M. Grabowski, UMN Extension
Orange leaf spots on the upper leaf surface are accompanied by raised reddish brown spore producing structures on the lower leaf surface.
Hollyhock rust, caused by the fungus Puccinia malvacearum, is a common and destructive disease affecting hollyhocks. The fungus, unlike many other rust fungi, completes its entire life cycle on one host. The fungus survives winter in plant material infected the previous year. In the spring, spores are rain splashed or wind blown to hollyhock leaves, starting new infections.
Infection first occurs on the lower leaves and then progresses upward to involve the entire plant. Initial symptoms of hollyhock rust are yellow to orange spots on upper leaf surfaces and brown, elongate lesions on the stems. Later, red to brown pustules develop on the lower surface of infected leaves. Orange-red spores are released from the pustules and rain splashed or wind blown to nearby leaves, starting new infections throughout the summer. Eventually, leaf spots brown and drop out leaving holes. Severely infected leaves turn brown and dry.
Hollyhock rust can be managed during dry years using cultural control practices. Remove all plant debris in the fall to reduce the amount of inoculum the following spring. Remove any mallow weeds in the surrounding area, as this weed can be infected by the same rust and serve as a source of inoculum. Use healthy transplants or start disease-free seeds each year. Keep plants growing vigorously in a well-spaced, sunny location, and provide sufficient water and fertilizer. Avoid overhead watering to prevent rust spores from being splashed from one plant or leaf to another. Routinely check hollyhocks for leaf spots and remove any infected leaves as they appear.
Michelle Grabowski, Chad Behrendt, Crystal Floyd