In past years, cedar-apple rust has drawn attention to rust galls on junipers and leaf spots and defoliation on apples and crabapples. Now, we see a related rust causing galls and stem distortion on hawthorn. The damage is far worse than the temporary defoliation on the crabapples. I do not know why this disease has spread in the landscape.
Cedar-hawthorn rust (fungus name, Gymnosporangium globosum) causes the typical yellow-orange, rusty-colored pustules on the leaves of hawthorn. The spots appear on the upper leaf surface, and eventually the cuplike structures of the fungus (aecia) appear on the underside of the leaves. These cups are only about 1/4 inch in diameter but when massed are quite visible and spectacular (to pathologists anyway). These structures produce spores that are released in the morning hours or with rain. Often, homeowners first notice this disease when they see an orange, spore-covered sidewalk or lawn below their hawthorns. If the disease is severe, the leaves turn yellow and drop early. Cedar-hawthorn rust causes the leaf spots described. Occasionally, it also infects the fruit. The other host of this disease is cedar, most often eastern red cedar. The galls produced on the cedar are similar to the cedar-apple rust galls, but smaller. They are spherical, flattened on the stem side, perennial, and reddish brown. Resistant varieties would be ideal against these rusts. English hawthorn is reported to be resistant to cedar-hawthorn rust. Also, some junipers are resistant to this fungus.
The disease that causes more damage on hawthorn is cedar-quince rust. The causal fungus is still a Gymnosporangium, but a different species--G. clavipes. The cedar-quince fungus does not usually cause a leaf spot. It may infect a large leaf vein, causing the leaves to curl and die. It typically infects the fruit, petioles, and stems, causing much tissue distortion. The fruit become covered with fungal aecia, giving them an orange, fringed appearance. The fruit then dry and drop. These fruit infections were common in 2002.
Presently the fruit are still attached, but dried. You can still see the rust disease on these fruit. The stems develop spindle-shaped galls that may girdle the stems and kill all tissue beyond that point. The winter host for cedar-quince rust is juniper. Many species can be infected, but eastern red cedar is the most common in Illinois. The stems become roughened and swollen at the point of infection, but no large gall is visible. Typical rust sporulation occurs on the swollen stems in the spring.
Both juniper and hawthorn are necessary for these diseases to occur. Both are used commonly in the landscape, so it is not a practical management tool to separate the hawthorns from their alternate host (junipers). Protection of the hawthorn with fungicides is an option that may be considered if these rusts are problems on specimen or high-value hawthorns. Sprays are used in the spring to protect new growth and developing fruit. Consult the Illinois Homeowners' Guide to Pest Management, Commercial Landscape &Turfgrass Pest Management Handbook, and Report on Plant Disease, no. 802, for more information. If your hawthorns have a chronic problem with rusts, mark your calendar now to spray next spring. Sprays in summer do not provide disease control for the hawthorns. As a general rule, cedars are not targeted for disease control.