We love hydrangeas. Oakleaf hydrangeas, hardy hydrangeas, smooth hydrangeas - we love them all. Most of the year they’re easy to care for as long as they have enough water, but late in the season some weird looking things can show up on the leaves: hydrangea rust and leaf spot. The good news is that these fungal leaf diseases pose no long-term threat to the health of your plants, but there are some steps you can take to minimize the aesthetic damage.
Rust on hydrangeas shows up as powdery orange spots on the undersides of leaves. As the disease advances, the leaves will turn yellow and then brown, and the flowers may turn brown prematurely. We have noticed this most often on varieties of smooth hydrangea (Hydrangea arborescens), including such varieties as Annabelle and Incrediball.
There are many fungal leaf spots that affect many different plants and have different appearances, but the one we are seeing on hydrangeas right how looks like a small tan or whitish dead spot a quarter inch or smaller surrounded by a purple or brown ring. The spots may expand and eventually cause the leaf to turn yellow and fall off. We are seeing this a lot on varieties of mophead hydrangeas (Hydrangea macrophylla), such as the Endless Summer series (and any of the varieties that change flower color with soil pH) and oakleaf hydrangea (Hydrangea quercifolia).
These fungal diseases spread by spores in hot, humid, wet conditions when water can carry spores from one leaf to another. Neither rust nor leaf spots will do any serious harm to your hydrangea, and both are treatable and preventable with the same methods. While you may lose leaves early this year, the hydrangea will come back just fine in the spring.
Once you notice the first signs of rust or leaf spots, you can spray your hydrangeas with a fungicide labelled for that disease (ask for help from a garden center employee if you're having trouble finding the right one) to help prevent its spread. We usually will spray for rust, but we often leave leaf spots alone, especially this late in the season. Rust can quickly defoliate a whole group of hydrangeas while the leaf spots are much slow to cause damage. Since both diseases are spread by water, you can help prevent infection by watering your hydrangeas at the base or using soaker hoses. Of course you can't stop rain from spreading the fungi, but you can keep your hose from spreading it.
Hopefully with these tips you can keep your hydrangeas happy and healthy through the end of the summer!