Those jelly-like orange masses on the junipers produce spores that infect the apple hosts. Once infection occurs, leaf spots on apple leaves develop in 1-3 weeks. Eventually, fungal spores are produced in these leaf spots on the apple tissues. The spores are spread by wind and rain back to junipers starting in about July. Without both hosts, the fungus can’t complete its life cycle.
The disease looks dramatic on junipers, but it does not cause any harm. The rusts can cause problems in the apple host, however. If infection is severe, many leaves drop off early and the tree is weakened due to reduced photosynthesis. If your tree only gets a small amount of rust each year, it probably won’t be an issue for long term tree health.
Management options (for apple hosts):
1) Resistance: For new plantings of fruiting or flowering apples, consider planting a rust-resistant variety. Information on crabapple cultivars is available at:
2) Tree care: For any apple tree, proper pruning will allow air movement through the canopy. This practice reduces the leaf wetness that promotes disease. Maintaining overall tree health will also help prevent the disease.
3) Fungicides: Homeowners with a bad history of this disease (severe defoliation), might consider preventative fungicide sprays on the apple hosts when leaves are out and the orange galls are active. For best control, applications should continue through May or as long as the orange galls are active.
Products with the active ingredients myclobutanil or propiconazole are examples of materials labeled for cedar apple rust management in flowering crabapples and non-fruiting apples. Propiconazole products include Bonide Infuse Concentrate and Fertilome Liquid Systemic Fungicide.
Some myclobutanil products are labeled for fruiting apples. However, in all cases, make sure you check the label carefully. For example, the myclobutanil product “Immunox Plus” is labeled for rust on flowering crabapples, but not for fruiting/eating apples, as it contains an insecticide along with the myclobutanil ingredient. In contrast, “Immunox Multi-Purpose Fungicide” is labeled for fruiting apples. If your local store does not carry products for fruit trees, you can find internet sites which carry different products.
Commercial fruit growers should consult the 2017 Midwest Tree Fruit Spray Guide, available here: http://www.extension.iastate.edu/Publications/PM1282.pdf
More information on cedar apple rusts is available at this site: http://www.plantpath.k-state.edu/doc/extension-factsheets/apple-rust.pdf There is also a video on rust diseases at: https://www.youtube.com/watch?v=yQdwSPtvhH8 (Megan Kennelly)