My peonies are covered in what looks like a white powder. What is that and will it hurt my plant? PF

Peonies are susceptible to powdery mildew and what you are describing sounds like powdery mildew. Peonies are one of many plants that can be affected by powdery mildew including azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, English ivy, photinia, blueberry, pecan, cucumber, and squash. The degree to which it affects the plant depends on a variety of factors including the overall health of the plant and the weather conditions.

Powdery mildew is a fungus that gets it nutrients through small, root-like appendages called haustoria. These haustoria penetrate the outer layer of the leaf to gain access to the nutrients in the leaves. Eventually, this removal of nutrients will cause the leaves of the plant to turn brown, die, and fall off if left untreated.

Powdery mildew can spread from plant to plant, but it may not depending on both the plant and the type of powdery mildew. While there are a variety of powdery mildews, they all follow the same life cycle.

As a fungus, it overwinters as small, black, spore bearing structures, or as fungal threads called mycelium. This overwintering happens in leaf debris, dormant buds on plants, or stems. When temperature rise above 60 degrees in the spring, the overwintering fungi begin to produce spores which are then moved by the air in search of an appropriate host plant. It can also be splashed by the rain up onto the plant and to other locations on the plant. Powdery mildew can spread pretty fast since it only takes about 48 hours for a newly germinated fungal spore to be able to start producing new spores which then continue the spread.

While high humidity can contribute to an environment that encourages powdery mildew growth, areas where plants are crowded together with poor air circulation can also be contributors to its development.

There are a couple of things you can do to help minimize the occurrence of powdery mildew. First, you can start by purchasing disease resistant varieties. Check with your favorite nursery when purchasing plants.

Also, if you tend to have powdery mildew each year on the same plant, consider thinning the plant out a bit to increase air circulation. Increased air circulation can help deter powdery mildew formation. Also, it’s a good practice at the end of the growing season to clean up any plant debris from an infected plant. This will help minimize the overwintering of the fungus in this plant debris.

Since we know that the disease can spread with splashing water like rain, pay close attention to how you water your plants. If possible, it’s always better to water in the morning so that any water that lands on leaves, can evaporate. When watering occurs in the evening, moisture can remain on the plants throughout the night which can encourage fungal development. It’s also a good idea to concentrate your watering efforts on the ground, around the roots and not on the leaves.

You’ll have the most success with powdery mildew treatment if you begin treatment at the first sign of disease. Unfortunately, once the plant gets powdery mildew, there is no cure. At that point it becomes a disease management issue.

At the first sign of the disease, you can remove the first leaves that show signs of powdery mildew. However, if the disease has spread, you can’t remove all the leaves of the plant and expect a good outcome.

When discovered, it’s also a good idea to begin a treatment program with an anti-fungal such as copper fungicide. Spray the plant on the frequency suggested on the bottle. Timing may vary depending on the fungicide. It’s also not a bad idea to rotate between two different fungicides to avoid the possibility of the disease building up a resistance to the fungicide. This treatment will help contain the spread of the disease to other parts of the plant and other plants in your landscape.

If you have a plant or a grouping of plants that seems to get powdery mildew each year, it would be a good idea to begin your fungicide treatment when the leaves first appear on the plant. This would kind of be like our taking a vitamin supplement at the beginning of cold and flu season. A little prevention never hurts. I have a grouping of tall garden phlox that seems to get powdery mildew each year so this is a good reminder to me to begin some preventive fungicide treatments.

If you don’t see any symptoms of powdery mildew until the fall, you probably don’t need to do anything since the plant is likely nearing the end of its seasonal life cycle. Just remember to do a good job of cleaning up the plant debris at the end of the season to minimize overwintering spores. See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.  Photo: Tom Ingram