Managing impatiens downy mildew in the landscape
Impatiens downy mildew is a disease caused by a type of mold (Plasmopara obducens). It results in impatiens losing their leaves and flowers. Impatiens downy mildew was first observed in Minnesota in 2011. Nursery and landscape impatiens are affected by the disease.
- Leaves first appear light yellow or stippled yellow & green.
- Leaf edges curl downward; leaves appear wilted.
- Fluffy white growth occurs on the lower surface of leaves.
- Blossoms drop first. Next leaves fall off leaving bare green stems. Finally stems collapse and lay flat on the ground.
- Plants infected young are stunted and have small leaves.
- In very wet conditions, infected plant tissue becomes water soaked, soft and mushy.
All varieties of Impatiens walleriana and any hybrid with I. walleriana in its background are susceptible to impatiens downy mildew. Touch-me-not (I. balsamina) and several wild species of impatiens can also be infected. New Guinea impatiens (I. hawkerii) is highly resistant. Bedding plants of different genera are not susceptible to impatiens downy mildew.
Impatiens downy mildew is caused by Plasmopara obducens. This pathogen is not a true fungus but is a member of the Oomycota, often referred to as water molds. The pathogen can be introduced into a garden on infected transplants. In addition, impatiens planted into beds that were infected in previous seasons may become infected through oospores, long term resting structures produced by P. obducens. Although little is known about oospores of P. obducens, oospores of the closely related pathogen Plasmopara halstedii, which causes downy mildew of sunflower, can survive 8 to 10 years in the soil in zone 3. It is highly likely that P. obducens will survive the winter in infected beds in Minnesota.
Plasmopara obducens also produces sporangia (structures that produce spores) on the lower surface of infected leaves. Sporangia can be splashed short distances to spread from plant to plant and can also become airborne and travel long distances on moist air currents. Plasmopara obducens thrives in cool (63-73°F) moist conditions. Four hours of leaf wetness is necessary for sporangia to form. Under hot dry conditions, infected plants may show no symptoms of disease and produce no sporangia on the lower leaf surface.
What to plant in beds that have been infected with impatiens downy mildew
Do not plant Impatiens walleriana or any hybrid containing I. walleriana in previously infected beds. The disease will survive from one season to the next. Alternative plants include coleus, caladium, begonia, and New Guinea impatiens.
Plants for beds with no history of impatiens downy mildew
Impatiens can be planted into beds with no history of downy mildew but care should be taken to purchase disease free plants.
- Thoroughly inspect all transplants for yellowing foliage and downy growth on the lower leaf surface.
- Reject any plants with evidence of infection. Once a plant is infected, it cannot be cured.
- Choose a supplier that uses a regular fungicide treatment program to protect plants from downy mildew.
- Purchase transplants grown in colder northern states to further minimize the risk of receiving infected plants.
- Infection of landscape impatiens is still possible from windblown spores.
Reduce moisture and humidity
Space plants so that air moves easily between plants and leaves dry quickly. Set sprinkler irrigation for early morning watering and providing deep and infrequent irrigation to reduce leaf moisture. Avoid evening applications of sprinkler irrigation. Use drip irrigation if possible to keep foliage dry.
Several fungicides will protect plants from infection, but no fungicides will cure the disease once infection has occurred. Home gardeners should contract with a licensed pesticide applicator to manage impatiens downy mildew fungicide applications.
If disease appears
If infection is found, bag and remove the infected plants, any fallen leaves, blossoms and the closest neighbors. Remove the entire plant including roots. Infected plants can be placed in a compost pile that heats up to 148°F or brought to a municipal compost facility. Alternatively, plants can be buried in an area of the yard where impatiens will not be grown. Fungicides will not cure an infected plant and it is better to remove the plant to reduce spread of the pathogen to other impatiens in the area. At the end of the season completely remove all plant material to prevent overwintering of the pathogen.