By Bev Johnson

Master Gardener

Our hot, damp weather is perfect for the growth of that yucky white powder that seems to be on every plant in the garden. Woody shrubs can have a light dusting of powdery white spots to leaves completely covered with what looks like white felt. Ninebark, especially the dark leaved variety, hawthorn, arrowwood viburnum, and lilac seem to be the most affected. Even the most inexperienced gardener can recognize this stinker.

  Powdery white fungal mycelia and spores can be found on the surface of the infected leaves, stems, flowers, and fruit. If the disease starts on mature leaves, it will have just a few spots or a light coating of white on otherwise healthy-looking leaves. This is considered a minor infection and won’t have much effect on the overall health of the shrub. If, however, it shows up on developing new leaves, shoots or flowers, the leaves may be severely affected. The young leaves can be crinkled, cupped upward, or distorted in other ways. These leaves often turn yellow or are covered in white, fungal growth. Young shoots may even be killed. Young flowers and fruit may be completely covered in a thick, white coat.

  This disease even survives our nastiest winters. It colonizes young tissue within the plant buds. Then the fungus starts a new infection as soon as the buds open in the spring. A shrub that has a few severely infected young shoots next to a completely healthy shoot, probably had the fungi living in the buds all winter.

  Powdery mildew fungi like mild weather and high humidity. They thrive in shade and cause the most severe infections on young succulent shoots. It will only affect one family of plants. That means the mildew on your lilac won’t make your rose bush sick. She will have her own species of mildew fungi.

  Most infections of this fungi don’t do much damage to the plant. Lilac leaves in particular look really ugly when coated with the white gunk. Mature trees may lose some lower leaves to the infection, but this is such a small portion of the canopy the tree just shrugs it off.

  If the leaves and shoots are distorted, yellowed and stunted, or the flowers and stems are infected, just cut them off and remove them from the area. It’s a good idea to have a plastic bag you can tie shut handy as you remove the infected parts to keep from spreading the spores.

  Your goal is to reduce the humidity around the plant as much as possible. Start by increasing air circulation. You may have to remove some plants to achieve this goal. Mulch – it keeps the moisture down in the soil. Don’t over-fertilize an infected plant. That just results in a new flush of succulent growth that is easily infected.

  A remedy that has been used in New Zealand for melons stops a light infection in its tracks. It’s simple. A cup of whole milk (the fats seem to be important) in a gallon of water and a few drops of liquid dish soap to help it stick to the plant will do the trick. It should be applied either early in the morning or during cool evening hours as a spray. If that doesn’t do it, a fungicide based on sulfur is a low impact fungicide. Do be careful as it can burn some plants like viburnum, so try it on a small area first. If the plants are sensitive to sulfur, or have a severe infection of young growing shoots, the systemic fungicide thiophanate methyl can be used. Do read the label.