Hollyhock Rust
By Brett Kerley
Every year I’m asked about a certain problem, ‘What on earth is happening to my hollyhocks? They look terrible! I’ve tried using fungicide and cutting off the infected leaves. What else can I do’?
About Hollyhock Rust
Well, why don’t we delve into this problem a little more? Hollyhock rust (or mallow rust) is a common fungal leaf disease of hollyhocks (Alcea rosea), it’s caused by the fungus Puccinia malvacearum. Also susceptible are other plants that are in the Mallow family, such as Hibiscus and Globe Mallows.
Firstly, let’s identify it. Signs of this fungus include orange-coloured rust pustules on the undersides of leaves. These progress to bright-orange spots with red centres on the upper leaf surface and on the shoots of hollyhocks. Eventually the rust pustules on the underside turn a reddish chocolate-brown colour. When the disease is severe, it can cause stunting, and premature leaf drop. A severe infection could kill your plant.
How To Prevent It
Sanitation is the key to preventing hollyhock rust. During the growing season, it is important to remove any leaves that become infected, because this fungus produces new rust spores that continue to cause infections. Sanitation also includes weeding out of round-leaved mallows (Malva rotundifolia), another host for this fungus
Remove all infested plant parts at the end of the season to limit the amount of fungus that will survive the winter. If not cleaned up, the infested plant debris will be the source for infecting the remaining hollyhocks in your garden come spring, as well as any you plant in the future, those that reseed, and of course, your neighbour’s hollyhocks. Once removed, the infected debris should be burned, bagged and added to your garbage, or taken to a yard-waste disposal site.
In addition to sanitation, keeping water off the hollyhock foliage and providing room for good air movement helps in rust prevention.
As for fungicide controls, products containing chlorothalonil, mancozeb, and sulphur can help. But since infections start in spring and continue during rainy weather into the fall, numerous preventive applications at 10 to 14 day intervals may be necessary to completely prevent rust. This is why proper sanitation is so critical. If you do use a fungicide, apply it only to foliage and stems, and of course use it cautiously and strictly according to the directions. By spraying any fungicide on the flower part of the plant it can cause our beneficial insects to die, thus creating more problems down the line.
Avoiding Future Problems
If you have hollyhock rust it is essential not to use those seeds for future plantings. Buy fresh seeds. Whilst purchasing plants at the garden centre, inspect closely as they may already have the disease. Always water the base of the plant so you don’t give rust the opportunity to grow on wet leaves. Drip irrigation or a soaker hose works great for these plants. Only fertilise when needed, according to the plants growing instructions on the seed packet. And lastly avoid planting hollyhocks densely to allow better air circulation that will promote more rapid drying of plants, as well as reduced humidity.
Happy gardening everyone!
