Just when you think you can identify any rust disease, a slightly different version pops up. Hollyhock rust has actually been with us for many years. It is caused by the fungus Puccinia malvacearum and is most severe in the spring and fall. Severely rusted leaves turn yellow, wither, and drop early. Plants may become ragged in appearance, but rarely do they die from a rust infection. Rust first appears--primarily on the undersides of the lower leaves--as lemon-yellow to orange waxlike pustules that turn reddish to chocolate brown with age. At first these waxy pustules resemble pycnidia or other flask-like fruiting bodies rather than rust pustules and can be confusing to diagnose. As larger spots of bright yellow to orange with reddish centers develop on the leaf surface opposite the pustules, symptoms become more typical of rust. The rust quickly spreads to other leaves, stems, and flower bracts. In humid weather, rust continues to spread from leaf to leaf until the entire hollyhock plant becomes infected and loses its leaves one by one.
This rust is also unusual in that it is microcyclic, producing only two types of spores (teliospores and basidiospores). The tree rusts (such as cedar-apple rust and cedar-quince rust) need two hosts to survive. Hollyhock rust does not need an alternate host.
The first rusted leaves should be picked off and destroyed. As soon as flowering is over, all rust-infected hollyhock leaves and stalks should be collected, then destroyed by burning, burying in a compost pile, or hauling away with the trash. Because rust may survive the winter on some weeds, they also should be removed.
In cases where disease control cannot be maintained with sanitation, preventive fungicides may be used, starting when new growth begins in the spring. All above-ground parts of the plants should be treated, and five or six applications are necessary at seven- to ten-day intervals. Registered chemicals include those listed on page 17 of the Illinois Homeowners’ Guide to Pest Management and page 125 of the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998-1999. Be sure to read labels carefully and always follow directions given.