Cedar-apple rust is a well known challenge in most of our apple varieties. Lesser known, however, is one of this year’s more noticeable diseases: cedar-hawthorn rust. Affecting both ornamental and fruiting pear varieties, cedar-hawthorn rust pressure has been high this year.

While there are definitely differences in the diseases, they look very similar on the leaf: rust colored pustules that discolor leaves and may cause them to drop. Control is exactly the same as well. This disease is usually more of a nuisance than anything, seldom causing significant harm. Control is typically optional unless you see substantial leaf drop.

Cedar-hawthorn rust is only active early in the season. In fact, once May is over, it’s too late to try and control it, anyway. At this point in the season, it’s time to look ahead to control next season.

What does control look like? First, how does the disease work? The rust spores that result in the leaf infection have to come from juniper (cedars) trees. During rainy periods in April and May, you’ll sometimes see the orange, gelatinous looking galls appear. At this point, spores are moving to the host tree – in this case pear – to begin infection.

Second, you have to know what the fungicide products labeled for control can actually do for you. While there are numerous fungicide control products that could be used for control, most would require you to apply at seven to ten day intervals from the beginning of April until the end of May to prevent rust infestations. However, products that contain myclobutanil can be applied with a good chance of killing the rust spores for as many as four days after the rust germinates. That means that instead of multiple spray applications for prevention, we can wait until we actually see evidence of spores being released before we apply. Watching for the spores on cedar trees will allow you to apply fungicide products in a timely manner next spring.

Rust diseases are typically little more than an aesthetic nuisance. Since control programs have to be implemented well before now, do what you can to keep trees with the heaviest infections as healthy as you can through the summer and in to next spring. Periodic watering is a big part of making sure that trees affected by cedar-hawthorn rust don’t succumb to other stressors such as drought.

For more information, see our KSU Plant Pathology information sheet on the topic at: https://www.plantpath.k-state.edu/doc/extension-factsheets/apple-rust.pdf .