There are various rust diseases on landscape trees. The cedar–apple and related rusts are most universal and therefore receive the most attention, at least in Illinois. Pine–pine rust galls and pine–oak rust galls also had some recent appearances in Illinois nurseries and landscapes. In addition, white pine blister rust has some historical significance but is currently rare in Illinois landscapes.
The three major cedar–apple and related rusts that we see in Illinois are cedar–apple rust, cedar–quince rust, and cedar–hawthorn rust. These are appropriately named for the alternate hosts on which they occur or the hosts on which they were first reported. Landscapers and homeowners are most frequently concerned about crabapples with cedar–apple rust and hawthorns with cedar–hawthorn rust or cedar–quince rust. The cedar host (red-cedar and therefore actually a juniper species) is not significantly harmed, and management strategies are usually concentrated on the crabapple or hawthorn host. Hawthorns have been plagued by cedar–quince rust in the past 5 years. This rust does not affect the leaves but causes unsightly galls or swellings on the hawthorn stems. The galls turn black and may girdle and kill the twig from that point outward. Cedar–hawthorn rust causes only leaf spotting.
Details can be found in the Illinois publication, Report on Plant Disease, no. 802, “Rust Diseases of Apple, Crabapple, and Hawthorn,” available in an alphabetical listing under horticulture publications at http://www.ag.uiuc.edu/%7Evista/. Illinois Extension offices should also have this publication.
Management strategies include the use of resistant cultivars, removal of galls on junipers where feasible, and use of preventive fungicide. There are many choices of resistant crabapple cultivars. At this U of I Web site, http://www.extension.uiuc.edu/IPLANT/plant_select/trees.htm, you will find a very informative article on selecting crabapples, including a listing of cultivars that can be grown in Illinois. This list rates plant disease reaction to cedar–apple rust, as well as to scab and fire blight. An Ohio publication, Cedar Rust Diseases of Ornamental Plants, lists some resistant junipers, crabapples, and hawthorns. This publication can be found at http://ohioline.osu.edu/hyg-fact/3000/3055.html.
Chemical options are also available for disease control. Where problems have occurred in the past and resistance is not available, fungicides may be sprayed annually to prevent infection. The sprays must be applied before or soon after spores arrive on the crabapple or hawthorn. Most recommendations tell us to make the initial spray when crabapples are in pink bud and then to continue sprays through petal fall. For hawthorns, the recommendation is similar but lasting 2 weeks after petal fall. Read product labels for recommended spray intervals. The idea is to protect the plants from spores that are moving from junipers to the susceptible plants when those plants are most vulnerable to infection. If you have some infected junipers with rust galls, you can observe when the horns emerge and spores form. Protective fungicide sprays should be applied at that time. As this article is being written (April 16), cedar–apple rust galls in Champaign already have horns emerging from the galls. There are many chemical options available in the 2004 Home, Yard, and Garden Pest Guide and the 2003 Commercial Landscape & Turfgrass Pest Management Handbook. For longer fungicide activity, a systemic product should be considered. Tables at the end of appropriate chapters provide mobility information on each chemical listed.