Lacecap hydrangeas are ideal for the home garden because they are less dense with a graceful appearance, creating a more subtle overall effect in a landscape than the more popular mophead hydrangea variety. Easily recognized by its flower head comprising a center cluster of small, fertile flowers surrounded by a ring of much larger flowers, the lacecap hydrangea is stunning grown alone or in mass.
When fungus attacks a lacecap hydrangea, you will see it affecting the leaves of the plant. A fungus will generally begin on only a few hydrangea leaves but can quickly spread to infect the majority of leaves on your plant. Although a fungus attacking your hydrangea will not kill your plant, the effect of a fungus can dramatically alter the beauty of a hydrangea if not rectified early on.
Three major fungi commonly infect a lacecap hydrangea under certain conditions. Cercospora leaf spot fungus is a common ailment of landscape lacecap hydrangeas, often resulting in a scraggly, leaf-barren bush. Powdery mildew becomes present on the underside of lacecap leaves initially, but can rapidly spread to the leaves on the entire bush. Rust fungus also begins on the underside of hydrangea leaves, but with small rust colored spots. Eventually rust fungus increases on the plant, causing leaves to turn brittle, discolor and drop.
Powdery mildew will start with small whitish spots on the underside of your lacecap hydrangea. The spores will quickly multiply and spread, soon covering most of the plant leaves with a whitish gray, powdery layer, sometimes dusty looking with purple spots. Buds and new growth can also be affected. Cercospora leaf spot begins on the leaves at the base and interior of a lacecap hydrangea with small circular brown or purple spots. The spots will turn a lighter color with a dark ring, or halo, surrounding the spot. The fungus spores spread throughout the leaf canopy, causing leaves to turn yellow and drop. Rust fungus can easily be identified by the bright rust or orange spots that appear on the underside of lacecap hydrangea leaves.
Powdery mildew can appear on your lacecap hydrangea any time the weather becomes cooler and humidity, or moisture, is high. Commonly appearing in mid- to late summer, cercospora leaf spot fungus infects a lacecap hydrangea by fallen diseased leaves. The spores will be splashed upon the lower leaves of the plant. Rust overwinters on plants, infecting neighboring plants by airborne transportation the following season.
Powdery mildew can be controlled at first sighting by weekly spraying of your lacecap hydrangea with garden sulfur fungicide. Creating good air circulation around the plant by not overcrowding can help prevent or decrease infection. Treating cercospora leaf spot with fungicide is not generally used unless it appears mid-summer. Cercospora leaf spot commonly develops toward the end of the growing season and the preferred treatment is removal of infected leaves. All fallen leaves should be picked up and destroyed to avoid further infection. Rust on your lacecap hydrangea leaves and branches should be pruned out immediately and destroyed. Spray the bush weekly with a garden sulfur until the rust disappears. All dropped, infected leaves should be removed and destroyed to prevent the spores from overwintering and reinfecting the bush.