Hydrangea shrubs reward gardeners throughout the summer with large and colorful blooms. Typically these shrubs are pest-free. However, under certain conditions, your hydrangea may show signs of a fungal or rust infection. Know the common diseases that affect hydrangea, and incorporate prevention and control into your gardening plan.
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Description of Hydrangea
Hydrangea is a leafy shrub or vine that bears plush blooms in mid to late summer. The plant loves water and thrives in a partially shaded site with moist soil. There are a variety of hydrangeas in landscapes. Panicle hydrangea (Hydrangea paniculata) is considered the most common type in gardens, and provides white blooms that turn pink as the season progresses. Oakleaf hydrangea (Hydrangea quercifolia) develops upright white blooms and has leaves that resemble oak leaves. Other types bear either pink or blue flowers depending on the pH level of the soil.
Powdery mildew, caused by the fungus E. polygoni, affects Smooth, Bigleaf and Panicle hydrangeas. Symptoms begin with a light gray coating on the undersides of leaves. Eventually, the fungus coats the entire surface of leaves and young shoots. Severe infestations reduce flowering. The fungus takes hold in landscapes during humid and warm conditions and when leaves are dry. To prevent fungal spread, the Alabama Cooperative Extension suggests removing debris around plants that could harbor spores from the previous season. Consider controlling powdery mildew with fungicide.
Cercospora Leaf Spot
Symptoms of Cercospora leaf spot on hydrangea usually begin in mid summer. Caused by the fungus Cercospora hydrangeae, this type of leaf spot affects most hydrangeas in the landscape, including Smooth, Panicle, Bigleaf and Oakleaf types. Early symptoms occur on leaves at the base of the plant and include circular or angular marks that are purple or brown with a light-colored center. Leaves on the entire plant are eventually affected. Removal of dead leaves and surface watering control the spread of this fungus.
If you see brown pustules on the underside of the leaves, then your hydrangea probably has leaf rust. Pustules may also be yellow, orange or purple. Rust proliferates in a wet environment with moderate temperatures. To slow down the spread of rust in your garden, the University of California suggests surface watering to avoid getting leaves wet.