Gardening Tips: Hollyhock Rust
This year, an early warm, dry period followed by late rains resulted in one of the worst cases of hollyhock rust I’ve ever experienced.
The University of California Integrated Pest Management program (http://ipm.ucanr.edu/), provides information about rust diseases on various plants such as roses, chrysanthemums and geraniums, broad-leaf trees like poplar and pear, and conifers such as cedar, pine and juniper. In general, rust organisms cause leaf spots and orange blotches that contain the spores for new infections. Rusts are happiest when it’s wet and mildly warm. And the spores can blow for miles, so when conditions are right, it’s almost impossible to avoid infection.
The University of Wisconsin Horticulture Program contains a pest note specific to hollyhocks (https://hort.extension.wisc.edu/articles/hollyhock-rust/). Hollyhock rust (Puccinia malvacearum) is the most common fungal leaf disease of hollyhocks and also can infect both cultivated and wild members of the mallow family. Usually, the disease effects are limited to premature leaf death and drop, but sometimes (rarely) it can kill the plant.
So, what can be done if you experience an unusually heavy infestation of hollyhock rust? According to the University of Wisconsin, “Once symptoms of hollyhock rust appear, control can be difficult.” Airflow is key to preventing the wet, warm environment fungi love.
Recommendations include: remove common mallow weeds from the area. They may be a host for the fungus. Avoid overhead watering; splashing water scatters the spores. Remove diseased leaves and stems. Dispose of damaged plant materials in the municipal trash (if allowed) or by burying them deeply. Do not compost or allow them to lie on the ground. Do not take the damaged plant materials to the yard waste recycling site. That can spread the disease to other areas.
Fungicides are available that are listed for use on hollyhocks but need to be sprayed repeatedly using different products with different modes of action to prevent fungicide resistance. According to UC, “The frequent applications required to provide good control of rust may not be warranted in many landscape situations.”
If you prefer less chemical control methods, the University of Wisconsin recommends not using the seed from infected plants. Avoid planting hollyhocks densely to allow the plants to dry during warm, wet weather. Water properly with a soaker or drip hose and only fertilize when indicated by a soil test. Lush, healthy growth provides an attractive site for fungus to grow.
As for me? I may not grow hollyhocks next to my fence next year. I’ve already decided that I won’t use fungicides, so I may have to take a break from the disease cycle.
Rebecca Miller-Cripps is a University of California Cooperative Extension Master Gardener of Tuolumne County.
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