Honeysuckle (Lonicera spp.) grows as a bush or, more commonly, a vine in U.S. Department of Agriculture plant hardiness zones 4 to 10. It's hardy and robust, twining and vining all over whatever is in its path, so much so that some forms, including Japanese honeysuckle (Loniceria japonia) are considered invasive parasites that if left unchecked will literally smother other plants. For all its vigor, honeysuckle is attractive to a large variety of pests and susceptible to a number of diseases. When the leaves develop black spots and turn yellow, act quickly to mitigate the problem.
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What Causes It
Although black spot is considered a rose (rosa spp.) disease, commonly affecting those USDA zone 4-to-8 perennials, it attacks other ornamentals as well, and honeysuckle is one of them. Black spot is caused by a fungus that overwinters in piles of leaves and on branches. The spores germinate is spring and summer when temperatures are in the mid-60s and foliage remains wet for six to nine hours. Spores are spread by wind and also through rain splashing on the soil. Although black spot won't kill the honeysuckle, it may weaken it, leaving it susceptible to attack from pests and other pathogens. It also ruins the beauty of the vine.
The first symptoms that appear are spots on the honeysuckle's foliage. These spots may be black or deep purple with irregular edges. The spots grow larger as the disease progresses. Finally, the foliage turns yellow and eventually falls from the vine. If a substantial amount of foliage falls, the honeysuckle vine becomes weak. The initial spots mimic other common ornamental plant diseases. For a definitive diagnosis, deliver or mail a foliage sample to your county cooperative extension office.
Remove infected foliage and stems. Bag the items and remove them from the garden to avoid spreading the disease. Apply lime sulfur spray to the honeysuckle to create an acidic environment on the honeysuckle's foliage. Black spot spores can't germinate under these conditions. The spray is only effective before the spores land on the foliage so spray the honeysuckle immediately after pruning off the infected portions. Make sure to completely cover the leaves with the spray.
When you prune the honeysuckle, also take a look at other plants nearby. Increasing sunlight on the vine helps prevent black spot and other fungal pathogens. Spread mulch on the soil beneath the vine to avoid having infected soil splashed onto the leaves during irrigation or rain. Use a fungicide spray or powder labeled for control of black spot in early spring, and repeat the treatment, evenly spraying or dusting leaves every week, unless otherwise noted on the fungicide label.