Philadelphus lewisii is a type of mock orange shrub that's sometimes known as wild mock orange or Lewis' mock orange. Homeowners prize it for ornamental purposes. It grows up to 10 feet tall and is covered by hairy leaves dotted with fragrant white blossoms during the summer months of May through July. Though the shrub is hardy, several problems may arise while growing it.
The shrub requires moist, well-drained soil, according to the University Corporation for Atmospheric Research. Giving the shrub too much water can cause root rot. Too little, and the shrub may become stunted in growth or fail to blossom adequately. Use the shrub's appearance as an indicator, applying water when the shrub begins to exhibit slight signs of drought stress, including slight wilting or curling of the leaves. In many climates, expect to water bi-weekly. Apply enough water to soak the soil to a depth of 12 to 15 inches to encourage deep root development.
Fungal Leaf Spot
Fungal leaf spot can mar the appearance of all types of Philadelphus shrubs, according to Michigan State University. Symptoms include brown, red or black spots on the leaves. It is most common during extended lengths of wet weather. Without treatment, the disease can spread throughout the mock orange shrub and cause widespread leaf loss. A fixed copper or mancozeb spray will kill the fungus that causes the disease. All fallen leaves should be swept up and removed to keep the spores from spreading.
The Botrytis cinerea fungus causes gray mold on mock orange shrubs, as well as on most other types of flowering shrubs. It can cause brown lesions on the mock orange's blossoms and leaves. The disease favors moist conditions. Minimize the chances of your mock orange shrub contracting the disease by pruning back surrounding vegetation to increase air flow and sunlight on the plant's foliage. If cultural changes don't kill the mold, treat it with a standard fungicide.
Powdery mildew leaves a thin, white coating on the flowering mock orange's leaves, according to Michigan State University. Like most fungal diseases, it exhibits itself after prolonged wet weather. A potassium bicarbonate or sulfur fungicide spray, applied during humid weather as soon as the telltale white powder appears on the mock orange's leaves, will help control the disease.