Written by Kimberley Pacholko
Updated on Sept. 25, 2021
Juniper Rust (Gymnosporangium juniperi-virginianae) better known as Cedar Apple Rust, s a fungal Disease requiring 2 years and 2 host plants to complete its life cycle. In the first group of host trees you have Apple, Quince and Hawthorne. In the second group you have Eastern Red Cedars and many varieties of Junipers. The disease can be quite destructive and disfiguring. The disease at maturity is easy to spot. In junipers and eastern red cedars there are galls with orange, gelatinous protrusions or horns coming out of them in the spring. In apples the upper surface of leaves develops bright yellow/orange spots with small bulges on the backside. Their branches develop swollen corky galls on the current year’s growth and fruit develops large rust spots that spoil the fruit.
Symptoms and Life Cycle
The fungus requires 2 years and 2 host plants to complete it’s life cycle. The cycle begins in the spring when rains and damp weather set in motion the growth of orange gelatinous tendrils or spore horns on infected junipers. These horns develop from the pitted areas of the galls that formed on the juniper the previous year (Morton Arboretum, n.d.). During dry and windy conditions, the spore horns dry up and release spores that can travel a few miles away to infect apple, crabapple and hawthorn trees, just as their vulnerable new growth is emerging (typically blossom time). Once the spore lands on the leaf or branch surface infection can take place in as little as 4 hours, (Wikipedia, 2021).
The apple leaves that have been infected begin developing small yellow/orange spots on their upper surfaces. These spots continue to enlarge, and small bulges can be seen on the underside of the leaves. These bulges produce hair like projections (hyphae), which release spores that are blown back to the junipers in mid to late summer. Apple branches can also be infected. Infected branch areas appear as swollen corky galls on the current year’s growth, usually no more than 1 inch in length (Morton Arboretum, n.d.). The galls swell and develop cylindrical fruiting bodies. Seriously affected twigs are stunted and may die. These branch fruiting bodies also release spores that blow back to the junipers. These spores germinate on the juniper and greenish galls are produced the next spring, but these do not mature to release spores to infect apple until the following spring (Omafra, n.d.). At that time mature galls, that are reddish/brown, varying in size from 1-3cm (1/2″ to 1″) begin to develop spore horns during the rainy damp season of spring and the 2-year cycle begins again.
Ideally do not locate your junipers near apple trees and vice versa. On the practical side though it difficult for home gardeners to necessarily know what their neighbours have planted. Planting resistant varieties can help. For a list of resistant varieties of juniper, apple, crabapple and hawthorn click. Pruning out galls and cankers will also help by reducing transmission to the alternate host. Cleanup fallen infected leaves and dispose of them.
Fungicides work best when applied just before and during the infection period. After infection they are of little to no use. Thus, the best time to spray apple, crabapple and hawthorn trees is in April and May. Begin spraying at bud break, spraying every 10-14 days (as per manufactures instructions), until pedal fall. There is no need to spray after that as there is only one infection period per year. For junipers and eastern red cedar begin spraying in July every 10-14 days until the end of August.
A Few Fungicides Available to Homeowners: (check your municipality for what is permitted for your area.)
“Myclobutanil (Immunox) Note there are several formulations of Immunox but only one is labeled for fruit. Before treating any plant, especially edible plants, check the label “(Kelly Feehan, n.d.).
SERENADE Garden This broad spectrum bio-fungicide uses a patented strain of Bacillus subtilis that is registered for organic use.
*Most of the fungicides registered for scab control also give good control of rust diseases.
Feehan K., (n.d.), Extension Educator; Nebraska Extension in Knox County, ‘Cedar Apple Rust & Apple Scab Spray Now’, Retrieved on Jan 14, 2021 from: https://extension.unl.edu/statewide/knox/cedar-apple-rust-apple-scab-spray-now/
Morton Arboretum, (n.d.); Retrieved on Jan.13, 2021 from: https://www.mortonarb.org/trees-plants/tree-and-plant-advice/help-diseases/cedar-apple-rust
Omafra, (n.d.); Excerpt from Publication 310, Integrated Pest Management for Apples, ‘Cedar-Apple Rust and Quince Rust’; Retrived on Jan. 14th,2021 from: http://www.omafra.gov.on.ca/english/crops/facts/cedarap.htm#cedar
Wikipedia, (2021); ‘Gymnosporangium juniperi-virginianae‘; Retrieved on Jan. 14, 2021 from: https://en.wikipedia.org/wiki/Gymnosporangium_juniperi-virginianae
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