Cedar-apple and related rusts cause considerable spotting, leaf discoloration, and defoliation of crabapples, quince, and hawthorn. Landscapers are particularly concerned about hawthorns. Although cedar hawthorn rust causes leaf spots and early defoliation, it is cedar-quince rust on hawthorn that causes the most damage. That disease may cause fruit distortion, galls on twigs, and stem dieback. The alternate host is juniper (usually Juniperus virginiana, which is eastern redcedar). Damage to the juniper is usually minimal, so treatment is focused on hawthorns.
Cedar-quince rust on hawthorn rarely causes leaf spots. The fruit becomes covered with the aecial stage of the fungus, which appears as many tiny, white, cuplike structures protruding from the fruit. This stage is attractive but does not last. The fruits shrivel and drop from the tree. Next the petioles, thorns, and twigs swell and turn orange with rust spores. Twig galls can girdle stems, resulting in much dieback. Obviously, this stage is most undesirable for the landscaper.
Spores from the juniper blow to the hawthorn in the spring and cause new infection as long as juniper galls are active. In central Illinois, galls began to swell rapidly the first week of April in the abnormally warm weather. Cool temperatures the next week slowed progress. We cannot give exact dates of spore movement because this event is triggered by weather conditions. Find galls on local junipers and watch them daily if accurate timing is necessary.
There are no commercially available hawthorn species with resistance to cedar-quince rust. If you have had a problem with cedar-quince rust on hawthorns and wish to get the disease under control, here are a few suggestions. Try to locate susceptible juni-pers away from hawthorns. During the winter, it is a good idea to prune out all remaining galls on hawthorn twigs infected in the past. Most of the galls will be dead, but some will still produce spores; so removal is both aesthetic and practical. It is possible to control this disease with fungicides, but keep in mind that fungicides prevent infection only for the year a chemical is applied: This is not a permanent fix. The fungus will move from the junipers and infect new shoots and fruit of the hawthorn very soon. Fungicide protection must be provided from bloom to whenever the juniper galls dry up (usually 1 to 2 weeks after petals fall). If you have found some galls on junipers in your area, you can be certain of the spray timing. The application of fungicide on hawthorn should be made when the orange galls on the juniper are gelatinous. This is usually when the hawthorns are in flower. Chemicals should be repeated according to label directions. A systemic product requires fewer applications. Refer to page 63 of the 2003 Commercial Landscape & Turfgrass Pest Management Hand book for a list of fungicide options. Refer to page 79 for information on chemical mobility to determine which have contact or systemic action. Similar infor-mation is on pages 91 and 103 of the Home, Yard, & Garden Pest Guide.
For more information, consult Report on Plant Disease, no. 802, “Rust Diseases of Apple, Crabapple, and Hawthorns,” available on the Extension VISTA Web site or in local Extension offices.