve as a source of the fungus (Verticillium dahliae) that causes the disease.  These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants.  Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on wood mulch and tree health:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

White Mold

Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.
Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.

What is white mold?  White mold, also known as Sclerotinia stem rot, is a serious and typically lethal fungal disease that affects over 400 species of plants in many plant families.  White mold causes severe damage in commercially grown snap beans, kidney beans, lima beans and soybeans (see UW Plant Disease Facts D0099, Sclerotinia Stem Rot), as well as commercially grown sunflowers.  White mold also can be a serious problem in home vegetable gardens on tomatoes, potatoes, cucurbits (e.g., cucumber, pumpkin and squash), carrots, lettuce, celery, brassicas (e.g., cabbage, broccoli, and cauliflower), basil, and rosemary.  White mold is a common disease on many annual and perennial ornamentals as well.

What does white mold look like?  Symptoms of white mold vary depending on the plant infected.  White mold can lead to crown and stem cankers (i.e., localized infected areas), root rots, wilts, damping-off of seedlings, and blossom and fruit rots.  Plants affected by white mold can wilt rapidly due to stem-girdling cankers at or near the soil line.  Brown spots can appear on flower petals and buds.  Cottony masses of fungal threads (called mycelia) may appear on stems or on nearby soil.  Hard, irregularly-shaped masses (called sclerotia) develop within or on the surface of infected plants.  The sclerotia are white at first and then turn dark brown or black as they mature, resembling mouse or rat droppings.

Where does white mold come from?  White mold is caused by several species of the fungus Sclerotinia, most commonly Sclerotinia sclerotiorum.  White mold fungi overwinter as sclerotia in dead plant material or in infested soil.  Sclerotia can survive in soil for at least five years.  During periods of cool, wet or humid weather, sclerotia germinate to form either fungal threads (called hyphae) or tiny, mushroom-like spore-producing structures (called apothecia).  Apothecia can release millions of spores over a period of several days.  Spores are dispersed primarily by wind, but also by rainsplash and insects.  Both spores and hyphae can infect plant tissue, with infection often occurring through dead or declining plant parts (e.g., flowers, leaves), and then spreading to healthy tissue.  Infections may also occur through healthy plant parts that are growing near or in contact with the soil.  In some plants, seeds can be contaminated by sclerotia or mycelia of white mold fungi.

White mold can be destructive on ornamentals such as Liatris.
White mold can be destructive on ornamentals such as Liatris.

How do I save plants with white mold?  White mold is difficult to manage once infections have occurred.  Prune four to five inches below obviously disease tissue or remove entire plants if they are severely affected.  DO NOT compost these materials because of the risk of spreading white mold fungi via long-lived sclerotia.  Burn these materials instead.

How do I avoid problems with white mold in the future?  Check seed for sclerotia and remove these structures if you find them.  Make sure your soils are well-drained, and avoid overwatering as well as frequent, light watering.  DO NOT overhead water (e.g., with a sprinkler).  Instead water deeply early in the day with a drip or soaker hose.  Promote good air circulation in your garden so that plants more rapidly dry when they do get wet (e.g., from natural rain).  Better air circulation can be achieved by choosing cultivars or varieties of plants that have a more open growth form, by spacing plants farther apart, by avoiding excessive use of nitrogen fertilizers (which can promote excessive production of lush, thick foliage), and by keeping weeds under control.  Weed control also eliminates potential alternate hosts that white mold fungi can infect.  As leaves and flowers or your plants naturally wither and die, remove them, as well as any other plant debris that may harbor white mold fungi.  Finally, in beds where severe cases of white mold have occurred, consider removing and replacing infested soil.

For more information on white mold:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Fact Sheet

Tobacco Rattle

What is tobacco rattle?  Tobacco rattle is a common and potentially serious viral disease that affects a variety of herbaceous ornamentals including, but not limited to, astilbe, bleeding heart, coral bells, daffodil, epimedium, gladiolus, hyacinth, marigold, tulip and vinca.  Tobacco rattle can also affect vegetable crops such as beans, beets, peppers, potatoes, and spinach.  On potatoes, the disease is referred to as corky ring spot (see UW Plant Disease Facts D0034, Corky Ring Spot) and can make affected potato tubers unmarketable.

Tobacco rattle virus-infected plants often have leaves with yellow line patterns.
Tobacco rattle virus-infected plants often have leaves with yellow line patterns.

