Disease-Resistant Rose Tips

Struggling with rose diseases often converts affection to frustration for millions of gardeners who grow these beloved shrubs. But good plant selection and site conditions can help them flourish.

Choose disease-resistant varieties to avoid black spot, mildew and rust. Fungicides can be used to control these diseases, though in warm, humid summers, even resistant plants may be affected.

Water

Roses need full sun and well-draining soil to thrive, but they are more susceptible to diseases when water deprivation stresses the plants. The best strategy is to avoid overhead irrigation, and use soaker hoses or watering cans to water at ground level, allowing the foliage to dry completely before nightfall. Roses resent being waterlogged, and overwatering can also lead to root rot, which often looks like black spot.

All classes of roses are susceptible to fungi that cause mildew, but some types and varieties are more resistant than others. The best way to avoid mildew is to choose a variety that has good disease resistance, and mulch the soil around it. In conditions that favor mildew development, use a fungicide spray labeled for roses and apply according to the instructions on the label.

Some of the most serious problems that affect roses in landscape settings are caused by viral pathogens, which can greatly reduce their vigor and beauty. Two of these are rose mosaic and crown gall, both of which can kill the plant if not prevented.

Rose mosaic is a widespread problem that results in unusual mosaic or line patterns on leaves. It spreads through the rose garden by the movement of eriophyid mites, and once infected, the plant is infected for life. Roses with significant foliar disease are not worth keeping, and they should be removed and destroyed. Rose breeders are working hard to develop RRD-resistant cultivars, and several are available.

Fertilize

Even disease-resistant roses need modest care to thrive. Water them deeply, but sparingly. Overwatering encourages plant diseases, such as rust and powdery mildew. Overhead watering reduces air flow, which can also lead to plant disease. Plant in a sunny location that receives six to eight hours of sunlight each day, especially in the morning. The sun helps dry the soil and encourages healthy root growth, while proper spacing improves air flow.

Fertilize regularly, using a well-balanced slow-release fertilizer (e.g., 10-10-10). Make the first application during bud break in spring. Repeat every four to six weeks throughout the growing season. Stop the fertilization process by late summer, as roses are entering their dormant phase.

Organic fertilizers are best for roses, as they help promote healthier plants with less risk of disease and pests. Add a 1- to 2-inch layer of compost at planting time or apply a rich, natural fertilizer like manure or bone meal in the spring.

Follow the instructions on the label for the specific type of fertilizer you use. Over-fertilizing can cause nutrient imbalances and burning of roots or foliage. Incorrect ph levels prevent the absorption of necessary nutrients, so regular soil testing can help ensure that your roses are getting what they need. When applying fertilizer, mix it into the top 2-3 inches of soil and water thoroughly to activate and incorporate the nutrients.

Pruning

Roses need regular pruning to enhance their architecture and ensure vigorous first-bloom growth. Pruning also helps prevent fungal diseases from overtaking the plants by eliminating old, diseased growth that can harbor pathogens. In addition, pruning reduces insect damage and controls the spread of diseases and pests such as thrips and aphids.

Some roses are very susceptible to black spot, mildew and rust; others are nearly immune; and many fall somewhere in between. Selecting disease-resistant varieties and bushes is an important first step, and fungicides formulated for your climate are essential.

When pruning, make sure to remove any twigs that are skinnier than a pencil, and all leftover leaves. These can drain energy from the plant, and carry over diseases into the next season. Removing these twigs also increases airflow around the center of the rose bush, and opens it up to more sunlight – both of which are important in preventing fungal diseases next spring.

Similarly, any new growth that is black or brown needs to go. This is a symptom of rosette disease, which causes malformed growth and a proliferation of thorns. It is also a sign of poor health in the plant and can lead to its eventual death.

This is also the time of year to spray your roses with a horticultural oil to control thrips and aphids. Both of these pests are common in the warm, humid North Texas climate.

Mulching

In addition to providing an attractive garden, mulching helps to keep soil cool and moist, minimizing weeds and reducing the need for watering. This also aids in the reduction of fungal diseases like powdery mildew and black spot. Mulch helps to lock in moisture, which is especially important during the summer months when the roses are most susceptible to disease.

While any organic material can be used as mulch, shredded leaves, grass clippings, or a compost pile are usually best for your roses because they provide the highest levels of nutrients. Wood chip mulch, made from a variety of hardwoods or softwoods, such as oak, maple, hickory, or ash is another good choice. It can be purchased at garden centers, or you can make your own by shredding woody yard waste and adding it to a compost bin. Sawdust is another option, but it should be well-aged, since fresh sawdust depletes nitrogen from the soil.

In general, a 2 to 3 inch layer of organic mulch is ideal for your roses. It will help conserve water, reduce the temperature of the soil, and return many of the nutrients that were removed from the ground when it was cultivated for other purposes. For the best results, apply mulch in late winter or early spring before your roses begin to grow.