Extension

Gardeners need to keep an eye out for more than thorns when working in their rose garden. Black spot is a common disease of roses caused by the fungus Diplocarpon rosae, and it can wreak havoc on rose bushes.

 

Black spot is a serious disease that readily attacks young expanding leaves and canes. It occurs when fungal spores on infected fallen leaves or canes are splashed on the new lower leaves of roses. This happens via rainfall or the sprinkler. This is why it’s important to remove the diseased parts of the plants quickly.

 

Black spot looks exactly like it sounds – dark spots with feathery edges about a half-inch in diameter appear on some cultivars of roses, while the spots are smaller with more distinct edges on other cultivars. A yellow halo may appear on some spots. Canes will feature small, purplish spots on the current year’s growth.

 

When plants are badly infected, leaves drop prematurely and canes may become completely defoliated, thereby reducing the quantity of the flowers and eventually killing the plant.

 

Because the spores must remain wet for several hours for spores to germinate, it’s best to irrigate the rose bushes with a soaker hose at the base of the plant. Avoiding dense plantings will increase airflow around the plants so the leaves will dry out quicker. Also, keep weeds in check to help with air circulation.

 

The fungus is tough and survives from year to year, so it’s important to remove and destroy diseased leaves and canes. Do not compost these materials. Gardeners may need to apply a fungicide to protect the plant’s young tissue.

 

For chemical control on disease-susceptible cultivars, visit your local garden center/nursery or contact the local OSU County Extension Office. Weekly fungicide applications must begin with new growth in the spring and continue until frost in the fall. Sprays may be omitted during hot, dry periods in the summer. Many general-purpose rose dusts and sprays are available that contain compatible materials for control not only of black spot and powdery mildew, but also for common insect pests. These materials have been well formulated and, if used properly, will give excellent results. During the dormant season, spray with lime sulfur.

 

There are disease-resistant varieties available on the market. Check with your local nursery or plant store to see which varieties grow best in your USDA Plant Hardiness Zone.

 

More information about growing roses in Oklahoma is available from Oklahoma State University Extension.