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 fall of the leaves.
Rust occurs most often in the spring and autumn. Early detection is important to gain control and stop the rust 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 stocks when plants are finished blooming. Rust spores overwinter in ground debris, and if not removed by early spring, can spread more hollyhock rust. Removing ground debris helps control the fungus. If a garden is mulched, the mulch should be removed and replaced each spring.
Chemical control may be necessary if rust reoccurs. Sulfur, applied as a dust or liquid spray, is effective and must be aimed at the underside of lower leaves. Using sulfur, providing good air circulation and avoiding overhead watering, especially in the evening, will reduce the severity of hollyhock rust. Some plant varieties are rust resistant.
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Updated Friday, April 19, 2013