Hollyhocks are infamous for being plagued by a fungal disease called rust, which produces reddish colored pustules (spots) on the upper and lower surfaces of leaves, stems, and green flower parts and causes early leaf drop. There are nearly 4,000 species of rust, and these fungi infect everything from grains to pine trees. Hollyhock rust (Puccinia malvacearum) attacks only hollyhocks and other members of the hollyhock family (Malvaeceae), including albutilon, hibiscus, lavatera, and malva. Weeds in the mallow family, such as cheese weed (Malva parviflora), velvetleaf (Albutilon theophrasti), and bristly mallow (Modiola caroliniana), also host hollyhock rust.
Some gardeners won't grow hollyhocks because of rust, but I say never give up on a favorite flower! Especially when you can take a few easy steps to prevent the disease from recurring and spreading. Rust, like most fungi, needs water to germinate and grow. So it's important to keep your hollyhocks' foliage and flowers as dry as possible, though this can be difficult in very humid climates. Hand water at the base of the plants, or use soaker hoses to keep leaves dry when watering. Space plants farther apart to ensure good air circulation, and avoid working around plants when they are wet. Don't plant hollyhocks by other susceptible host plants, and be sure to pull any weeds in the mallow family.
If your plants do become infected, good garden sanitation practices are the best remedy. Remove affected leaves immediately and burn or dispose of them in the garbage. Don't place affected plants in your compost pile. Spores overwinter in infected leaves and stems. Most hollyhocks are biennial, which means that once they produce seed, they die. In late summer, when you see the rosettes of foliage that will provide next year's display, promptly pull up any finished stalks and dispose of them. At the end of the season, be sure to remove any dead plant matter remaining in the bed, because it harbors rust spores and perpetuates the problem. In spring, mulch around the base of the hollyhocks.