Spectacular blooms and diverse types and varieties make roses a favorite of many Kentucky gardeners. However, warm, humid growing conditions create an ideal environment for serious problems each year with black spot and powdery mildew.
Gardeners can nip these fungal diseases in the bud by planting resistant or tolerant varieties and creating an unfavorable environment for disease development. It may be necessary to use fungicides throughout the summer, especially on susceptible varieties.
To reduce foliar diseases, try to avoid conditions where rose leaves remain wet for an extended period of time. Do not wet foliage when watering plants and allow sufficient time for leaves to dry before nighttime. Prune out shading vegetation from overhanging trees and provide space between rose bushes to improve ventilation and sunlight penetration.
Sanitation also is important for managing rose diseases. If you have not already removed and destroyed old leaves, winter-damaged canes and debris, do it as soon as possible. These items are a source of disease-causing organisms.
Many fungicides are labeled to control rose diseases. Always check the label to be sure the product controls black spot and powdery mildew and read and follow application instructions. To maintain disease suppression, repeat fungicide applications at 10- to 14-day intervals throughout the growing season.
Black spot produces dark, circular spots with fringed borders on the top or bottom side of leaves. Infected leaves often turn yellow and drop, reducing flower numbers and quality.
White, powdery fungal growth is a sign of powdery mildew. It is easy to locate on such plant surfaces as leaves, stems and flower buds. Infected leaves may be small and deformed.
Two other important, but less common, foliar diseases of roses are downy mildew and rust. Downy mildew produces lesions that are an off-color, later turning purplish brown. It leads to defoliation. Rust-colored spots on leaves and stems indicate the disease, rust. Severely infected leaves may shrivel and turn brown.
Another summertime disease is rose rosette, which affects roses throughout Kentucky. It is not a fungal disease.
This disease is spread by a microscopic mite. The primary host is multi-flora rose, a thorny plant native to the Orient and introduced into the United States as a conservation plant and “living fence.” The disease also affects cultivated roses.
Early symptoms are increased growth of shoots, which appear more succulent than normal and develop excessive thorns, and distorted, dwarfed leaves. The affected shoots are not winter hardy and produce few blooms. Rose plants eventually die.
Early disease detection is essential to keep rose rosette from spreading. Remove and destroy any infected roses to keep the disease from healthy plants nearby. Carefully remove diseased plants to avoid scattering disease-carrying mites to other plants. Since multi-flora roses might be a disease reservoir, remove and destroy any located within one-eighth of a mile from the rose bed.
For more information, contact the Harlan County Cooperative Extension Service at 573-4464.
Jeremy Williams is the Harlan County extension agent for Agriculture & Natural Resources. Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.