Cedar Apple Rust

Mt. Home, Ark. –

For those who are not familiar with cedar-apple rust, you can see it on  apple tree leaves in the spring and summer. Small, pale yellow-orange spots develop on the upper leaf surface shortly after bloom. These spots enlarge and turn orange (rust).

            The chief damage by this disease occurs on apple trees, causing early leaf drop and poor quality fruit. This can be a significant problem to commercial apple growers but also harms the appearance of ornamental crabapples in the home landscape. On apple, symptoms first appear as small green-yellow leaf or fruit spots that gradually enlarge to become a yellow-orange color. On the upper leaf surface of these spots, the fungus produces specialized fruiting bodies called spermagonia. On the lower leaf surface (and sometimes on fruit), raised hair-like fruiting bodies called aecia appear as microscopic cup-shaped structures. Wet, rainy weather conditions favor severe infection of the apple. The fungus forms large galls on cedar trees in the spring, but these structures do not greatly harm native redcedar and ornamental cedar, although some twig dieback may occur.

            The life cycle is complex and involves two plants (apple and cedar) and their fruiting structures (telia, aecia and pycnia). The pathogen requires two years to complete its life cycle. The fungus overwinters in reddish-brown galls on the cedar tree. In the wet spring, the galls extrude gelatinous tendrils consisting of two-celled teliospores. Air currents carry the teliospores to the apple tree where they infect within four hours under favorable conditions. In July and August, windborne aeciospores from apple infect cedar leaves. Rust lesions develop in one to three weeks. The galls mature the second year after infection.

            Resistant varieties of apple and crabapple are the best method of control. Because it is impractical to keep enough distance between native cedar trees and cultivated apples or crabapples in the state, fungicides can be used to protect apples against infection. Several fungicides are highly effective against rust diseases. Fungicides should be applied just before blossoms open and for the next six weeks to protect the emerging leaves and developing fruit. Fungicides such as myclobutanil, Bacillus subtilis, ziram, penthiopyrad, kresoxin-methyl, Pristine, Adament, triflumizole, and trifloxystrobin will control rust; Captan will not do as good a job. Be sure to read and follow label directions.

            For more information on spraying fruit trees, contact the University of Arkansas System Division of Agriculture Cooperative Extension office at 425-2335

By Mark Keaton
County Extension Agent - Staff Chair
The Cooperative Extension Service
U of A System Division f Agriculture

Media Contact: Mark Keaton
County Extension Agent - Staff Chair
U of A Division of Agriculture
Cooperative Extension Service
3 East 9th St. Mountain Home AR 72653
(870) 425-2335
mkeaton@uada.edu

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