The Treatment for Hydrangea Rust

About Hydrangea Rust

• Hydrangea rust is caused by the fungus Pucciniastrum hydrangeae, which visits only hydrangea and hemlock trees (Tsuga spp), the latter a plant in the pine family and hardy in USDA zones 4 through 8, depending on the species. Many rust fungi require more than one plant to complete their life cycles, and hydrangea rust is no different. After overwintering on hemlocks, the spores of hydrangea rust float on the wind, and the lucky ones land on hydrangeas. If conditions are just right, the spores will start a rust infection. Fortunately for other plants in your landscape, hydrangea rust won't go beyond these two plants, so other members of the garden have nothing to fear from rust on your hydrangeas.

Common Rust Symptoms

• Hydrangea rust is easy to diagnose when the rust-colored pustules begin to appear on the undersides of hydrangea leaves. Upper surfaces usually have yellow or orange spots corresponding with the locations of the pustules. As the fungal infection matures, these spots spread across both sides of the leaf, often growing together into one large yellow or rust-colored area. Leaves with heavy rust loads may drop and die prematurely, but those that survive will develop red-brown spores on the upper leaf surfaces in late summer.

Managing Rust with Cultural Modifications

• One of the things that makes hydrangeas so prized in the landscape, besides their billowy blooms, is their thick layers of leaves. But it is this bushiness that makes them so susceptible to fungal diseases like rust. These dense leaf configurations trap moisture and interrupt air flow, giving rust spores ideal conditions under which to germinate. You can help your hydrangea by thinning the inside of the bush during its dormant period and plucking any rust-infected leaves as they appear. If you have a hemlock nearby, cleaning up the fallen debris around both it and the hydrangea can help break the cycle of infection. When trimming, be sure to sterilize cutting tools with a disinfect such as rubbing alcohol.

Chemical Treatment for Rust

• Chemical control is rarely warranted for hydrangea rust, but if the infection is widespread and causing serious concern, a weekly spray of neem oil mixed at a rate of two tablespoons per gallon of water can be applied to affected leaves. Make sure to coat the leaves thoroughly, being careful to spray both sides until they're moist, but not dripping. Take caution when using neem -- although it's a safer chemical for gardeners, it's not without risk -- be sure to read the entire label before applying it to your hydrangeas.