Many of you are seeing (or will soon see) rust on your hawthorns. There are two major rust diseases that we see on hawthorn in Illinois, both of which were mentioned in issue No. 2 of this newsletter in the “Cedar-Apple and Related Rusts” article.
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 together are quite visible and spectacular. 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 will turn yellow and drop early. Cedar-hawthorn rust causes the leaf spots described. Occasionally, it will also infect 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. Not many choices are available. English hawthorn is reported to be resistant to cedar hawthorn rust. Also there are some junipers resistant to this fungus.
Cedar-quince rust also infects hawthorn. 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. 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. There are many species that can be infected with eastern red cedar, the most common in Illinois. The stems become roughened and swollen at the point of infection, but a large gall is not visible. Typical rust sporulation will occur on the swollen stems.
It is not a practical management tool to separate the hawthorns from their alternate host (junipers). Both are used commonly in the landscape, and the fungi can infect trees as much as a quarter of a mile away. 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 2000, and RPD No. 802 for additional 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.