Honeysuckles (Lonicera spp.) are evergreen or deciduous shrubs or vines prized for their clusters of fragrant flowers attractive to hummingbirds. Hardiness varies between species and cultivars. One widely cultivated specimen, the trumpet honeysuckle (Lonicera sempervirens), grows in U.S. Department of Agriculture plant hardiness zones 4 through 10. One disease occasionally troublesome on a prized honeysuckle is powdery mildew, which overwinters in buds and on bark and spreads to infect other plants via wind. Powdery mildew prefers shady conditions, high relative humidity and temperatures between 60 and 80 degrees Fahrenheit, so a change in weather or small adjustments to the honeysuckle growing environment often curbs this disease sufficiently.
Selectively prune off overhanging or adjacent tree or shrub branches to increase the amount of sunlight reaching the infected honeysuckle and improve air circulation around the plant. Direct sunlight or leaf temperatures above 95 degrees Fahrenheit can kill powdery mildew.
Prune off and dispose of infected portions of the honeysuckle as the powdery mildew is noticed, if it is contained to just a few branches or leaves. You can also wait to prune off infected portions of the powdery mildew when the honeysuckle is dormant and leafless if the specimen is deciduous.
Rinse the honeysuckle foliage with a water spray or irrigate the plant in early morning using a sprinkler or other overhead watering. This splashing water can knock spores off the plant or kill spores it contacts. Unlike many other fungi that attack plants, the spores of powdery mildew do not require free water to germinate.
Prepare a suitable horticultural oil mixture to spray on the honeysuckle. Both plant- and petroleum-based oils are least toxic fungicides and pesticides able to eradicate a mild or heavy powdery mildew infection. Dilution rates vary between oil types. Always follow manufacturer recommendations for dilution rates. Stylet oil is more highly refined than other oils, making it less likely to injure the honeysuckle. To prepare a suitable stylet oil spray, blend the oil and water together at a rate of 1 ounce of oil per gallon of water.
Spray the prepared horticultural oil mixture onto a small, inconspicuous area on the honeysuckle and monitor that area for a week, looking for any injury. If damage occurs, further dilute the oil spray or choose a different product. If no injury is apparent, treat the entire honeysuckle.
Spray the powdery mildew-infected honeysuckle thoroughly, coating the upper surfaces and undersides of leaves and twigs that show disease symptoms. As long as conditions favor powdery mildew growth, repeat treatments every 7 to 10 days may be necessary.