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| 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.