Downy mildew on impatiens is of great concern throughout Florida. Winters in south Florida provide high humidity paired with cool nights, creating ideal conditions for disease development. Downy mildews are caused by several different species of pathogens that tend to be plant-host specific. Plasmopara obducens is the pathogen that affects impatiens. Some literature indicates that nighttime temperatures of about 50°F to 72°F favor downy mildew, but the disease has been reported on impatiens year round in south Florida.
Downy mildew affects all hybrids and varieties of Impatiens walleriana (gardening impatiens), also called busy Lizzie. Other species of impatiens that are susceptible include Impatiens balsamina (balsam impatiens, garden balsam and rose balsam) and native wild impatiens Impatiens pallida and I. capensis (Jewelweed). There is currently no evidence for seed-borne transmission of the downy mildew pathogen on impatiens. Note that New Guinea impatiens, Impatiens x hawkeri, are considered very tolerant, and the disease is yet to be reported on this host in Florida.
Young plants and new growth are most susceptible and typically show symptoms first. Initially, leaves may look a little yellowish or speckled (Figure 1). In fact, these symptoms can look very similar to nutritional deficiencies. Faint gray lines may be seen on the tops of leaves, or leaf edges may curl downward (Figure 2). Sometimes the yellowing is not visible before leaf curling begins.
As the disease continues to progress, whitish, downy looking growth is visible on the undersides of leaves (Figure 3). This whitish growth consists of spore-containing structures that have emerged from the lower leaf pores (stomata). Next, leaves and flowers will drop quickly, leaving mostly stems (Figure 4).
Downy mildew is a common and destructive fungal disease that can plague balsam plants This disease thrives in damp, humid conditions and can quickly spread, causing extensive damage If left untreated, downy mildew can severely impact the health and appearance of balsam plants. Fortunately, there are several effective solutions for getting rid of downy mildew and restoring balsam plants to their former glory.
Identifying Downy Mildew on Balsam Plants
The first step is learning how to spot downy mildew on your balsam plants. Here are the key symptoms to look for:
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White Powdery Coating The most telling sign of downy mildew is a white, powdery coating visible on the upper leaf surfaces This powdery substance consists of fungal spores that can rapidly spread the disease.
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Yellowing Leaves: Infected leaves will turn yellow or brown and can eventually wilt and fall off.
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Stunted Growth: Plants plagued with downy mildew often exhibit stunted growth and have fewer blooms.
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Damp Conditions: Downy mildew thrives in damp, humid environments. Be extra vigilant after rain or when humidity levels are high.
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Leaf Curling: The edges of infected leaves may curl or distort.
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Leaf Spots: Small, water-soaked spots may appear on the leaves.
Carefully inspect your balsam plants regularly to catch downy mildew early before extensive damage occurs. Know what to look for and take quick action at the first signs of infection.
Removing Infected Parts
If you spot downy mildew on just a few leaves or stems, start by pruning away the infected parts. Use clean, sterilized pruners and cut well below any visible signs of the disease. Promptly remove and discard the infected plant material – do not compost it. Pruning can stop the disease from advancing if caught early.
Improving Air Circulation
One of the best defenses against downy mildew is improving air circulation around your balsam plants. Downy mildew thrives in stagnant, humid conditions. Boost airflow by thinning out any crowded vegetation and giving your balsams ample space. Trim back nearby plants to allow better air movement. Providing good ventilation deprives downy mildew of its preferred damp habitat.
Avoiding Overhead Watering
Similarly, avoid wetting the leaves when watering your balsam plants. Water that lingers on the foliage creates prime conditions for downy mildew growth. Instead, water balsam plants at the soil line rather than from overhead. If watering by hand, use a wand attachment to target the base of the plants. When using irrigation systems, water early in the day; this gives any moisture on the leaves time to dry out in the sun.
Applying Fungicides
For moderate or severe cases of downy mildew, the use of fungicides is recommended. There are many effective fungicides made specifically for combatting downy mildew on ornamental plants like balsams. Always carefully read and follow label directions when applying fungicides. Thorough coverage of both upper and lower leaf surfaces is key. Repeat applications may be needed at 7-14 day intervals for full disease control.
Trying Natural Remedies
For a more natural approach, various homemade treatments can also fight downy mildew. Options like neem oil, baking soda, and potassium bicarbonate solutions have proven antifungal characteristics. Use care when preparing and applying these DIY remedies. Test on a small area first and repeat applications are typically needed. These natural solutions may be slightly less effective than commercial fungicides.
