Over 100 species of rose plants bloom in an array of colors and grow as bushes, shrubs and even vines. Cultivation of rose plants has become a bit of an art in some horticultural circles and information on how to grow and care for them is abundant. Rose plants, like all plants, are subject to several problems that any serious grower needs to be aware of.
Japanese beetles, aphids, thrips and rose scale are the most common pests that attack rose plants. Japanese beetles, active in summer, will eat entire flowers and a substantial amount of foliage. If the leaves of your rose plant are curling and turning brown, the culprit might be aphids--very tiny, usually light green insects. They leave a sappy residue in their wake.
Thrips will attack the blooms and seem to prefer buds. These insects are active from late spring through summer, and like areas that are hot and dry. If leaves change color and drop off prematurely it may be rose scale. This insect can be devastating to your rose plant, so keep a close eye out for it. Cut off any infected parts of the rose plant. The best way to protect your rose plant is to keep the planting bed free of weeds and debris and use an insecticidal soap at the first sign of an infestation.
Powdery mildew is a fungal disease, the symptoms of which are easy to spot: a white, powdery substance on leaves and stems. Powdery mildew is a common rose disease and can be treated with a commercial fungicide, according to package directions.
Black spot, also caused by a fungus, is common in humid climates. This fungus grows on the leaves and spreads, eventually, to the canes. You will be able to identify this disease on your rose plant as the leaves will develop brown spots, turn yellow and then drop off. It's important to remove and destroy all infected leaves. Apply sulfur dust every week to prevent this disease.
Rust is a very serious fungal disease, identified by reddish brown spots under the leaves and on the cane. Cut off infected leaves and canes. Use sulfur powder on the soil in early spring to prevent rust.
Iron deficiency, or chlorosis, is a common rose malady and symptomized by yellow leaves and stems. Perform a soil test and if the result is lower than 7.0, apply a handful of chelated iron around the base of the plant, or purchase a fertilizer fortified with iron.
If leaves turn yellowish-green with spots, or the flowers are smaller than normal, the rose plant could need more nitrogen. Add some well-rotted manure to the soil around the plant and mix it in well. If the leaves are turning yellow and appear to be dying, the rose plant may need magnesium. Epsom salts work well for this; sprinkle 1/2 cup around the base of the plant.