What does tobacco rattle look like?  Symptoms of tobacco rattle can vary depending on the plant species or variety/cultivar affected, and depending on environmental conditions.  Infected plants may exhibit mottling (i.e., blotchy light and dark discoloration of leaf tissue), yellow ring spots or line patterns, localized chlorotic (i.e., yellow) spots or streaks, necrotic lesions (i.e., dead spots), or leaves with notches.  Leaf discoloration symptoms are often quite attractive and can be misidentified as variegation, a natural variation in leaf color due to plant genetics.  Plants that become infected with TRV at a young age can exhibit a variety of leaf and stem deformities.  In potato, the disease typically manifests itself as a series of necrotic, corky spots, arcs or rings in tuber tissue.

Where does tobacco rattle come from?  Tobacco rattle is caused by the Tobacco rattle virus (TRV).  The virus is often introduced into a landscape in infected plants.  Once established in a location, the virus can be spread to other plants by stubby-root nematodes, a group of microscopic, worm-like organisms in the genera Trichodorus and Paratrichdorus.  These nematodes feed on the roots of infected plants, acquiring TRV, then move to non-infected plants where their subsequent feeding spreads the virus.  TRV also can be spread from plant to plant mechanically when pruning tools (e.g., shovels, trowels, knives, pruners) that are used to trim or divide plants become contaminated with the sap from infected plants.  Grafting is another means by which the virus can be transmitted.  Finally, TRV can be found in seeds from infected plants.

How do I save a plant with tobacco rattle?  Once plants have become infected with TRV, they remain infected indefinitely.  Infected plant cannot be treated in any way to eliminate the virus.  They should be removed and disposed of by burning (where allowed by local ordinance), burying or hot composting.

Ringspot symptoms on epimedium due to tobacco rattle virus. Photo courtesy of Anette Philbbs of the WI DATCP.
Ringspot symptoms on epimedium due to tobacco rattle virus. Photo courtesy of Anette Philbbs of the WI DATCP.

How do I avoid problems with tobacco rattle in the future?  The best way to prevent problems with tobacco rattle is to grow plants that are not susceptible to TRV.  Such plants include, but are not limited to, annual phlox, carnation, sweet William, zinnia and plants in the genus Datura (e.g., devil’s trumpet, downy thorn-apple, zombie cucumber).  Before buying either annual or perennial ornamentals, carefully inspect plants susceptible to TRV for symptoms caused by the virus.  DO NOT buy symptomatic plants.  Keep in mind however, that infected plants may not show symptoms at all times.

To prevent mechanical spread of TRV when pruning or dividing plants, decontaminate tools by treating them for a minimum of one minute with:

  • 2.75 tablespoons Alconox® (a lab detergent) plus 2.5 tablespoons 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 decide to use SLS (SDS), be sure to wear gloves, safety goggles and a dust mask, and mix the solution in a well-ventilated area as SLS (SDS) is a known skin and eye irritant.  Once treated, rinse tools with sufficient water to remove any residues.  Also, when working with plants, thoroughly wash your hands on a regular basis with soap and water to deactivate any of the virus you may pick up on your hands.

Control of the nematodes that transmit TRV is not practical in home garden settings and is not recommended.

For more information on tobacco rattle:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Fact Sheet

Powdery Mildew – Herbaceous Ornamental

Plants such as phlox, Monarda and zinnia are very susceptible to powdery mildew.
Plants such as phlox, Monarda and zinnia are very susceptible to powdery mildew.

What is powdery mildew?  Powdery mildews are diseases that occur on the above-ground parts (especially the leaves) of many herbaceous ornamentals, as well as deciduous trees and shrubs, indoor houseplants, and many agricultural crops.  Conifers are not affected by these diseases.

What does powdery mildew look like?  The name of these diseases is descriptive.  The upper and lower surfaces of leaves, as well as stems of infected plants, have a white, powdery appearance.  They look as though someone has sprinkled them with talcum powder or powdered sugar.

Where does powdery mildew come from?  Powdery mildews are caused by many closely related fungi that survive in plant debris or on infected plants.  These fungi are fairly host specific.  The powdery mildew fungus that infects one type of plant (e.g., phlox) is not the same powdery mildew fungus that infects another (e.g., lilac).  However, if you see powdery mildew on one plant, then weather conditions (high humidity) are favorable for development of powdery mildews on a wide range of plants.

How do I save a plant with powdery mildew?  DO NOT panic!  For many plants, powdery mildews are cosmetic, non-lethal diseases.  For other plants [e.g., phlox, beebalms, zinnia (see University of Wisconsin Garden Facts XHT1175)], powdery mildews can cause severe leaf loss.

Annuals such as zinnias are also very susceptible to powdery mildew.
Annuals such as zinnias are also very susceptible to powdery mildew.

