Problem: Small, fringed, black-to-brown spots on leaves come first, then leaves yellow and drop off. Symptoms of this fungus disease start at the bottom of the plant, then work their way up. Common in warm, humid weather.
Solution: Remove and discard infected leaves. To reduce infection, try a baking soda-oil spray (mix 2 teaspoons baking soda and 2 teaspoons summer oil in a gallon of water). Use often to protect new growth.
Problem: Patches of white fungal filaments and spores disfigure leaves, buds, and stems. Thrives in humid air but ― unlike other rose diseases ― needs dry foliage to become established.
Solution: Overhead watering (in the early morning) may wash off fungal spores and reduce infection. Baking soda-oil sprays can also be effective (see black spot). For the worst cases, spray with a fungicide such as triforine or benomyl.
Problem: Small rust-colored spots form on leaf undersides. Leaf tops show yellow mottling; in advanced cases, leaves yellow and drop. Warm days, cool nights, and moisture encourage this fungus disease, which is spread by spores.
Solution: Keep fallen leaves picked up, and during winter, pick off any rust-infected leaves that remain on the plant. During the growing season, spray foliage with a wettable sulfur, or a fungicide such as triforine.
Problem: Small green, red, pink, or black oval insects cluster on new leaves, leaf undersides, and buds, distorting them by sucking plant juices.
Solution: If infestations are light, do nothing; beneficial insects such as ladybugs will feed on them. For heavier infestations, dislodge the pests with a strong spray of water from a hose, or spray with insecticidal soap.
Problem: Barely visible, spiderlike insects hang out on leaf undersides, often weaving delicate webs. They suck plant juices, turning the leaves stippled yellow.
Solution: Spray leaves with water in early morning. For heavy infestations, spray with insecticidal soap or summer oil.