rust on hollyhocks
For the past three years, my hollyhocks have been afflicted with rust. They start out all right but as they mature, the rust comes in and covers their leaves. The blossoms are okay but the leaves wither up. I have read that copper sulfate can alleviate this problem. Is this so, and if it is, should I use liquid copper sulfate or crystals? When should it be applied? Should I pull up existing plants and destroy?
The rust disease overwinters in infected plant debris and new spores form in the spring and move into newly emerged leaves. Because rust spores will continue to overwinter and return every spring, it is important to break the disease cycle to reduce hollyhock rust problems. Hollyhocks should be planted in areas with good air circulation. Good fall clean-up is important, cut the plants back to ground level and remove any fallen leaves. Remove the plant material from the area. When watering avoid wetting the foliage and place a 1-2 inch layer of mulch underneath the plants. Also removing the first leaves that have rust in the spring will help reduce the disease problem.
You can manage the bed culturally and if not effective, decide if you want to spray in the future. Plants can be sprayed with a labeled fungicide as a preventative before the rust appears on your plants. Keep in mind most fungicides are preventative and will not cure the leaves that are infected. Fungicides are applied as the new leaves emerge. Contact your extension for chemical recommendations, as we cannot recommend pesticides across state lines.