Impatien Downey Mildew is spreading throughout Canada and it has been indentified in our area. It is caused by a fungus-like pathagon that is specific to the host plant of garden variety impatiens (Impatien walleriania). It affects double impatiens and mini impatiens but does not affect New Guinea impatiens or other shade loving annuals.
Early symptoms include yellowing or yellow stippling of the leaves and downward cupping of upper leaves giving the appearance that plants need to be watered. As the disease progresses, flowers and leaves drop, eventually leaving bare stems with a few small leaves at the top. Finally, the stems completely collapse and the plant dies. After heavy rains, leaves may have a water-soaked appearance, resembling frost injury. Plants infected when young will be stunted.
During humid or rainy weather, a white coating of spores will be produced on the lower surface of some leaves. Turn over the leaves to look for the white coating of spores; not all leaves will have this. These spores are spread by wind, wind-driven rain, and splashing water. Cool night temperatures (58 to 62 ̊ F) which encourage heavy dews are ideal for disease development, even if it is hot and dry during the day. New infections occur when leaves stay moist for a few hours. Downy mildew tends to be worse in very dense plantings, where there is overhead irrigation and areas where leaves stay wet for extended periods of time. New infections develop as the short-lived spores are spread by water splash for short distances or by wind currents for longer distances. Another type of spore, a resting or survival spore (called an oospore), is produced within infected plants just before they die. For this reason, it is important to remove infected plants from garden beds as soon as you see them. Oospores are capable of surviving in the soil through the winter and can cause new infections of garden impatiens the following year.
To manage this disease, it is recommended to remove infected plants, including leaf debris and roots) to a bag and it should be disposed of immediately. Do not compost diseased plants! If infected plants remain in your garden or compost pile, there is a high risk the spores will overwinter and affect future plantings.
This disease is more common in gardens and flower beds, but since the spores move on wind, there is a chance that the disease may be in your container soil too. Fewer studies have been done on the impact of this disease to container plantings. This year, if you wish to try planting Impatiens in your containers, change your soil completely, change your coco mat (if used for wrought iron containers) and wash with a diluted bleach solution your ceramic, fibreglass or plastic pots. Fresh soil will be required and monitoring for the spores (see symptoms above) will be necessary. If it presents itself in your containers, removal at once will be necessary.
In your gardens and flower beds, if you have already experienced Impatien Downey Mildew (perhaps last year your plants did poorly?), you must use non-host plant choices for the next 1-2 seasons. These plant choices can include:
New Guinea Impatiens – colourful and will perform in shadier locations too, do not grow into a mass, will look like individual plants in the garden, suggested to mulch in between the plants for a finished look
Sunpatiens – a spreading New Guniea Impatien that has great coverage, can tolerate shadier locations too. More info: www.sunpatiens.com
Begonias – fibrous begonias, also called wax begonias (sold as a box plant) will grow into a mass if planted close together (4-6 inches apart), tuberous begonias (sold in a 4” pot) will be striking, but will not grow as a mass, suggest to also mulch them like the suggestion above for the New Guinea Impatiens
Caladiums – a summer bulb, striking leaves that are paper thin
Ferns – tropical Sword Fern (Kim Queen) or perennial varieties
Upright Lobelia – a spreading box plant that can tolerate shadier locations
Coleus – box plant or specialty varieties are available in 4” and 6” pots
Sweet Potato Vine – can actually be used in a flower bed! Spreads quickly
Iresine – a cousin to Coleus, it is bright and lovely in the shade
Perennials – hosta, lamium, coral bells (heuchera), tiarella, astilbe, anenome, iris, vinca minor, columbine, lady’s mantle, some ornamental grasses, Creeping Jenny and Solomon’s Seal
Adding perennials to your garden space will change the landscape, but maybe change is good! Many people are looking at this “problem” as an opportunity to try something new, to change it up a bit. The alternatives are more expensive; unfortunately nothing will beat the value one gets from a box plant Impatien. Rest assured, this disease is not something the gardening industry has fabricated to make more money! As growers and retailers, we really wish this was not happening. But, collectively, if we all take caution with our plantings for a year or two, we may be able to eradicate Impatien Downey Mildew from our local landscape and return to using them again in flower beds and containers.
