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Cedar Apple Rust- what is it, how to treat it

To a gardener, juniper trees & apple trees are like oil & water- they don’t mix. Well, they DO mix, but not in a good way.

Cedar apple rust (Gymnosporangium juniperi-virginianae) is a fungal disease that requires juniper plants to complete its complicated two year life-cycle. Spores overwinter as a reddish-brown gall on young twigs of various juniper species. The name came from the discovery of the fungus on red cedar, or Juniperus virginiana

While it doesn’t cause too much harm to the host juniper tree or shrub, the fungus will wreak havoc on your apple or crab-apple trees. trying to mind their own business and grow nearby.

In early spring, during wet weather, these galls swell and bright orange masses of spores are blown by the wind where they infect susceptible apple and crab-apple trees. The spores that develop on these trees will only infect junipers the following year. From year to year, the disease must pass from junipers to apples to junipers again; it cannot spread between apple trees.

On apple and crab-apple trees, look for pale yellow pinhead sized spots on the upper surface of the leaves shortly after bloom. These gradually enlarge to bright orange-yellow spots which make the disease easy to identify. Orange spots may develop on the fruit as well. Heavily infected leaves may drop prematurely.

Treatment

Choose resistant cultivars when available- here’s a link to a list of them

Rake up and dispose of fallen leaves and other debris from under trees.

Remove galls from infected junipers. In severe cases, juniper plants should be removed entirely. Keep in mind however, the fungal spores can travel in the wind a long distance so the infected juniper may not even be in your yard.

Apply preventative, disease-fighting fungicides labeled for use on apples weekly, starting with bud break, to protect trees from spores being released by the juniper host. This occurs only once per year, so additional applications after this springtime spread are not necessary.

On juniper, rust can be controlled by spraying plants with a copper fungicide at least four times between late August and late October.

Safely treat most fungal and bacterial diseases with Fungonil by Bonide

Always follow the manufacturer’s instruction on any treatment.

If you’re unsure about anything with application, or to identify this or other types of fungus, feel free to stop in the garden center and speak with one of our knowledgeable experts. If you can’t make it in, you could send us a message under the Contact Us tab, with a couple clear photos of the damage in question and the best contact number to reach you. We look forward to helping your plants thrive!