Master Gardener Coordinator’s Corner – Sep 2017

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Plant Identification

Powdery Mildews and Powdery Mil-Don’ts

Plant Identification

by Nate Walton, MSUE Master Gardener Coordinator, MSU Extension – Leelanau County

Epipactis helleborine. Photo from Minnesota Wildflowers

Have you seen this plant?  This is Epipactis helleborine, or helleborine, a terrestrial orchid that was introduced to Michigan from Europe sometime towards the end of the nineteenth century.  It has recently been reported as a problem weed in Michigan lawns and gardens.  It is quite a difficult weed to manage, so if it is in unwanted areas around your home you might want to take some measures to control it.

Removing the flower heads before they go to seed is a key control measure for this plant, so you’ll want to take care of it right away.  At my house in Traverse City, the helleborine started to bloom in July.  If you’d like to eradicate the plant altogether, digging it up roots and all is the recommended method.  However, the roots go deep and it will re-sprout from the rhizome if any pieces are left behind, so you will want to be thorough.

Photo by BerndH via Wikimedia Commons

For more information on identification and control of helleborine, read the MSUE article “Homeowners battling a weedy orchid invading lawns and flowerbeds,” which can be found at:


Powdery Mildews and Powdery Mil-Don’ts

Click on photo for enlarged image. Photo by Nate Walton

The high humidity this summer has made it a really good year for powdery mildew.  You have probably noticed a white coating with a powdery appearance on a variety of annuals and perennials this summer.  The powdery mildew pathogen is a fungus, so it’s related to mushroom producing fungi such as the Morchella spp. that produce morel mushrooms.  Unlike morels, however, the mycelia of powdery mildews do not grow in a network under the soil.  Instead, they grow over the upper surface of the plant leaves.

In the summer, this mycelium produces tiny spores (conidia) that give the leaf its white powdery appearance.  The spores can be spread from leaf to leaf or plant to plant by wind or splashing water.  Lucky for us, most powdery mildews are very host specific, so they will not spread from, for example, your rosebush to your lilac.

When powdery mildew is found on adjacent plants that are not closely related, it just means that both species are susceptible and that the conditions are right for mildew in that location.  Choosing resistant varieties or growing susceptible varieties in locations with good air circulation and plenty of sunlight are some ways to prevent powdery mildew from gaining a foothold in your garden.  Also, avoid overhead irrigation and growing susceptible varieties (e.g. Monarda spp.) in crowded and/or shady areas.

Beginning in late summer, the powdery mildew pathogen starts to get ready for winter.  To do this, it forms a tiny black structure called a chasmothecium.  Chasmothecia show up as tiny black spots on the leaves infected with powdery mildew (see photo).  It is these chasmothecia that will produce fresh spores next year to reinfect the green foliage in the spring.  Removing and destroying the leaf litter under infested plants will help reduce the amount of powdery mildew attacking your garden next year.

If you would like more information on how to manage powdery mildew, read the MSUE article “P is for powdery mildew on ornamentals”, contact your local extension office, or call the MSUE garden hotline Monday through Friday from 9am to noon, and 1pm to 4pm.  The phone number for the hotline is 1-888-MSUE-4-MI (1-888-678-3464).  Thanks, and happy gardening!

  • If I spray my plants with a fungicide, will that reduce the amount of powdery mildew showing up next year?

    Thank you,
    Michael O’Brien

  • Hi Michael, great question! Powdery mildew can be a tricky disease to manage even for plant nurseries. Some plants such as Lilac are more susceptible yet remain resilient. Others can be easily devastated by the fungus. When it comes to treating with fungicides, be aware that powdery mildew can adapt and overcome the sprays so it is an important practice to be selective. The thought of ‘more is better’ can actually allow the fungus time to adapt, mutate, and continue to grow. One good option is that if you see the fungus on a leaf, remove it to prevent spreading.

    Now for specifics: there are hundreds of types of powdery mildew. Thus, hundreds of types of treatments. I have attached two links below for further information. The first is a document that describes powdery mildew and a basic spraying option. The second is to the main link regarding powdery mildew, and you can search by your specific “crop” (including ornamentals). You may have to cut and paste the links into your browser.

    Finally I would like to suggest creating a native ecosystem in your yard can help tremendously with disease, and pest, management. When you build a native ecosystem, you use plants that have adapted to our area over thousands of years. Which brings arachnids, insects, and mammals that may in fact take care of that powdery mildew FOR you (such as eating the leaf). While this system is not fool proof, especially for powdery mildew, science supports maintaining a healthy native balance to fight off a host of other problems that are sure to come along if given time. A great website for more information on native plants is, and a great spot in northern Michigan to see some natives is the Boardman River Nature Center in Traverse City.

    I hope this information helps you create an effective plan for next year, and that you are successful with whatever plants you choose.

    -Whitney Miller, Advanced Master Gardener, MGANM

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