Cedar-Apple Rust

Cedar-apple rust, a fungal disease caused by Gymnosporangium juniperi-virginianae, is a serious and disfiguring disease of ornamental and fruit apples.

Rust fungi are complex organisms that require two hosts to complete their life cycle. The first host of this disease is the Eastern red cedar, Juniperus virginiana, where the rust forms ball-like structures with fungal telial horns, which 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 stays over the winter.

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.

Brad Gurr is an I.S.A. Certified Arborist and Plant Health Care Specialist dedicated to the care and preservation of trees. For more information on this subject or any plant health care related questions please feel free to contact Brad online at [email protected]

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