Rust of Hollyhock

Overview

Rust, Puccinia heterospora, is a fungal disease that infects the leaves and stems of hollyhocks. Lower leaves show the condition first, and the disease progresses upward during the growing season. The extent and severity depend on weather conditions.

Symptoms and Diagnosis

The first symptoms appear on the leaves as individual, yellow, circular spots about one-eighth inch in size that enlarge to one-quarter inch or more in diameter. On the underside of the leaves, swellings or blisters soon emerge within these spots. At this stage, the symptoms resemble insect or mite galls found on many other plants; however, such galls are rare on hollyhocks. The swellings continue to develop and in a few more days, they release masses of reddishbrown spores, covering the entire undersurface of the leaf.

You can easily identify the disease in the field by the round pustules that are covered by loosely attached red spores. Leaves that are heavily infected eventually turn gray or tan and die. Rust spots may also occur on the stems and leaf petioles and occasionally on green flower parts.

Life Cycle

The reddish spores are easily spread by splashing water, rain, and wind. The fungus overwinters in plant debris and possibly in overwintering, living plant tissue. Symptoms will appear very early the following spring when weather conditions are favorable.

Hollyhock rust is not extremely common throughout the state. Fortunately, we can usually enjoy colorful hollyhocks in spite of the disease, though the foliage may become unattractive in wet years.

Integrated Pest Management Strategies

1. Use disease-free plants. Start plants from seeds and place them some distance from the infected stock.

2. Improve air circulation. Space plants out in a dry sunny location so that moisture is quickly evaporated from the foliage.

3. Keep plants healthy. Maintain the plants in a vigorous growing condition through a good fertility program. Irrigate during dry periods but keep the foliage dry, if possible.

4. Remove infested material. Remove old plant material and bury, burn, or compost it at the end of the flowering season. Picking off the first infected leaves as they appear may also be helpful. However, this practice alone often is not successful in controlling rust.

5. Control weeds. Destroy common mallow weeds in the vicinity as they may be hosts to rust disease and a source of hollyhock infection.

6. Use fungicides if necessary. Consider using fungicides to supplement cultural control methods. Direct the applications primarily at the lower surface of the leaves and treat at approximately 7 to 10-day intervals. Reapply if rainfall exceeds 1/2 inch during the period. Pesticides registered for use include chlorothalonil, sulfur, and mancozeb. All are protective fungicides and prevent rather than eradicate the fungus. Start using a fungicide prior to the first sign of disease development.

Organic Strategies

Strategies 1, 2, 3, and 4 are strictly organic approaches. Using an appropriate organic herbicide (or removal by hand) would be a viable organic approach to Strategy 5.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against rust. Not following the pesticide label before usage is a violation of federal law.