Cedar-Apple Rust, caused by the fungal disease, Gymnosporangium juniperi-virginianae is another serious and disfiguring disease of ornamental and fruit apples. Rust fungi are evolved complex organisms that require two hosts to complete their life cycle.
The first host of this disease is Eastern red cedar, Juniperus virginiana,where the rust forms ball like structures with fungal telial horns that forcibly eject the spores that spread the disease to apples. Although these bright orange horned goo balls are quite alarming they are usually not noticed on the juniper or mistaken for fruit or cones. Once the spores from the juniper travel to the leaves of apples the real unsightly damage occurs.
On apples the spores germinate and after a week or two cause orange pustules on the leaves of susceptible apples. Rainy wet weather in the spring is ideal for the germination on junipers and the spread and infection to apples.
Once in the apple leaf, given a short period of wet weather, the leaf lesions form fruiting structures in mid to late summer and re-infect the juniper host where the disease over winters.
Cultural control, includes removal of junipers in the vicinity of apples as well as planting disease resistant varieties of apples. Avoiding mass plantings of apples and junipers in landscapes reduces the amount of disease inoculum and the potential for severe outbreaks.
There are a number of fungicides that are labeled for control of Cedar-Apple Rust. Typically they require several applications starting at bud break and continuing every two weeks to get suitable control. Fungicides may also be applied to junipers in mid to late summer to reduce the back transmission from the apples to the junipers.
Your I.S.A. Certified Arborist is up to date on the latest research on Cedar Apple Rust and can help preserve your apple trees.