Monitoring for Recurrence
Once you have treated your balsam plant for downy mildew, be on the lookout for any signs of the disease returning. Continue inspecting your plants routinely and have a treatment ready to apply at the first indication of infection. Maintain optimal growing conditions – proper spacing, air circulation, watering practices, etc. Staying vigilant is key to preventing a new outbreak.
Removing Severely Infected Plants
In severe cases where the majority of the plant is affected, removal may be your best option. Heavily infected plants likely will not recover, even with treatment. Pull up the entire plant, including all roots. Bag it securely and discard with the trash – do not compost. Severely infected plants can serve as a source of spores to spread downy mildew to healthy plants nearby. Completely removing them protects your other balsams.
Disinfecting Tools & Hands
Practicing good sanitation habits is crucial when dealing with downy mildew. The fungal spores can stick to tools and hands, spreading the disease as you garden. Routinely disinfect your gardening tools, pots, and pruning shears using a diluted bleach solution. Always wash your hands after handling infected plants. Meticulous sanitation prevents you from inadvertently transmitting downy mildew spores.
Removing Nearby Weeds & Debris
Weeds and plant debris around your garden can harbor downy mildew spores over winter. Removing these potential disease reservoirs can reduce the amount of innoculum that may infect your balsam plants next season. Keep the garden free of weeds. Clean up any dropped leaves and spent plants after the growing season ends. Tidy gardens are less likely to foster diseases like downy mildew.
Choosing Resistant Varieties
When shopping for balsam plants, look for cultivars labeled as downy mildew resistant or tolerant. Plant breeders are working to develop varieties that are less prone to this disease. Resistant selections will still require proper growing conditions but are less likely to succumb to downy mildew. As availability increases, planting resistant varieties will be a key prevention tip.
Using Preventative Fungicides
Preventative fungicide applications can stop downy mildew before it takes hold. If downy mildew has been an ongoing issue in your area, treat plants with a fungicide labeled for downy mildew control at the start of the season. Repeat per label instructions for ongoing protection. Preventative fungicides are especially important for susceptible varieties.
Growing From Seed
Some sources indicate plants started from seed may be less vulnerable to downy mildew than transplants. This is likely because fungal spores are not present right away when seeds are sown. If suitable growing conditions persist, however, downy mildew can still develop. But growing balsams from seed can be part of an integrated approach.
Increasing Spacing
Allowing ample space between your balsam plants improves air circulation and lowers humidity levels around the foliage. These conditions make it harder for downy mildew spores to thrive and spread. Wider spacing also reduces crowding so plants dry out faster after rain or watering. Proper spacing is an easy, effective way to make conditions less favorable for downy mildew development.
Staking Plants
Staking up tall or droopy balsam plants opens up air flow around the leaves and stems. Improved circulation makes it more difficult for downy mildew to take hold. Staking also prevents plants from sprawling on the ground where fungal spores may gather. Keep plants upright with properly sized stakes and hoop supports when necessary.
Timing of Planting/Pruning
Try to time planting or pruning balsams so that longer periods of warm, dry weather follow these activities. This allows any wounds time to heal before cool, damp conditions arrive which favor downy mildew. Quickly treat any pruning cuts with a sealant. If weather is consistently wet, delay major pruning jobs until drier periods.
Managing Irrigation
Careful irrigation management can create less conducive conditions for downy mildew development. Water early in the day so leaves have time to dry before nighttime. Soaker hoses or drip irrigation applied at soil level are preferable to overhead watering. Avoid excessive wetness on foliage. Let the soil partially dry out between waterings. Monitor soil moisture to guide your watering schedule.
Choosing Sunny Sites
Selecting a sunny, open garden location avoids shaded, more humid sites where downy mildew thrives best. Sunny spots allow better air movement around the plants and dry out quicker after rain or watering. Proper site selection sets the stage for growing vigorous, healthy balsams less prone to diseases like downy mildew. Full sun is ideal but a few hours of light afternoon shade is acceptable.
Removing Fall Foliage
Clean up any dropped leaves and other plant debris around your balsams in the fall. Removing this material eliminates a source of fungal spores that could overwinter and re-infect your plants next spring. Rake thoroughly and dispose of the debris – do not compost. Starting the growing season with a clean slate helps interrupt the disease cycle of downy mildew.
