Hollyhock plants don't have many disease problems, but one fungus that plagues them is rust. All of the green parts of the plant are susceptible to infection. The plant rarely dies from rust, though severe infections cause yellowing and early leaf drop.
Rust occurs most often in the spring and autumn. Early detection is important to gain control and stop it from spreading. Rust first appears on the underside of the lowest leaves as lemon-yellow to orange, almost waxy, pustules that turn reddish brown with age. Pustules are small, blister-like formations. On the leaf surface opposite the pustules, larger bright-yellow to orange spots with reddish centers develop. Once on the leaves, rust can quickly spread to stems and other leaves.
Pick and destroy infected leaves at the first sign of rust. Cut down and destroy infected stalks when plants have finished blooming. Rust spores overwinter in fallen plant debris,so rake up and remove this material by early spring to control the disease. Rake up and remove this material by early spring.
Chemical control may be necessary if rust reoccurs. Sulfur, myclobutanil and chlorothalanil are examples of fungicides registered for use on hollyhock. Avoid using sulfur when temperatures are predicted to reach 85 degrees F within 24 hours of application. Leaf burn can occur under those circumstances. Providing good air circulation and avoiding overhead watering, especially in the evening, will also help reduce the severity of hollyhock rust. Some hollyhock varieties are rust resistant.
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Updated Monday, February 01, 2016