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Garden Q&A: Rust can take away from your hollyhock's beauty

| Friday, Jan. 4, 2013,
If you look at the upturned leaf between two of the flowers on this old-fashioned hollyhock, you'll see signs of a rust infection -- orange pustules. Jessica Walliser

Question: My old-fashioned hollyhocks were a complete disaster this year. By old-fashioned, I don't mean those new-age carnation types; I mean those single-petaled types we had as kids. This past spring, the leaves developed normally, but then a brown, ugly fungus took over. The stalks grew tall, but there were only puny buds and no flowers. Then the leaves and stalks died. What caused the fungus? What should I do in the spring to avoid this problem again?

Answer: Rust is a common fungal disease among hollyhocks, and the symptoms you describe are classic. There are thousands of different species of rust, some of which are major agricultural pests. The species of rust that attacks hollyhocks is Puccinia malvacearum and, unlike some other rusts, it needs only a member of the hollyhock family to complete its lifecycle. Other susceptible plants include mallows, rose-of-Sharon, lavatera, and flowering maples ( Abutilon sp.). The good news is that rust seldom kills hollyhock plants, but it does, as you discovered, make them look terrible. Here's what to do about it for next year.

The spores of hollyhock rust are spread from plant to plant primarily on the wind, though they can be spread by humans, animals or insects. Initial signs of rust are small, pale splotches on the upper leaf surface. They quickly turn into brilliant orange bumps on the lower leaf surface. These are the spore-producing bodies. They can also occur on the stems and even flower buds. In severe cases, as you discovered, affected leaves often shrivel up and drop off the plant entirely and the flowering is reduced.

As with all fungal diseases, wet leaf surfaces promote infection. Be sure to give your hollyhocks plenty of room to optimize air circulation around the plants. Always irrigate the root zone only and avoid wetting the leaves whenever possible. Since the spores easily survive winter on infected leaves and stems, right now you should head out to the garden and remove every piece of old foliage and all the stalks. Dispose of them in the trash or burn them. During the growing season, remove and throw away infected leaves as soon as possible.

You also can use an organic biofungicide such as Serenade to aid in the prevention of rust next spring. It must be applied at 10-day intervals to upper and lower leaf surfaces before the initial signs of infection occur.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212

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