Roses, Disease Control

One of the most destructive rose diseases in Georgia.

You can recognize black spot by the nearly circular black spots with frayed or fringed margins. Spots vary from one or two to a dozen or more on a leaf, usually on the upper side.

Black spot, a fungus disease, is the most widely distributed and most destructive disease of roses in Georgia. This disease, combined with powdery mildew, causes rose growers a lot of concern. Black spot and powdery mildew cause leaves to shed prematurely. Continuous shedding of leaves shortens the life of the bud and limits the number and quality of the flowers the plant can produce.

You can recognize black spot by the nearly circular black spots with frayed or fringed margins. Spots vary from one or two to a dozen or more on a leaf, usually on the upper side. The spots enlarge from pinhead size to the size of a dime and as they enlarge they may run together to form irregular blotches on the leaves. An entire leaf may become yellow or there may be a yellow area around individual spots.

Premature leaf shedding is serious.

Leaf shed follows, often with two or three sets of leaves being lost in a season. Although the leaf spotting is ugly, the most serious effect on the plant is the premature shedding of leaves. This reduces the vigor of the plants.

Black spot overwinters. It lives through the winter on fallen leaves and in dead areas of diseased canes. In late fall, you should prune all dead or dying canes or stems. This removes the overwintering source of the fungus that causes the black spot disease.

Make pruning cuts four-to-six inches below any obvious sign of damage and close to a bud. This reduces the likelihood of common canker development.

Burn pruned material…

Rake up and burn the material that has been pruned. Burn all dead rose leaves, too. Sanitation will help prevent the spread of the disease, but it will not completely control black spot.

Powdery mildew usually appears in the late spring or early summer and is considered a hot weather disease. The white covering of the fungus growth covers young buds which results in no blooms or distorted flowers. Leaves may curl slightly or become blistered and then show signs of the white powdery fungus growth. The disease can occur on leaves, buds and canes causing a stunting and twisting of the new growth.

Spray at the first of the season.

To control black spot and powdery mildew on roses, you must follow a regular, weekly preventive fungicide spray schedule during the growing season. Begin spraying when leaves first appear in the spring.

There are many fungicides available which may be used alone or in combination for black spot. For further information regarding disease control in roses contact your local county Extension office.