Avoiding Overcrowding
When planting your balsam bed or border, resist the urge to space plants too closely. Overcrowding prohibits air circulation and retains high humidity levels around the plants – a downy mildew welcome mat. Leave adequate room between plants for airflow and to allow sunlight penetration. Give balsams sufficient elbow room to thrive with lower
Life Cycle of the Pathogen
The pathogen that causes downy mildew is a type of water mold and more closely related to algae than to fungi. Other similarly classified pathogens include Phytophthora and Pythium spp. Downy mildew pathogens can spread by two different types of spores. One type, zoospores, moves through water. Sporangia that contain and can release zoospores are easily windborne. This explains why this disease is spread by splashing overhead irrigation, rainfall, and wind. The other type of spore, the oospore, forms inside plant tissues, where the pathogen can survive for years. Downy mildew is very aggressive and can rapidly spread, so action should be taken quickly if it is found.
It is critical that high levels of sanitation be maintained in the nursery. Plugs or any other Impatiens walleriana plants brought into the nursery should be carefully inspected and rejected if they appear diseased. Hard surfaces in the growing area between crops should be sanitized with quaternary ammonia compounds (e.g., Greenshield, KleenGrow, etc). All impatiens plant tissue should be removed between crops and should not be composted. Plant debris should be disposed of in dumpsters that are emptied off-site.
Frequent scouting for this pathogen should be conducted, especially as evening temperatures cool. High relative humidity in Florida makes downy mildew extremely difficult to control. Plants should be spaced to allow as much air movement between them as possible to hasten leaf drying after irrigation or rainfall.
If affected plants are detected, they should be quickly removed and any fallen leaves and flowers from growing areas should be disposed of off-site. Treat remaining plants with preventive fungicides as indicated in Table 1. Wash hands thoroughly after handling problematic plants and before handling non-affected impatiens.
This pathogen may erupt under high humidity, cool temperatures, and overcrowding. Overcrowding reduces the wind movement between plants that helps leaves to dry more quickly. Prevention is the only effective management strategy against downy mildew. Elimination of overhead and nighttime watering, excessive fertilization, overcrowded planting beds, and other stresses on the plants would all be very helpful in avoiding or reducing occurrence of this pathogen. However, our tightly arranged mass planting practices in the landscape, coupled with whole-farm irrigation systems and periodic rainfall often make ideal conditions difficult to achieve. Inspect impatiens leaves and stems before purchasing plants for the landscape and before installing them, and reject any affected plants. Scouting will not entirely eliminate the risk of downy mildew because windborne spores can still infect healthy plants, but it is always better to avoid introducing the pathogen to the landscape on new plant materials.
Impatiens showing early symptoms of downy mildew should be removed and thrown away off-site, and any surrounding impatiens should be treated with a protective fungicide. When pulling out an infected impatiens landscape planting, attempt to remove as much plant tissue as possible from the planting bed, which will help to reduce inoculum of the pathogen (e.g., oospores). Consider replacing heavily diseased plantings of impatiens with another bedding plant species that is not susceptible to downy mildew.
Resistance to a fungicide occurs when a pathogen develops a genetic mutation at the target site that reduces its sensitivity to a specific fungicide. Using a single fungicide repeatedly over time suppresses only the portion of the pathogen population sensitive to that fungicide, with the result that only the resistant portion of the population remains. This population will go on to reproduce and become the majority. Eventually, the fungicide becomes ineffective because the majority of the population is no longer susceptible to it. If a pathogen is resistant to a fungicide, it is usually resistant to all fungicides in that class.
To minimize the potential for fungicide resistance, the Fungicide Resistance Action Committee (FRAC) provides codes to facilitate proper rotation of chemical families that have similar modes of action. FRAC codes make proper fungicide rotation easy and are included with the chemical recommendations in this fact sheet (Table 1). When purchasing fungicides for managing downy mildew, fungicides with different FRAC codes should be selected and the manufacturers label followed.
The landscape management recommendations described above should be followed and the fungicides listed in Table 2 can be used.
For the management of downy mildew in the nursery and landscape, professional use fungicides are listed in Table 1. Table 1.
Fungicides for use in managing downy mildew by commercial operations.
