This is a very common disease of hawthorns and juniper shrubs. The disease requires two hosts to complete its life cycle, and the symptoms look very different on the two host species. In the spring, the hawthorn leaves emerge and appear healthy. Soon after flowering, the rust spots start appearing on the leaves and the “hairy-looking” fungal fruiting bodies can be seen on the underside of the leaves. Spores from the fruiting bodies infect a juniper host, and brownish galls form on the juniper twigs. The following season the galls produce spores, which are dispersed by wind to other hawthorn hosts, and the disease cycle continues.
The rust fungus requires two hosts to complete its life cycle, so it is sometimes possible to reduce the incidence of disease by removing one of the hosts from your yard. People generally favour the hawthorn over the juniper. However, if one of your neighbours has susceptible junipers in their yards, your hawthorn can easily be re-infected from their plants. Remove juniper galls by pruning in the dormant season.
New landscapes should not include both hosts.
Protective sulphur sprays can begin on hawthorn in spring, beginning at blossom time (around the time the orange spore horns are emerging from the juniper galls), and continuing at 10-day intervals. We have also found that copper oxychloride provides protection against this disease.
(c) 2015 Shane LePage, Wild Rose Garden & Tree Service Inc., Red Deer, AB.