I just removed a tall nanny berry viburnum from my garden. It had suffered from bad powdery mildew for a few years, and the shrub was too big for me to treat and control it. As the spot is in partial shade, I would like to replace it with a suitable large bush or small tree, such as a serviceberry. My friend who is also a gardener says that serviceberry, and indeed a number of flowering shrubs are also susceptible to mildew. He suggested waiting until next spring to replant, and that the powdery mildew spores would die off over this year. In your opinion, can I replant sooner, and what type of shrub might you recommend?
The good news about powdery mildew, a white fungal growth that forms on the upper surfaces of leaves, is that it is a host-specific fungal disease. This means that the types of fungal pathogen that affect members of the Viburnum family, Erysiphe or Phyllactinia, will not affect shrubs that are not in this family. The bad news is that there are many different pathogens of powdery mildew that affect many different shrubs and plants, and the conditions in which one pathogen thrives are most often the conditions in which others will thrive. So if your growing conditions are not optimal, your next planting, if susceptible, may be affected by a different powdery mildew strain.
Powdery mildew is generally considered more of a cosmetic problem since it does not typically kill its host, although it can be unsightly, and in severe cases can reduce photosynthesis in affected plants. Powdery mildew thrives best when cool nighttime temperatures give way to warm temperatures in the daytime. It can be most severe when growing conditions are crowded and shaded, with poor air circulation. The powdery mildew pathogens that affect Viburnum overwinter in a dormant state in buds and shoot tips, where they infect new growth in the spring. Other powdery mildew pathogens overwinter on leaf debris where their spores are dispersed by wind in the spring.
Viburnum are part of the Caprifoliaceae family which also includes honeysuckles, abelias, and weigelas, so these are plants you may wish to avoid in replanting this spring.
When you are deciding upon a replacement shrub you should think about selecting a cultivar that is resistant to powdery mildew. For instance, it is a common disease of dogwoods, but most of the Japanese dogwood (C. kousa) cultivars and most Japanese interspecific hybrids (C. kousa × C. florida) are resistant. Acer Palmatum, or Japanese maple, suitable for partial shade, is in general not considered susceptible to powdery mildew (although it is susceptible to other fungal diseases). The Missouri Botanical Garden’s “Plant Finder” website is a useful online tool that provides a comprehensive description of plants including their resistance or vulnerability to pests and diseases.
If you choose a replacement shrub that is susceptible, proper cultural practices can make a difference: make sure there is enough sun, and enough spacing between plants for good air circulation, so that humidity levels may be reduced. You may need to think about pruning surrounding plants. Fertilizing affected plants is not recommended since this may stimulate growth and promote infection of new shoots. Clean-up of leaf debris in the fall is a good practice all around.
Here is a Toronto Master Gardeners guide that contains lists of shrubs for different growing conditions. You could use this as a basis for some further research into what appeals to you: