By Adam Champ, WVDA, Plant Regulatory Officer II
Hollyhocks are favored by many gardeners for their cheerful summer blossoms. Unfortunately, these graceful perennials are afflicted with a common and destructive disease called hollyhock rust. The fungi, Puccinia malvacearum, is the pathogen responsible for hollyhock rust. Many plants in the mallow family, Malvaceae, are susceptible to infection of hollyhock rust, which includes several common weeds. Even though many species are susceptible, hollyhock rust is an autoecious rust, meaning it does not require an alternate host to complete its life cycle like many rust fungi.
The pathogen survives the winter in hollyhock debris from the previous season and begins producing infectious spores in spring. Spores can be wind-blown, rain-splashed, or moved by people, animals, or tools to nearby hollyhocks. Existing infections will produce more spores from pustules during favorable environmental conditions, leading to secondary infections throughout the growing season.
Symptoms of hollyhock rust include yellow to orange spots on the upper surface of the leaf. Beneath these spots, raised brown pustules will later develop on the undersides of the leaf. As the disease progresses, leaves will wither and eventually defoliate. Plants may blight rapidly under moist and cool environmental conditions, which favor the development of the pathogen.
Cultural controls are important in controlling hollyhock rust. One of the most important cultural controls is removing/destroying hollyhock vegetation at the end of the growing season to remove inoculum for the following spring. Ideally, you want to destroy old vegetation by burning or deep plowing/burying, but you can also double bag and send to the landfill.
There is a wide variation of genetic susceptibility between varieties of hollyhocks and using more resistant varieties is encouraged. Remember that ‘resistant’ doesn’t necessary mean ‘immune.’ Inspect plants at the nursery and only purchase apparently healthy plants.
Avoid irrigation over the plant to keep moisture off the foliage. Irrigating by drip or hose at ground level will help keep the foliage dry. Irrigating in the morning will promote rapid drying of leaves as opposed to evening. Keep proper spacing around the hollyhocks to allow for good air drainage, which in turn will decrease humidity. Controlling weeds around your hollyhocks have the dual advantage of not only improving air drainage, but also removes weed species that may also be hosts of hollyhock rust.
Fungicides are available when chemical control is necessary. Chemical control frequently can’t overcome poor cultural controls. A balanced approach of cultural practice and fungicide application is critical to success. Most fungicides are preventative in nature and need to be applied before infection to be effective. Even systemic fungicides which may have some curative properties, will only help very early in an infection. Apply fungicides before or shortly after infection is observed. Achieving control in the primary infection period early in the growing season will result in reduced disease pressure from secondary infections later in the summer. As with all pesticides, read and follow all directions and precautions found on the label.
Adam Champ is a Plant Regulatory Officer II with 15 years of experience with the WVDA. His responsibilities include nursery inspection and phytosanitary certification.
He holds a Bachelor of Science in Horticulture, minor in Environmental Microbiology from WVU and Associate Degrees in Horticulture and in Forest Resource Management from Potomac State College.
