`Impatiens walleriana’s, future as a shade tolerant annual flower is threatened by downy mildew (Plasmorpara obducems). The disease’s first appearance in New York was 2009. Last year several more states reported the disease. It is now widely distributed in many areas of NYS, including the Greater Rochester area.
Impatiens infected with downy mildew first exhibit yellowing leaves, similar to what looks like a nitrogen nutrient deficiency. The infected leaves appear white on the underside that is a layer of spores that can travel through the air, infecting other impatiens plantings. The leaves eventually fall, leaving pale green stems that melt away. Practically, there is no landscape treatment, according to Brian Eshenaur, Cornell IPM Plant pathologist. Container grown plants where the foliage is kept dry may delay infection.
As a systemic disease, it infects the entire plant and flower growers also find it very difficult to manage. Bill Chase, owner of Chase Greenhouses in Rush is encouraging his customers to select other annual flowers such as coleus, marigolds, petunias, fibrous begonias or New Guinea impatiens, (which are not susceptible to the disease). Chase plans to reduce the numbers of impatiens he offers for sale next year.
‘Impatiens are popular shade garden annuals’ says Walt Nelson, horticulturist with Cornell Cooperative Extension Monroe County. ‘Movement of cuttings across our Nation is making it difficult to combat the disease.’ He believes some growers will not offer impatiens next year rather than disappoint customers with disease prone plants.
Gardeners are advised by Nelson not planting impatiens in an area where impatiens were killed by the mildew, as the disease survives the winter in soil and will re-infect impatiens in that site. Hopefully future varieties will have disease resistance resulting from current plant breeding work. It will be several years before resistant varieties are available. Until then gardeners should celebrate diversity and plan on planting different annual flowers next spring.