Caraganas, also called Siberian peashrubs, that exhibit a mildewy growth on their leaves may be infected with powdery mildew. Powdery mildew is a widespread fungal disease that infects caraganas all over the United States including Alaska. Though caragana is a hardy tree or shrub, this disease can compromise its health if left untreated, according to the University of Rhode Island.
There are numerous types of powdery mildew. Each type is host specific, which means the powdery mildew infecting your caragana shrub cannot spread over to your rose plants or nearby vegetation, according to Colorado State University. Typically, powdery mildew germination and spread is favored by humidity and temperatures between 60 to 80 degrees F. Powdery mildew feeds by sending rootlike structures into the cells of the leaves and sucking up nutrients.
Gardeners may find white to grayish-brown patches on caragana leaves. This mildew-looking growth occurs on the tops and bottoms of leaves. As the disease progresses, leaves turn yellow, curl, deform and defoliate off of the caragana shrub. Powdery mildew spreads through the wind and rainwater. Overhead watering can spread the disease to healthy portions of your caragana.
Treat your infected caragana with a fungicide when you first detect the disease. You can use potassium bicarbonate, neem oil, myclobutanil, sulfur or biological fungicides, as suggested by the University of California. Avoid using any fungicide when temperatures exceed 90 degrees Fahrenheit. Refrain from spraying a water-stressed caragana shrub with fungicide. Water the plant in the morning for a few days before applying fungicide.
Planting your caragana shrub properly can prevent powdery mildew episodes. Plant your caragana in a sunny location; powdery mildew thrives in shady environments. Make sure that you space your shrubs at least 15 feet apart. Caragana shrubs can spread to a width of 12 feet. Prune off damaged shrub areas to keep powdery mildew from spreading and clean up all debris underneath your caragana shrub.