Controlling Powdery Mildew

Powdery mildew has been seen on a wide variety of woody shrubs in Minnesota this year. Infections range from a light dusting of powdery white spots to leaves completely covered with dense white felt. Ninebark (Physocarpus opulifolius; especially varieties with dark leaves), Arrowwood Viburnum (Viburnum dentatum), and Lilac (Syringa spp.) are just a few of the ornamental shrubs that have been observed with this fungal disease.

Powdery Mildew on a Ninebark Shrub

Powdery mildew is easy to recognize because it looks just like its name. Powdery white fungal mycelia and spores can be found on the surface of infected leaves, stems, flowers and fruit. If the disease starts on mature leaves, the fungus is often only a few spots or a light coating of white on an otherwise healthy looking leaf. These infections are typically considered minor infections, and have little effect on the overall health of the shrub. If the disease comes in on young developing leaves, shoots and flowers, however, leaves may be severely affected. It is not uncommon to find young leaves that are crinkled, cupped upwards or otherwise distorted by powdery mildew infections. These leaves often turn yellow or are covered in white fungal growth. Young flowers and fruit may be completely coated with the white fungus and often fall off prematurely. Young shoots that are severely infected may even be killed.

One way that the powdery mildew fungi survive Minnesota’s harsh winters is by colonizing young tissue within plant buds. In these cases the fungi starts up new infections as soon as the buds open in the spring. A shrub that has a few very severely infected young shoots next to other, completely healthy shoots, most likely had powdery mildew fungi surviving in its buds all winter long. This type of infection can be seen on many ninebark shrubs this year.

Powdery mildew fungi are favored by mild weather (60-80F) and high humidity. They thrive in shade and cause the most severe infections on young, succulent shoots.  In addition, most powdery mildew fungi can only cause disease in one genera or one family of plants. The powdery mildew fungi on lilac will therefore not spread to the rose bush in the yard. A different species of powdery mildew fungi must be present to infect the rose bush.

What can be done to control these powdery pests?

First, many powdery mildew infections do not significantly harm the plant and therefore do not need to be controlled. Many lilacs bloom beautifully year after year despite repeated infections with powdery mildew. Mature trees may have powdery mildew on the lowest leaves, but this is such a small proportion of the canopy that the tree’s overall health is not affected.

In cases where leaves and shoots are distorted, yellowed, and stunted, action should be taken. If only a few shoots are severely infected, these can be pruned off and removed from the garden. This is true for infected flower clusters as well as leafy shoots. In addition, infections on stems should be removed, since these often turn into bud infections and survive the winter in that form.

Humidity around the plant should be reduced as much as possible. By mulching around the shrub with woodchips or other organic material it will help to keep moisture in the soil. Improve air circulation around the plant by removing any weeds crowding the plant. Place shrubs in areas of the garden where there is good air movement and sun appropriate for the shrub’s needs.  Do not over fertilize shrubs suffering from powdery mildew. Fertilizer often results in a flush of new succulent growth that is easily infected by the fungus.

Rose leaves affected by Powdery Mildew

If all cultural control practices are not enough to control the disease, a protective fungicide can be used to control powdery mildew. Copper fungicide can be used to help control powdery mildew, as well as if any of your plants have early or late blight and/or leaf spot. Another product that works great is Fung-onil; it is a great all-purpose fungicide that can be used on any plants from your ninebark to your roses. If you’re looking for an organic option use EcoSmart Organic Garden Fungicide; it’s environmentally safe and works on contact. If your roses are the only plant that seems to be affected by powdery mildew Rose RX 3 in1 is a great multi-purpose product; two way triple action: insecticide – miticide – fungicide. Rose Rx 3 in 1 does contains Neem oil organic insecticide and is approved for organic gardening. And as always please remember to read the labels and follow the instructions as directed when you’re using any chemical. 

 

Information provided from the University of MN Extension Services.