Apple and crabapple (Malus), hawthorn (Crataegus), quince (Chaenomeles), Eastern red cedar (Juniperus virginiana), and some junipers (Juniperus)
Description & Symptoms
Hawthorns, Apples, Crabapples, Quince (deciduous, broadleaf host)
Rust is a two-host fungal disease that moves between evergreen cedars or junipers and deciduous hawthorns, apples, or quince. Orange, rusty-colored spots appear on the upper surface of leaves in mid- to early July in the Chicago area. Short orange tendrils emerge from the infected spot on the underside of the leaf. Eventually similar symptoms appear on individual fruits, which become encased with threadlike protrusions. Small sections of twigs become covered in a hard, thickened, black fungal growth. In severe cases, the lawn or pavement beneath an infected tree may be covered with bright orange spores.
Cedars and Junipers (needled evergreen host)
Yellow or orange gelatinous tendrils emerge in spring from brown galls that form on the branches of cedars and junipers the previous summer.
Timing & Life Cycle
During wet spring weather, spores are released from orange tendrils that appear to drip from galls on junipers. These spores are blown to hawthorns, apples, or quince where they land on leaves, twigs, and fruit. The spores turn into orange lesions with short, fuzzy protrusions. These protrusions release spores that are blown back to junipers, where in late summer they germinate and form brown galls that start the cycle of infection over again.
Rust is extremely unsightly on ornamental plants. Fruit is stunted and misshapen. Trees defoliate early.
Treatment & Solutions
When possible, avoid planting eastern red cedar and other junipers within several hundred feet of apple or hawthorn trees. However, the two hosts can infect each other from a quarter-mile away. Although the galls do not usually affect the health of red cedars, they are the source of another year of infection and should be pruned out. Early spring preventative fungicide treatment of hawthorns may be warranted in some cases.
Please contact Plant Information Service at (847) 835-0972 or via e-mail at email@example.com for more information about chemical treatments for rust.