Plant disease-resistant varieties. Some roses are very susceptible to these fungal diseases, others are virtually immune, and many fall somewhere in between. So the first line of defense is to select only disease-resistant varieties. Many reference books, catalogs, and Web sites include this information.

Grow healthy plants. Plant roses in full sun in rich, well-drained soil. The ideal soil pH is between 5.8 and 6.2. Space plants about 3 feet apart to allow good air circulation, and mulch roots in summer to help maintain even soil moisture and to prevent competition from weeds.

Water regularly. Apply enough water to moisten the entire root zone, which may be as deep as 18 inches and as wide as the crown of the plant. Get to know your soil and how water penetrates and travels within it. Water when the soil is dry to a depth of 2 to 3 inches.

Water roots, not leaves. Wet leaves encourage some fungi, so avoid this unless you are washing off dust or aphid infestations with a hose. Drip irrigation is better than a sprinkler for watering roses.

Fertilize correctly. Rose plants that are either under- or overfed are more susceptible to problems. One simple method is to buy a fertilizer packaged for roses and follow the label directions.

Prune correctly. Different kinds of roses need different kinds of pruning. Most 3- to 5-foot-tall rose plants need one annual pruning in early spring. At that time, remove any dead branches, as well as branches that crowd the center of the plant. Prune so that the plant has a vase shape.

Inspect leaves. Inspect your plants regularly for signs of disease. It’s best to do this when the foliage is dry because you can unwittingly spread diseases by touching wet plants. Remove any diseased leaves and apply a preventive spray.

Use preventive maintenance. In the fall, remove and dispose of all fallen leaves on the ground below the plants. In late winter or early spring, prune plants and apply horticultural oil to smother disease organisms that may have overwintered on branches. Spread a fresh layer of mulch to cover any disease spores that might be on the surface of the old mulch.

Common Rose Diseases and Controls

Black spot causes black spots on leaves and stems. Remove all winter prunings, a common source of reinfection. In early spring before growth begins, spray a combination of dormant oil and lime sulfur.

Powdery mildew causes grayish white fuzz on new leaves and flower buds. It spreads on dry leaves, usually during periods of low rainfall and warm, humid nights. Use a spray containing potassium bicarbonate, basically baking soda but in a form that is more plant-friendly. You can make your own powdery mildew spray by combining 3 teaspoons baking soda with 2 tablespoons highly refined summer oil and 1 gallon of water. Spray every 10 days to 2 weeks.

Another kind of mildew, downy mildew, causes round, dark purple spots with yellow edges. It’s more serious than powdery mildew but less common. It needs moisture to spread, so control it by watering only in early morning.

Rust causes rust-colored spots on leaf undersides and is most common during periods of warm days and cool nights. Remove winter prunings and spray dormant oil combined with lime sulfur.

Where you live makes a difference when it comes to rose problems. A variety that is disease resistant in one location may be disease prone in another, so choose plants wisely.

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