Follow Best Cultural Practices

Yes. This is most likely powdery mildew. If you follow best practices, powdery mildew might not find a foothold in your rosebush:

At the first spot of powdery mildew, remove the leaves from the rose bush itself and from the ground around the plant. Throw the infected leaves and twigs in the trash, never into a compost pile. Also, be careful not to touch healthy leaves with your pruning shears so as not to spread the disease and sterilize your pruners with rubbing alcohol after pruning.

Give your rosebush space between it and other plants, wall or fences to allow for good air circulation so mildew doesn't have the high humidity it needs to grow.

Give your rose the right care to keep it healthy, such as full sun, good soil drainage and regular watering, with a deep soaking once a week during the growing season.

Avoid overhead and evening watering, which cause high humidity. Instead, water in the morning so the soil surface can dry out.

Avoid over-fertilizing, which produces an abundance of new growth and fewer actual roses. The young growth is especially susceptible to disease.

Remove all leaves, branches and twigs from around the plant when you prune your rosebush.

Place a layer of mulch over the soil in the spring to cover any fungus spores on the surface of the soil.

Spray your rose with a strong spray of water from the hose in the morning now and then to wash off any fungus spores that have not yet embedded themselves into the leaves.

Reapply powdery mildew treatments often, every two or three weeks, throughout the growing season. Commercial products designed to kill powdery mildew come in liquid or powder forms. Mix the fungicides with water according to the product directions and apply them to the tops and undersides of leaves until the leaves are completely covered with the solution. Choose a dry, calm day to apply the products and reapply after a rain or every seven to 10 days for the most effectiveness.

Roses with shiny or waxy leaves are less prone to powdery mildew than some other types, and rugosas (Rosa rugosa), which grow in USDA zones 2 through 7, seems to have a natural resistance to the disease. Other rose varieties are bred for disease resistance, including 'Radrazz' (Rosa 'Radrazz'), for USDA zones 4 through 9, in the trademarked Knock Out rose series.

Hope this helps!