Tuesday, June 11, 2013

Juniper-Hawthorn Rust by Andrew Vogt

Cedar apple or Juniper Hawthorn rust
It is late May or early June and, after some much-needed spring showers, your juniper and cedar trees look as if they were used as a backdrop for a contest in which paint ball guns were used to shoot globs of gelatin.  Globs of a tan or orange gelatinous substance of various sizes cling stubbornly to many of the smaller branches and twigs of your plants, often surrounding them and encasing some of the needles, especially on the upper and inner foliage surfaces.  Some of the globs, especially the larger ones, have noticeable protrusions that look like horns.  Within a few days, the globs of gelatin will turn the color of chocolate and begin to harden into unsightly galls that will range in size from 1/16 inch to more than 2 inches.  What happened?

The answer is that your junipers and cedars have become hosts to a fungus, “Gymnosporangium”, which is also commonly known as juniper-hawthorn rust or cedar apple rust gall.  The gelatinous globs on your trees were actually exuded from tiny pocket-like depressions on the surface of existing galls that were already on your trees.  The gelatinous globs are called “telial horns” or spore horns from which billions of fungus spores are ejected and spread by the wind to other hosts.
Will these galls compromise the health of your juniper or cedar trees?  No, but left unchecked they will be unsightly, and they may lead to damage to the health of certain other nearby species of trees that are susceptible to the fungus and are essential to the life cycle of the fungus.  These other susceptible trees, called alternate hosts, include hawthorn, apple, crabapple and mountain ash.  These alternate hosts are essential to the life cycle of the fungus because the fungus must spend 4 to 6 months of its 2-year life cycle on an alternate host after maturing for 18 to 20 months on a juniper or cedar host.  Infected alternate hosts can suffer premature defoliation, leading to winter damage, and failure to set fruit.
Unfortunately, no fungicide is effective in eliminating the galls or their gelatinous “blossoms” once they have appeared on a juniper or cedar tree.  However, the life cycle of the fungus can be broken by removing existing galls from the tree before next year’s gelatinous globs appear, and applying a fungicide containing Triadimefon at 2-week intervals from July through September.  Nearby alternate hosts should also be treated with fungicide.
For more information about juniper-hawthorn fungus, call Jefferson County CSU Extension at 303-271-6600 and speak with a Master Gardener.