Downy mildew is a disease caused by fungal-like organisms that can affect a wide variety of popular bedding plants. While this fact sheet will focus primarily on common bedding plants, downy mildews affect many other commercially important crops.

The downy mildew pathogens that infect bedding plants are a range of mostly host specific species found in the genera Basidiophora, Hyaloperonospora, Peronospora, and Plasmopara. These organisms closely resemble fungi but, among other differences, have distinct cell wall compositions and modes of reproduction, which classifies them in a different taxonomical kingdom than fungi and are most closely related to brown algae. Downy mildew pathogens are obligate parasites, meaning they require a host plant to reproduce and cannot be cultivated in the laboratory.

A wide range of bedding plants are susceptible to downy mildew, including Impatiens, China aster, snapdragon, Coleus, spider flower, Lisianthus, Gerbera daisy, primrose, common sage, red salvia, Texas sage, Viola/pansy, Verbena, sweet alyssum, and garden stock. In North Carolina, Coleus downy mildew is the most common based on records in the NC State Plant Disease and Insect Clinic.

Most downy mildew pathogens are host specific. For example, Plasmopara obducens only infects impatiens and Peronospora antirrhini only infects snapdragons. Certain other downy mildew pathogens have a broad host range. For example, Hyaloperonospora parasitica, which infects both sweet alyssum and stock, infects many other crops in the Brassicaceae family.

The optimum environmental conditions for downy mildew is specific to each downy mildew pathogen. However, in general, disease is favored by cool temperatures and high humidity conditions (60 to 80°F and humidity greater than 85%). Under these conditions, the infection can progress rapidly.

Symptoms of downy mildew vary based on the host plant. Often the first symptom is chlorotic (yellowing or losing green pigment) or necrotic spots on leaves, but the severity and pattern can vary by host plant. For example, infected leaves of Coleus will exhibit larger necrotic areas (Figure 1), whereas the chlorotic spots on infected autumn sage leaves are typically smaller, patchier and more dispersed (Figure 2). Water-soaking or a greasy appearance also may be observed depending on the conditions or the host (Figure 3).

Downy mildew pathogens produce sporangia (spores) on the underside of leaves only and can help in diagnosing the disease (Figure 4, Figure 5). Sporulation can appear as patches of white, purplish, or gray fuzz. Other symptoms include leaf distortion and stunting, especially when the plant is infected at a young age. As the disease progresses, leaves may fall off and eventually the plant may die. However, the infection may also remain localized, depending on plant species, age, and health, as well as environmental conditions.

There are many other abiotic and biotic problems that can look like downy mildew, so it is important to contact your local extension agent and/or submit samples to the NC State Plant Disease and Insect Clinic for assistance.

Figure 1. Downy mildew on Coleus.

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Figure 1. Downy mildew on Coleus.

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Figure 2. Downy mildew on autumn sage

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Figure 2. Downy mildew on autumn sage

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Figure 3. Water-soaking symptoms on leaves of tickseed with downy mildew.

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Figure 3. Water-soaking symptoms on leaves of tickseed with downy mildew.

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Figure 4. Sporulation (white fuzz appearance) on leaves of tickseed infected with a downy mildew pathogen.

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Figure 4. Sporulation (white fuzz appearance) on leaves of tickseed infected with a downy mildew pathogen.

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Figure 5. Sporangia and sporangiophore (structure that bears sporangia) of the downy mildew pathogen that affects tickseed (Plasmopara halsteadii).

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Figure 5. Sporangia and sporangiophore (structure that bears sporangia) of the downy mildew pathogen that affects tickseed (Plasmopara halsteadii).

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Sporangia are the asexual spores of the downy mildew pathogen that infect plants. Sporangia can be spread by air currents (long distance) or water splash (short distance). The sporangia germinate directly and enter the plant through stomata (respiratory pore on the undersides of leaves). The pathogen can grow throughout the leaf tissue and produce sporangia through stomata creating masses of sporangia on the undersides of the leaves. These sporangia are then dispersed to neighboring plants and the cycle continues. Some members of the downy mildew pathogens may form sexual, survival structures (oospores) that can remain in the soil and plant debris through the winter. In the spring, or when conditions are conducive, the oospores germinate into sporangia which then can initiate a disease outbreak.

The downy mildew pathogens tend to be aggressive and, thus, there are few effective cultural practices to reduce disease.

• Select plants resistant to downy mildew. This is the most effective way to reduce disease. There are some commercially available varieties of bedding plants known to be resistant or less susceptible to downy mildew (e.g., some varieties of Coleus and pansy) or select plants that do not get downy mildew, such as New Guinea impatiens.
• Minimize humidity. If planting in a greenhouse, try to maintain humidity below 85%. If planting outdoors, improve air circulation by widening plant spacing, thinning, staking, and pruning plants. Use a drip irrigation instead of an overhead watering system, if possible. These practices may reduce the severity of disease, but will not eliminate it.
• Remove infected plant material to reduce disease in subsequent years. Removing infected plant material is only effective for the downy mildew pathogens that form the survival structure (oospores). Discard infected material in the trash rather than compost since the pathogen can remain in the soil and plant matter through the winter. It is important to inspect new plant material for symptoms to avoid introducing the pathogen to a new area.
• Remove weeds in surrounding areas. Nearby weeds may be hosts for the pathogen, but this varies by pathogen.

Homeowners should focus on taking preventative measures against downy mildew by following the recommendations above. There are no effective fungicides to manage downy mildew for homeowners.

In addition to the recommendations above, fungicides can be used to control downy mildew. The following table lists commercially available fungicides known to mitigate downy mildew (Bremia lactuae, Peronospora spp., Plasmopora viburni) on floriculture crops in greenhouses. For more information on the efficacy of the products, see table 10-11B in the NC Agricultural Chemicals Manual. Before applying any of these chemicals, consult the product label and take necessary precautions. Also, be sure to rotate fungicides in different FRAC groups to prevent the pathogen from developing resistance to the chemical.

The 2025 North Carolina Agricultural Chemicals Manual provides information on the selection, application, and safe use of chemicals.

The NC State University Extension Plant Pathology Website provides information on crop disease Management.

The Southeastern US Pest Control Guide for Nursery Crops and Landscape Plantings provides information regarding control products used in nursery crops, greenhouse crops, and ornamental landscapes.

The NC State University Plant Disease and Insect Clinic (PDIC) provides diagnostics and control recommendations.

Backerman, J. (2022). Diseases of Landscape Plants, Downy Mildew. Purdue University Extension.

Chase, A.R., Daughtrey, M.L, Cloyd, R. A. (2018). Compendium of Bedding Plant Diseases and Pests. APS Press. College of Agriculture and Life Sciences, North Carolina State University. (2024). 2024 North Carolina Agricultural Chemicals Manual. NC State Extension.

Dicklow, M.B., Madeiras, A. (2018). Downy Mildews of Ornamental Plants. UMass Extension Greenhouse Crops and Floriculture Program.

Hausbeck, M., Harlan, B., Lopez, R. (2020). Don’t let downy mildew on impatiens surprise you. Michigan State University Extension.

Pollard, A. (2023). Addressing Downy Mildew and Powdery Mildew in the Home Garden. PennState Extension.

Rossman, A.Y., Palm, M.E. (2006). Why are Phytophthora and other Oomycota not True Fungi?. Plant Health Instructor APS.