What You Need to Know About Downy Mildew

Frequently Asked Questions

Q. What is Downy Mildew?

A. A foliar disease caused by a fungus-like organism known as water mold (plasmorpara obducens). Transmitted by windblown spores, there is no evidence of seed borne transmission. The airborne spores are short-lived. They will not survive long on dry inanimate objects, e.g. containers.

Q. Do I need to be worried about downy mildew affecting all of my plants?

A. No! This strain of Downy Mildew only affects Impatiens wallerina and Double Impatiens. It does not affect any other plant including Sunpatiens and New Guinea Impatiens.

Q. If I decide to use an alternative this season, what are my affordable options?

A. We have options for all locations! If you plant your impatiens in a sunny location, we are recommending Wave Petunias. They have a colour palette and spreading habit that rivals Impatiens. The ever-popular Fibre Begonias are an excellent choice for your shade location. For Sun/Shade locations in shades of red, we are suggesting you try Salvia Red. It is bright, bold and mass plantings are spectacular throughout the season.

Q. My Impatiens were beautiful last year, there were no problems and they looked very healthy. Is it safe for me to plant Impatiens again?

A. There is no guarantee either way. Downy Mildew spores are carried on air currents. The spores are easily dislodged from the underside of leaves. If a garden in your area is infected, there is a possibility spores will be transmitted.

Q. What environmental conditions favour Downy Mildew?

A. Wet foliage, cool (~60ºF) temperatures (especially at night), and moist or humid air are ideal conditions for disease development. Symptoms often show up after a period of heavy rainfall or prolonged wetness, usually in August.

Q. What chemical can I use to eradicate Downy Mildew if I find it in my Impatiens garden?

A. Due to the current regulations in Ontario, there are no chemicals available, at this time, for the gardener. However, the industry is currently involved in research to find solutions to aid our gardening community. We will provide updates on their progress as this becomes available.

Q. Is the disease more of a problem in beds that are in full shade?

A. Downy Mildew is a water mold. As the name implies, it likes and requires moisture to reproduce and cause new infections. Plants in heavily shaded locations where the leaves stay wet for extended periods will generally have a higher incidence and severity of disease because moisture promotes infection.

Disease tends to be worse in:

• Locations where leaves stay wet for extended periods of time;
• Very dense beds;
• Beds receiving overhead sprinkler irrigation, because the foliage does not dry quickly.

Plants in more open or sunnier areas with better air movement will generally have less disease because the length of time moisture remains on the leaves is reduced.

Q. If beds are cleaned up thoroughly, can Impatiens be safely planted there next year?

A. Impatiens replanted into beds with a history of Downy Mildew may be at higher risk of infection than Impatiens planted into beds with no history of the disease.

Q. What should I do with the infected plants that I remove?

• Avoid placing infected plants in the compost; spores may survive the winter;
• Bag and discard in a landfill if local regulations allow;
• Plants can also be buried to a depth below your till line.

Q. If Downy Mildew survives the winter in plant debris in the soil, is it safe to plant other flowering or foliage plants in affected beds next season?

A. The Downy Mildew strain infecting Impatiens attacks only Impatiens walleriana and a few species of wild impatiens. See our list for alternatives to plant in these locations.

Alternatives to Impatiens

Annuals:

Foliage, Ferns & Grasses:

Perennials:

We recommend the following Petunias for those who plant their Impatiens in a sunny location (6 hours of sun). These petunias offer the same impact of impatiens, having a wide variety of colours and fast spreading habit.