Q. I’ve grown fantastic hollyhocks in the past. This spring, healthy, vigorous plants came up but then soon developed small yellow nodules all over the leaves. Although the plants bloomed, the leaves turned brown and dropped off. My hollyhocks were not as tall as usual and the blossoms are small, dull and faded. Is there a remedy?
A. Most likely it is a disease called Hollyhock Rust. Hollyhocks are susceptible to a number of different leaf-spot diseases, but, by far, the most common and destructive is rust. This fungus begins by causing tiny pinhead-sized brown spots on the undersides of the leaves. At the same time, the top of the leaf shows a larger yellow-to-orange-to-tan spot. Eventually, the spots enlarge and join together as the disease spreads to the stems and even to the green parts of flowers. The leaves then shrivel and turn brown, giving the plants a blighted appearance.
The disease especially favors damp and/or humid weather, which we certainly had plenty of earlier this summer. Gardens that have been watered regularly during dry weather provide ideal conditions as well, but morning dew and/or high humidity can also provide enough moisture for infection. Removing infected leaves promptly and cleaning up all plant residue at the end of the growing season is critical to reducing the spread of the disease.
Some fungicides that contain chlorothalonil, mancozeb or sulfur are labeled for use in controlling hollyhock rust. However, keep in mind that fungicides are preventative, not curative. They can only protect healthy foliage from becoming infected. If the plant is heavily diseased, it is too late to apply fungicides. Always consult the label for recommended rates and safety information before applying.
See also the Purdue Plant & Pest Diagnostic Lab article, “Hollyhock Rust - NOT the same fungus as Soybean Rust!”