It's no surprise that roses are among the most popular ornamental garden plants: they're beautiful, fragrant, and easy to grow in most climates. However, many popular roses are also susceptible to three fungal diseases, whose names black spot, mildew, and rust are descriptive of their appearance on rose leaves. Here are steps to follow to keep the plants healthy and vigorous.
Some roses are very susceptible to fungal diseases, others are virtually immune, and many fall somewhere between. So the first line of defense is to select only disease-resistant varieties. Many rose reference books, catalogs, and Web sites include this information.
Plant roses in full sun in rich, well-drained soil. Soil pH is ideally between 5.8 and 6.2. If the pH is lower, add limestone. If it's higher, add sulfur. 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.
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Generally, you want water to reach everywhere within the plant's 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. If the soil around your roses is dry to a depth of 4 inches, you should definitely water, but it's best not to let it get that dry.
Wet leaves encourage some fungi, so avoid wetting them unless you are washing off dust or aphid infestations with a hose. Drip irrigation is better for watering roses than sprinklers.
Rose plants that are either under- or overfed will be more susceptible to problems. One simple method is to buy a fertilizer packaged for roses and to follow the label directions.
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Different kinds of roses need different kinds of pruning. Most 3- to 5-foot-tall rose plants need to one annual pruning in early spring. At that time, remove any dead branches or twigs, as well as branches that crowd the center of the plant. Prune so that the pruned plant has a V or vase shape.
Visit your roses regularly and check leaves for signs of disease. It's best to wait until the leaves are dry, midmorning through afternoon, since we can unwittingly spread diseases by touching wet plants. If you notice signs of disease, remove affected leaves and apply a preventive spray as directed on product label.