The rust diseases cause yellow to orange to bronze colored spores in open pustules on the plant surface. They give the plant a rusty appearance; thus, the name of the disease. The cedar apple type of rusts in our area of the country usually refers to cedar-apple, cedar-hawthorn, and cedar-quince rusts. These are also known as the Gymnosporangium rust diseases. Each requires two host species to complete its life cycle, which is why the disease name is hyphenated with the name of two hosts.
The evergreen host for all three of these diseases is juniper or red cedar. That host now harbors brown galls of the rust fungus on its stems. Galls of cedar-apple rust are rounded and about 1 inch in diameter. Cedar hawthorn rust galls are somewhat smaller and more irregular. Cedar-quince galls are actually more of a swelling on the stem of the junipers rather than a gall. Any day now, you will see these galls begin to develop yellow to orange, gelatinous tendrils covering the gall surface. The spores on these tendrils blow to nearby broadleaf hosts in wet weather in April or May. Apple, crabapple, hawthorn, and quince are the major deciduous hosts in our area. Leaves, stems, or fruit might be infected, depending on the rust disease and host. The rusty colored spores once again make diagnosis easy on these hosts.
In most cases, control measures are focused on the deciduous host. Resistant cultivars would, of course, be the preferred method of control, but that is not always an option in an established landscape. If the disease occurs each year and trees are valuable specimens, fungicide protectants might be warranted. Consult the Illinois Commercial Landscape & Turfgrass Pest Management Handbook 2000 or the llinois Homeowner’s Guide to Pest Management for chemical options. Landscapers who use fungicides complain most frequently about lack of disease control on hawthorns. The major issue is most often the timing of the chemical applications. The fungicides are used as protectants. Fungicide sprays to protect hawthorns must be started when new growth appears and flower buds begin to open. This varies with the location and weather conditions, so you will need to watch your hawthorns closely. Repeat applications also vary with the product chosen. Read the label on the chosen fungicide carefully for these details. If you have had problems with efficacy in the past, try one of the systemic fungicides that may give longer protection. Those listed in the commercial handbook as having some systemic activity include Banner, Bayleton, Duosan, Eagle, Spectro, Systhane, TwoSome, and Zyban.
More information on these rust diseases can be found in Report on Plant Diseases No. 802, Rust Diseases of Apple, Crabapple, and Hawthorn, available on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm