This disease of hollyhock is not extremely difficult to diagnose. It is a rust and therefore causes pustules with orangish spores, but it forms fruiting bodies on the underside of leaves in pustules that look like little galls at first. Severely rusted leaves turn yellow, wither, and drop early. And, although plants may become ragged in appearance, they rarely die from rust. Usually, we see this disease in spring and fall, but last week we were still receiving calls about this rust at the Plant Clinic.
Rust first appears on the underside of lower leaves as lemon yellow to orange, waxlike pustules that turn reddish to chocolate brown with age (its present stage). At first, these waxy pustules resemble pycnidia or other flasklike fruiting bodies rather than the open pustule of most rusts. 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. In humid weather, the rust fungus continues to spread from leaf to leaf until the entire hollyhock plant becomes infected and loses its leaves.
As a pathologist, I am intrigued by this disease because it is microcyclic, producing only two types of spores and needing no other host to complete its life cycle. Also, its pustules appear different from other rusts. In issue No. 1 of this newsletter, you learned that cedar-apple rust requires two host species and has several spore stages. Rust of hollyhock should be easier to control because we can concentrate on the plant. Pick off and destroy the first rusted leaves. As soon as flowering is over, collect all rust-infected hollyhock leaves and stalks, destroy them by burning, and bury them in a compost pile or haul them away with the trash. On some weeds, the rust fungus can survive the winter, so practice good weed control.
Preventive fungicides are available for this rust, but sprays must be initiated in the spring as new growth appears. If this is a perennial problem that you would like to control, mark your calendar now to spray next spring. Options are listed in the pest control handbooks as usual. You might also consider looking for a resistant variety. One friend said that his new, improved variety is loaded with rust while his neighbor’s hollyhock, which had been his grandmother’s, is unaffected. For more on this disease, read Report on Plant Disease No. 627.