When a highly valued plant has had severe leaf loss due to powdery mildew for several years, you may want to consider using a fungicide for control.  Fungicides containing chlorothalonil, copper, mancozeb, myclobutanil, triadimefon, sulfur or thiophanate-methyl are registered for powdery mildew control.  A combination of baking soda (1½ tablespoons) and a light weight (i.e., paraffin-based) horticultural oil (3 tablespoons) in water (1 gallon) has also been shown to be effective.  Most products should be applied every seven to 14 days from bud break until humid weather subsides.  DO NOT use myclobutanil, triadimefon, or thiophanate-methyl as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other listed active ingredients to help minimize problems with fungicide-resistant strains of powdery mildew fungi.  DO NOT alternate myclobutanil and triadimefon as these active ingredients are chemically related.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.  Also consider pretesting any product that you decide to use on a small number of leaves or plants before treating a larger area to make sure there are no toxic effects, particularly when treating during warmer weather.

How do I avoid problems with powdery mildew in the future?  Consider buying plant varieties that are powdery mildew resistant.  This will not guarantee that your plants will be powdery mildew free every year, but should result in less severe disease when it occurs.  Reduce the humidity around your plants by spacing them further apart to increase air flow.  Be sure not to over-water as this can lead to higher air humidity as well.  Finally, at the end of the growing season, remove and destroy any infected plant debris as this can serve as a source of spores for the next growing season.  You can burn (where allowed by local ordinance), bury or hot compost this material.

For more information on powdery mildew:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Root-Knot Nematode

Root-knot nematodes (Meloidogyne spp.) are small, soilborne, worm-like organisms that infect many agricultural and horticultural plants.  Root-knot nematodes are found worldwide, and are named for the swellings (called “galls” or “knots”) that they cause on plant roots.  Economically-important species of Meloidogyne include M. arenaria, M. hapla, M. incognita, and M. javanica.  Of these, M. hapla (commonly known as Northern root-knot nematode) is most likely to be found in Wisconsin soils.

Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.
Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.

Appearance:  Root-knot nematodes are about 1/10 the size of a pinhead and are typically embedded inside roots.  They are impossible to see with the naked eye.  Juvenile root-knot nematodes (both males and females), as well as adult males, are vermiform (i.e., worm-shaped) and live in the soil.  Adult females are spherical in shape and live inside roots.  Both males and females possess a thin, tube-like structure called a stylet that they use for penetrating root tissue.

Symptoms and Effects:  When root-knot nematodes enter roots, they release chemicals that cause nearby root cells to enlarge.  This leads to the formation of swollen, distorted areas in roots known as galls or knots.  The number and size of galls varies depending on plant species and cultivar, and the number of root-knot nematodes in the soil.  On some hosts (e.g., grasses) root swelling can be very difficult to detect.  Nematode feeding interferes with proper root function (e.g., water and nutrient movement).  Thus, infected plants may be stunted and wilted, may exhibit discolorations (e.g., yellowing) typical of plants with nutrient deficiencies, and ultimately (in field or vegetable crops) may have lower yields.  Because root-knot nematodes tend not to be uniformly distributed in the soil, symptomatic plants often occur in patches and are typically surrounded by plants of normal height and appearance.  Environmental factors such as slope, soil type, or soil moisture can cause similar patchy patterns, so identification of a root-knot nematode problem requires examination of symptomatic plants at a lab qualified to perform nematode diagnostics.

Life Cycle:  Root-knot nematodes (i.e., M. hapla) are native to Wisconsin and can be spread whenever contaminated soil or infected plants are moved.  Root-knot nematodes survive the winter as eggs in the soil.  Like insects, root-knot nematodes have several juvenile stages and the nematodes molt (i.e., shed their outer layers) as they grow.  The second juvenile stage of root-knot nematode is the most important, because at this stage the nematode seeks out and infects plant roots.  Once it has entered a root, a root-knot nematode molts three more times before becoming an adult.  A male root-knot nematode is able to move about freely and can leave a root.  A female root-knot nematode remains and feeds in a given location within a root.  Eventually, a female enlarges to the point where a portion of her body extends to the root surface and this allows her to lay her eggs in the soil.  In some hosts, eggs can also be found within the galls.

Control:  If you are having a root-knot nematode problem in your garden, consider crop rotation and the use of cover crops as management tools.  See University of Wisconsin Garden Facts XHT1210, Crop Rotation in the Home Vegetable Garden and XHT1209, Using Cover Crops and Green Manures in the Home Vegetable Garden for details.  If used properly, these techniques can be effective in reducing the number of root-knot nematodes in the soil.  M. hapla, the root-knot nematode species most common in Wisconsin, does not infect corn, wheat, oats or rye, so use of these crops in a rotation or as cover crops often provides great benefit.

