July 4, 2001
Stillwater Gazette
Betsy Halden

Black spot and other pests of roses

High humidity plus hightemperatures equals black spot, powdery mildew, and other undesirable conditions in gardens. Black spot. The leaves on my new J.P. Connell shrub rose are covered with black spots. The leaves are dropping hourly. The plant is in the same container that it was in when I bought it. I may have a plant that was infected before I bought it. Maybe. Most rose gardeners contend with black spot sometime during the season. but this year black spot is prevalent now.

Black spot is a disease caused by a fungus called Diplocarpon rosae. The optimal conditions for the disease to develop are 75-85 F and high relative humidity. Infection may be greatest on leaves that remain wet for six hours or longer. Leaves and canes can become infected.

Leaf symptoms are roundish, black spots with fringed margins that can be up to ½ inch in diameter. The spots form on the upper sides of leaves. The tissue surrounding the spots turns yellow. Infected leaves may prematurely drop from the plant. Usually lower leaves are infected first. Excessive leaf drop weakens the plant, predisposing it to other forms of injury such as those caused by temperature extremes.

Cane symptoms are blister-like, purple blotches that later turn black. The fungus probably will not kill the branches, but cane infections can be important in the pathogen’s survival through the winter. Good cultural practices help to manage black spot. We can rake fallen leaves and remove infected canes. Avoid wetting leaves when watering. Maintain air circulation around the plants to promote drying.

Several rose cultivars have resistance against black spot. The label will usually state if the plant is disease resistant. If a cultivar is not resistant, fungicide applications may be necessary for control of the disease. Informed help at a local garden center can tell you about products labeled for black spot. Some varieties may require spraying every 7-10 days through out the growing season. this is true if the weather is cloudy, warm, humid. Cover both sides of the leaves when spraying. If possible, alternate between different fungicides. It is important to read the label and follow the recommendations for applying the product. Fungicide treatments are most important when used in conjunction with good cultural practices.

There are other pests of roses that are abundant this year. Rose chafers eat petals and the interior of buds and flowers. One-sided blooms result. Handpicking is one way to control of this pesky insect. Rose sawflies feed on rose foliage. The larvae skeletonize rose leaves. Additional information on these and other pests is available through the University of Minnesota Extension Service.

There are a number of helpful documents available on the University of Minnesota Extension website www.extension.umn.edu. I put roses in the search box.. The search results included documents on selecting roses, planting roses, cultivation and care of roses, diseases and pests of roses, and integrated Pest Management (IPM) for insect pests of roses. The information is written for Minnesota gardeners in zones 2, 3, 4. This makes it more relevant than some of the information in general books on roses which seem to be written for zones 6 and 7. The University Extension materials are designed to help Minnesota gardeners be successful.

Betsy Halden is a Master Gardener with the University of Minnesota Extension Service. University of Minnesota Extension Service materials were consulted in the preparation of this column. You can reach Betsy Halden at eahalden@comcast.net