Most gardeners are familiar with powdery mildew, those grayish white patches that resemble talcum powder gone wild. Appearing during warm, dry weather, it afflicts a wide variety of plants. While it doesn’t need moisture to grow, it does require high humidity to germinate its spores — and North Carolina has its share of high humidity. Conversely, it will not occur when leaf surfaces are wet.
It’s host specific, needing a particular host plant if it is to survive so the powdery mildew that attacks Monarda, (bee balm), will not infect lilacs—and vice versa. The good thing about powdery mildew is that as awful as it looks, the fungal threads only grow on the surface of the leaf, never invading the plant’s tissues.
Take note when your plants develop powdery mildew: Those plants, like lilacs, that develop it late in the growing season will survive this fungal infection. My Phlox paniculata ‘John Fanick’ acquires it after it has bloomed at a time when I will cut it back. Conversely, those plants, such as begonias, that suffer from it early on in the growing season will display poor growth throughout the growing season.
There are certain things that we gardeners can do to cut down on the production of powdery mildew. Phlox and Monarda are two species that are highly susceptible to it so choose cultivars that have shown demonstrated resistance to it. Phlox paniculata ‘David’, ‘Delta Snow’, and ‘Nicky’ are three that do well. P. paniculata ‘John Fanick’ will succumb to it—but only later in the growing season. Many bee balms are now advertised as being resistant.
Avoid late summer applications of nitrogen, as new plant growth is more susceptible to infection. Practice good sanitation by destroying all infected plants and debris. Do not compost: the fungal spores can survive the winter. Provide good air circulation.
There are some chemical controls, such as sulfur, neem oil, and fungicides but I must confess I have never used them, as I find avoiding those plants that are prone to develop it is an easier path.
Researchers are looking into the germicidal properties of milk as there is evidence that milky sprays can control powdery mildew. Brazilian farmers are now using a formula of one-part milk to nine-parts water in their weekly sprayings on zucchini. Melon growers in New Zealand are also spraying their vines with milk rather than fungicides to encouraging results: Somehow spraying edible foods with a milky formula is more appetizing than using fungicides.
My suggestion if you have persistent trouble with powdery mildew is to take out that particular perennial, replacing it with a resistant cultivar. My first bee balms were inflicted with it but the problem was solved with I replaced them. I keep Phlox paniculata ‘John Fanick’ because it performs beautifully for me the whole month of July while perfuming the air. When the mildew arrives, it is time to cut it back so I have learned to live with it.
By all means try the milky formula—and let me know if it works.
Absent from their gardens, Kit and Lise enjoy roaming our region exploring the intersection of horticulture and suburban living. More on Instagram @AbsenteeGardener or email: firstname.lastname@example.org.