Cover crops of French marigolds (Tagetes patula) also have been shown to reduce the number of root-knot nematodes in soil.  This common garden ornamental releases a chemical (alpha-terthienyl) that is highly toxic to root-knot nematodes and prevents their eggs from hatching.  As an added bonus, root-knot nematodes are not able to develop properly in marigold roots.  When using crop rotation or cover crops, proper broadleaf weed control is critical because weeds can provide a place for root-knot nematodes to survive and reproduce.

Finally, consider amending the soil in your garden with organic matter such as compost or leaf mulch.  Such amendments tend to increase the diversity of microorganisms in the soil and can encourage the growth of certain soilborne fungi that ensnare and feed on root knot nematodes, and parasitize their eggs.

For more information on root-knot nematode:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Root Rots in the Garden

Brown discoloration of roots typical of root rots.
Brown discoloration of roots typical of root rots.

What is root rot?  Root rot is a general term that describes any disease where the pathogen (causal organism) attacks and leads to the deterioration of a plant’s root system.  Most plants are susceptible to root rots, including both woody and herbaceous ornamentals.  Root rots can be chronic diseases or, more commonly, are acute and can lead to the death of the plant.

What does root rot look like?  Gardeners often become aware of root rot problems when they see above ground symptoms of the disease.  Plants with root rot are often stunted, wilted, or have top-down dieback.  They may also have leaves with a yellow or red color, suggesting a nutrient deficiency.  Examination of the roots of these plants reveals tissue that is soft and brown.

Where does root rot come from?  Several soil-borne water molds (i.e., fungi-like organisms) and true fungi can cause root rots, including (most frequently) Phytophthora spp. and Pythium spp. (both water molds), and Rhizoctonia solani and Fusarium spp. (both true fungi).  These organisms have wide host ranges, and prefer wet soil conditions.  Water mold root rot organisms such as Pythium and Phytophtora produce thick-walled spores (called oospores) that can survive for long periods (years to decades) in soil.

How do I save a plant with root rot?  REDUCE SOIL MOISTURE!  Provide enough water to fulfill a plant’s growth needs and prevent drought stress, but DO NOT over-water.  Remove excess mulch (greater than four inches) that can lead to overly wet soils.

Stunting, top-down dieback, and red or yellow foliage can indicate a root rot problem.
Stunting, top-down dieback, and red or yellow foliage can indicate a root rot problem.

Chemical fungicides (PCNB, mefenoxam, metalaxyl, etridiazole, thiophanate-methyl and propiconazole) and biological control agents (Gliocladium, Streptomyces, and Trichoderma) are labeled for root rot control.  However, DO NOT use these products unless you know exactly which root rot pathogen(s) is(are) affecting your plants.  Contact your county Extension agent for details on obtaining an accurate root rot diagnosis and for advice on which, if any, fungicides you should consider using.

How do I avoid problems with root rots?  Buy plants from a reputable source and make sure they are root rot-free prior to purchase.  Establish healthy plants in a well-drained site.  Moderate soil moisture; add organic material (e.g., leaf litter or compost) to heavy soils to increase soil drainage, and DO NOT over-water.  Provide just enough water to fulfill a plant’s needs for growth and prevent drought stress.  Also, DO NOT apply more than three inches of mulch in flowerbeds.  Excessive mulching can lead to over wet soils, which favor root rot fungi growth and reproduction.  Finally, minimize movement of root/crown rot fungi in your garden.  DO NOT move soil or plants from areas where plants are having root rot problems.  DO NOT water plants with water contaminated with soil (and thus potentially with root rot organisms).  After working with plants with root rot, decontaminate tools and footwear by treating for at least 30 seconds with a 10% bleach solution or 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants).  If you use bleach to decontaminate metal tools, be sure to thoroughly rinse and oil your tools after you are done gardening to prevent rusting.

For more information on root rots:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Impatiens Necrotic Spot (INSV)

Ringspots on coleus leaf caused by impatiens necrotic spot virus. (Photo courtesy of Margaret Daughtrey)
Ringspots on coleus leaf caused by impatiens necrotic spot virus. (Photo courtesy of Margaret Daughtrey)

What is impatiens necrotic spot?  Impatiens necrotic spot is a viral disease that causes considerable losses to greenhouse-grown ornamentals and, to a lesser extent, vegetable crops.  In the southern United States, impatiens necrotic spot can also be a problem on field crops.  Ornamental crops affected by impatiens necrotic spot include impatiens, gloxinia, cineraria, cyclamen, exacum, petunia, begonia, primrose and ranunculus.  Susceptible vegetables include cucumber, tomato and pepper.  Impatiens necrotic spot also affects weeds including chickweed, jewelweed, oxalis, and gill-over-the-ground.

