Common flower diseases

Mid to late summer is the time diseases get a hold and become noticeable in gardens and landscapes. Powdery mildew, gray mold (Botrytis), and black spot on roses are some of the more common diseases to watch for on your flowers.

Powdery mildew, as its name suggests, resembles a white powdery mildew on leaf surfaces of plants. It also may attack stems and flowers if severe, according to a University of Vermont Extension leaflet. Leaves eventually turn yellow, then brown, then die. Although not fatal to plants, it makes plants unattractive and may weaken plants over several years. Annuals that may be infected most commonly include zinnias snapdragon, and verbena. Perennial flowers that may be infected commonly include delphinium, lungwort, and garden phlox. Choosing cultivars (cultivated varieties) resistant to this disease is one of the easiest methods of control.

Unlike most such fungus diseases spread by microscopic structures called “spores”, this one actually is inhibited not promoted by rain and wet leaves. High humidity will favor this disease, so keeping plants spaced properly will promote air circulation and lower humidity around plants.

Chemical controls also can be used. Make sure to read and follow label directions on all fungicides for best control, and for your safety and that of the environment. Least toxic controls shown effective in research at the University of Vermont and elsewhere include horticultural oil and baking soda. Least toxic sprays closely related to baking soda are registered for use on this disease. Make sure and begin applications at the onset of the disease, often in late June or July, and every two weeks after.

Gray mold is perhaps the most common disease of flowers, attacking many species under conditions of high moisture and cool temperatures. It too is well named, appearing as a gray mold on any plant part but primarily on old and dying leaves and flowers. It begins as water-soaked spots, growing into the gray fuzzy coating.

The spores on this disease as with most fungi are spread by wind and splashing water. So one control is to prevent splashing water, such as watering near the base of plants with drip irrigation. Water early so the foliage can dry during the day and not go into the night wet. Allow plenty of air circulation around plants, and remove any diseased flowers or leaves.

Roses are one of the most popular and widely grown flowers of all time in temperate areas, and black spot is one of their most important and common diseases. According to a University of Maine Extension leaflet, this disease begins as black spots and so the name. These are most prevalent on upper leaf surfaces, and are up to one-half inch across. Leaves turn yellow around the spots, then all yellow and fall off. Spots may also appear on rose canes, first purple and then turning black.

The black spot fungal disease requires at least seven hours of wet conditions for infection, and is inhibited above about 85 degrees (F).

So although you may not be able to keep plants hot in the garden, if you can keep them dry through proper watering and air circulation you can minimize the disease. Grow plants in an open and sunny location. Avoid watering during cloudy weather. Allow plenty of space between plants for air circulation.

Black spot overwinters in fallen leaves and infected canes, so pruning out infections and raking up leaves at the end of the season also will go a long way towards providing control. Fungicides can be used during the growing season.

Cultivars vary in their resistance, so choose resistant ones if possible. This should be indicated on plant labels, or ask professionals at your local nursery. Some of the shrub roses resistant to this disease, as well as to powdery mildew, include some cultivars from the Meidiland, David Austin, and Explorer series. Many other shrub and old-fashioned roses are resistant as well.

More on these and other diseases, pests, and controls can be found at an extensive online website (

Dr. Leonard Perry is Extension Professor, Department of Plant and Soil Science at the University of Vermont. Visit his website at