Q. I held off planting impatiens last year in hopes that I could start using them again this year. Unfortunately, the downy mildew that spoiled my impatiens two years ago seems to have spoiled them again this year. Will I ever be able to enjoy them again?
A. Downy mildew hit Western Pennsylvania gardens with a vengeance in 2012, and it continues to make it difficult to grow standard impatiens (Impatiens walleriana). The causal organism, Plasmopara obducens, can lie dormant in soils for many years. It may have remained in your garden from 2012, or it could have come in on infected transplants, or the spores could have blown in on the wind.
While there are many plants – annuals and perennials – that can be grown in shade, none of them comes close to providing the riot of color that impatiens bring to the shade garden. Given the importance of standard impatiens to gardeners and landscape professionals alike, land grant universities and plant breeders are working to develop resistant varieties. Right now you are better off growing other shade-tolerant plants to avoid disappointment.
The first symptoms of downy mildew on impatiens that most growers notice include yellowing and stunting of infected plants. Their leaves turn yellow and start to drop until all that is left is bare stems with a few pitiful leaves hanging on. The disease often starts with discrete yellow spots on the upper leaf surface that most people would barely notice. However, if you turn those leaves over and examine the underside, you would see the fuzzy white growth characteristic of downy mildew. Infected plants are reduced to barren stalks as they lose their leaves, and even those finally die. Cool, humid weather with temperatures in the 58- to 72-degree range favor development of downy mildew.
While the disease can be prevented with fungicide applications, those that are most effective against this disease are not readily available to home gardeners. And fungicides are not effective once plants are infected. Control focuses on sanitation and planting resistant species next year. Remove and bag infected plants as soon as they are noticed to reduce the spread to healthy plants. Allow adequate space between plants at planting to permit good air circulation, and avoid overhead irrigation, especially during cool weather.
Shade-tolerant annuals that are resistant include New Guinea impatiens (I. hawkeri), begonia (Begonia spp.), caladium (Caladium bicolor) and coleus (Solenostemon spp.). Shade-tolerant perennials include astilbe (Astilbe spp.), hardy begonia (Begonia grandis subsp. evansiana), pigsqeak (Bergenia cordifolia), Siberian bugloss (Brunnera macrophylla), coral bells (Heuchera spp.), turtlehead (Chelone lyonii), bishop’s hat (Epimedium spp.), wild geranium (Geranium maculatum), hellebore (Helleborus spp.), cardinal flower (Lobelia cardinalis), variegated Solomon’s seal (Polygonatum odoratum var. pluriflorum ‘Variegatum’), and Allegheny foamflower (Tiarella spp.).
Send questions to Sandy Feather by email at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.