What does impatiens necrotic spot look like?  Symptoms vary from species to species and from cultivar to cultivar.  Environmental conditions can also influence symptom development.  Infected plants may exhibit chlorotic or necrotic spotting; stem, vein or growing point necrosis; ringspots, mosaic or line patterns on leaves; color break in flowers; wilting and collapse.  Plants that are infected at a young age tend to exhibit more severe symptoms than those infected at an older age.  Some plants may remain symptomless but still can be a source of the causal virus, leading to infection other plants.

Where does impatiens necrotic spot come from?  Impatiens necrotic spot is caused by the Impatiens necrotic spot virus (INSV).  The primary means of plant-to-plant spread of INSV is by the western flower thrips (Frankliniella occidentalis).  This insect acquires INSV when feeding on infected plant sap, carries the virus to its next feeding site, and transmits the virus to the new plant after 15 to 30 minutes of feeding.  Only a few thrips are needed to spread INSV rapidly throughout a greenhouse.  Numbers of INSV-infected plants also can be increased simply by taking vegetative cuttings from infected plants.

How do I save a plant with impatiens necrotic spot?  After infection, INSV spreads throughout a plant.  Therefore plants remain infected indefinitely.  Infected plants cannot be treated to remove the virus and should be destroyed by burning (where allowed by local ordinance), burying or composting.

Symptoms of impatiens necrotic spot on Gloxinia. (Photo courtesy of Margaret Daughtrey)
Symptoms of impatiens necrotic spot on Gloxinia. (Photo courtesy of Margaret Daughtrey)

How do I control impatiens necrotic spot in the future?  Impatiens necrotic spot control focuses on excluding INSV and preventing its spread.  Inspect any plants entering a greenhouse (e.g., new plant shipments, plants moved in from outdoors) for viral symptoms and thrips.  Isolate new plants until they are determined to be thrips- and virus-free.  In commercial settings, isolate breeding and stock plants from production, and do not carry over plants from fall production into the spring.  Destroy any symptomatic plants.  Control greenhouse weeds, which can be symptomless reservoirs of INSV.  Monitor and control thrips populations.  Exclude thrips by screening greenhouse vents and doors (screening should have apertures that are ≤0.135 mm).  Set up yellow, white or blue sticky traps to monitor for thrips in growing areas and near greenhouse entry points.  Indicator plants such as Petunia x hybrida ‘Summer Madness’, ‘Super Magic Coral’, or ‘Red Cloud’ (with flowers removed) can provide early warning signs of a problem with thrips.  Petunia leaves on which INSV-free thrips feed will develop whitish feeding scars, while those leaves on which INSV-carrying thrips feed will develop small brown to black spots, turning tan with a dark border.  See also University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses, for additional pointers on thrips control, including insecticide recommendations.

For more information on impatiens necrotic spot and thrips:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Fact Sheet

Impatiens Downy Mildew

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.

White, fuzzy growth on the lower leaf surfaces of impatiens leaves is typical of impatiens downy mildew. (Photo courtesy of Kelly Ivors)
White, fuzzy growth on the lower leaf surfaces of impatiens leaves is typical of impatiens downy mildew. (Photo courtesy of Kelly Ivors)

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 your county Extension agent.

Disease - Herbaceous Ornamental, Fact Sheet

Hosta Virus X

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.

Blotchy leaf coloring (called mottling), discoloration along leaf veins and puckering of leaf tissue are typical symptoms of Hosta virus X. (Photo courtesy of Anette Phibbs)
Blotchy leaf coloring (called mottling), discoloration along leaf veins and puckering of leaf tissue are typical symptoms of Hosta virus X. (Photo courtesy of Anette Phibbs)

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 your county Extension agent.

Disease - Herbaceous Ornamental, Fact Sheet

Hollyhock Rust

Hollyhock rust causes orange to yellow spots on leaves of hollyhock and other plants in the mallow family. (Photo courtesy of Stephanie Porter)
Hollyhock rust causes orange to yellow spots on leaves of hollyhock and other plants in the mallow family. (Photo courtesy of Stephanie Porter)

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

Brown to dark-red bumps on the undersurface of leaves is typical of hollyhock rust. (Photo courtesy of Tom Creswell, Purdue University)
Brown to dark-red bumps on the undersurface of leaves is typical of hollyhock rust. (Photo courtesy of Tom Creswell, Purdue University)

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 agent.