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Hollyhock rust is the most common ailment of hollyhocks and shows up as tiny spots on the undersides of the lower leaves. These spots then grow larger and become orange or brown. With time, larger bright yellow to orange spots with reddish centers develop on the upper leaf surfaces. With appropriate environmental conditions (damp or humid weather), rust infections will quickly spread to other leaves until they become covered with chocolate colored pustules of spores of the rust fungus. These spore pustules may also form on leaf stems, main stalk, and green leaves at the base of the blossoms. Plants with rust are rarely killed, but they look unthrifty and ragged.
The fungus that causes this rust disease winters in infected plant material left over from the previous year. Be sure to remove and destroy infected leaves, stems, and other plant debris after the plants finish blooming to help reduce the amount of fungal spores that will be available to cause new infections next year. Don't neglect fall clean-up: cut down the plants to the ground and destroy the stalks and fallen leaves.
Some fungicides are effective against hollyhock rust. If you choose to use a chemical, the fungicide should be applied as soon as the first spore pustules are observed. Be sure to check the label for any precautions regarding use.
Plant and Pest Digital Library and Digitally Assisted Diagnosis, Purdue University, West Lafayette, Indiana.