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Products (active ingredients) |
FRAC Group2 |
Use1 |
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Subdue Maxx (mefenoxam) |
4 |
N, L & G |
|
Fenstop (fenamidone) Compass (trifloxystrobin) Disarm (fluoxastrobin) Heritage (azoxystrobin) Pageant Intrinsic (pyraclostrobin + boscalid) |
11 |
G N, L & G N, L & G N, L & G N, L & G |
|
Segway (cyazofamid) |
21 |
N, L & G |
|
Aliette (fosetyl-Al) Alude (potassium salts of phosphorous acid) Fosphite Fungicide (potassium phosphite) |
33 |
N, L & G N, L & G N, L & G |
|
Micora (mandipropamid) |
40 |
N & G |
|
Orvego (ametoctradin + dimethomorph) |
45 + 40 |
N & G |
|
Adorn (fluopicolide) |
43 |
N, L & G |
|
Cease or Rhapsody (Bacillus subtilis strain QST 713) Double Nickel (Bacillus amyloliquefaciens strain D747) |
44 |
N, L & G |
|
Segovis (oxathiapiprolin) |
U15 |
N, L & G |
|
Badge (copper hydroxide + copper oxychloride) Champ (copper hydroxide) COC (copper oxychloride) Cuprofix Ultra 40 Disperss (basic copper sulfate) |
M1 |
N, L & G N & G N, L & G N, L & G |
|
Protect (mancozeb) |
M3 |
N, L & G |
|
1N= Nursery; L= Landscape; G= Greenhouse 2 Fungicides within the same group (with same numbers or letters) indicate same active ingredient or similar mode of action. Additional Control: Actinogrow (Streptomyces lydicus WYEC 108), Soilguard (Gliocladium virens G1-21), Sporatec (clove oil+rosemary oil+thyme oil) and Trilogy (neem oil) may also provide protection against some forms of downy mildew in Florida landscapes, nurseries and greenhouses when applied prior to infection. |
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Fungicides for use in managing downy mildew in home landscapes.
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Non-Commercial Homeowner Products |
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Concern Copper Soap Fungicide (copper octanoate) |
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Ferti-lome Broad Spectrum Landscape and Garden Fungicide, Ortho Max Garden Disease Control (chlorothalonil) |
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Monterey Agri-Fos (phosphorous acid) |
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Southern Ag Liquid Copper Fungicide (copper ammonium complex) |
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Southern Ag Triple Action Neem Oil (extract of neem oil) |
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Remember, the label is the law; be sure to use products only in a manner consistent with the manufacturer directions on the labels. Please use pesticides safely. |
Release Date:November 9, 2020
Reviewed At:May 30, 2024
- DOI: https://doi.org/10.32473/edis-pp309-2013
- Critical Issue: 1. Agricultural and Horticultural Enterprises
The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition.
This document is PP309, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date September 2013. Revised May 2020. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.
N. A. Peres, professor, Plant Pathology Department, UF/IFAS Gulf Coast Research and Education Center; L. A. Warner, assistant professor, social marketing and program evaluation, Department of Agricultural Education and Communication; UF/IFAS Extension, Gainesville, FL 32611.
Prevent & Treat Powdery Mildew and 4 Home Remedies that Work!!
FAQ
Can plants recover from downy mildew?
What is the best remedy for downy mildew?
Can downy mildew be reversed?
How do I get rid of powdery mildew permanently?
How do you control downy mildew?
The best control of downy mildew is to make sure that your plants do not get it in the first place. Since downy mildew needs water to survive, the very best thing you can do to prevent downy mildew is to water your plants from below. Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant.
How does downy mildew spread?
Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant. The spore of downy mildews spreads by literally swimming through water until they come across live plant material to infect. If there is no water on your plant leaves, the downy mildew cannot travel to or infect your plants.
What is the best defense against downy mildew?
The best defense against Downy Mildew is to prevent it from taking hold in the first place: Choose Resistant Varieties: Some plant breeds have been developed with resistance to Downy Mildew. Proper Plant Spacing: Allowing proper air circulation reduces humidity around the plants.
What is downy mildew & how does it affect my garden?
Downy mildew grows on and into the leaves of preferred plants, living off the plant’s water supply. While small amounts of downy mildew don’t do major harm to your garden, they’re a symptom of a larger issue. Many food crops and some flowers and shrubs are susceptible to this infection.