Common Flower Diseases

Battling Mildew, Mold and Black Spot

Powdery mildew on a leaf.

Mid- to late summer is when most plant diseases start becoming noticeable. Powdery mildew, gray mold (Botrytis), and black spot are three of the most common flower diseases. Learn how to recognize them and what you can do to avoid them in your own garden.

Powdery mildew, as the name suggests, resembles a white, powdery coating on leaf surfaces. If severe, it also may appear on stems and the flowers themselves. Affected leaves eventually turn yellow, then brown. Dead foliage typically falls off the stem, though it will sometimes remain in place. Although not fatal to plants, powdery mildew makes the foliage unattractive and repeated bouts of the disease will gradually weaken the plant. Annual flowers that are particularly susceptible to powdery mildew include zinnias, snapdragons and verbena. Perennials that are commonly infected include delphiniums, lungwort, bee balm and garden phlox. Choosing cultivars (cultivated varieties) that have proven to be resistant to this disease is the easiest method of control.

Most fungal diseases are spread by microscopic structures called "spores" that are transfered on wet foliage. However, powdery mildew thrives in high humidity. Keeping plants well-spaced and removing weeds will help ensure good air circulation and reduce the humidity around plants.

The least-toxic controls for powdery mildew include horticultural oils and baking soda. If using horticultural oils, follow the instructions on the label. If using baking soda, mix it in warm water at a rate of 1 teaspoon per quart and spray it on plants every seven to 10 days. As with all fungus diseases, it is essential to begin application at the early onset of the disease—often late June or early July—and ensure that all susceptible foliage is treated. Repeated applications are usually necessary right through the duration of the growing season. Other options for control include Serenade Garden Disease Control, Garden Dust and Rose Rx 3-in-1. Chemical controls may also be used, but before using, read and follow instructions on the label.

Gray mold is perhaps the most common disease of flowers. It is especially problematic during periods of high rainfall and cool temperatures. Like powdery mildew, gray mold is well named. It appears as a gray mold, primarily on old and dying leaves and flowers. It begins as water-soaked spots and eventually develops into the characteristic gray, fuzzy coating.

As with most fungal diseases, gray mold is spread by wind and splashing water. One effective strategy to minimize gray mold is to prevent rain or irrigation water from splashing onto the leaves and flowers. When watering your plants, apply the water gently at the soil level, near the base of the plant. Or, better yet, use drip irrigation. In most cases, mold spores can only infect foliage if it is consistently moist for several hours at a time. Make a habit of watering in the morning, rather than the evening, so foliage has time to dry out during daylight hours. Thin plants and keep flower beds weeded to allow plenty of air circulation around plants. Any diseased flowers or leaves should be promptly removed. Another option for control is Serenade Garden Disease Control.

Black spot on roses.

Black spot is another common fungal disease. It is particularly problematic for roses. The disease typically begins as black spots on the foliage. These spots are most prevalent on upper leaf surfaces, and may be up to ½" across. Leaves eventually begin to yellow around the spots, then become all yellow and fall off. The spots may also appear on rose canes, first being purple and then turning black.

Black spot requires at least seven hours of wet conditions for infection, and it is inhibited at temperatures about 85 degrees F. Although you may not be able to turn up the temperature in your garden, you can minimize the disease by keeping foliage dry through proper watering and good air circulation. If possible, plant susceptible flowers like roses in an open and sunny location and avoid watering during cloudy weather.

Black spot fungus over winters in fallen leaves and infected canes. Pruning out infections and raking up leaves at the end of the season will go a long way towards providing control. Cultivars vary greatly in their resistance to black spot, so choose resistant ones if possible. If this information is not indicated on plant labels, you can research the variety online or check with horticulturists at your local nursery. Some of the shrub roses that have exhibited resistant to black spot—and powdery mildew—include cultivars from the Meidiland, David Austin and Explorer series. Many of the other shrub roses and old-fashioned roses are resistant as well. Neem oil is another way to prevent and control black spot. Try Rose Rx 3-in-1.

This article is adapted from a piece by Leonard Perry, a professor at the Univeristy of Vermont in Burlington, Vt. To see more of his articles, go to Perry's Perennial Pages.