Tree Facts: Rust diseases on trees and their control

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Rust fungi, in the genus Gymnosporangium, causes disease in the Rosaceous or apple family, commonly called juniper rust. Cedar-apple rust is the best known, but there are at least nine other related rusts.

Cedar–apple rust and other related rusts occur throughout the Dakotas. During the summer, fall and winter, cedar-apple rust and several other closely related rusts reside on host plants which serve as sources of nutrition for the fungi, but are not harmed. In the spring, the fungi go through their reproductive cycle and release spores that are dispersed by the wind that then infect other plants that can be harmed.

Cedar-apple rust hosts are eastern red cedar (ERC) and Rocky Mountain juniper (RMJ) and attacks the leaves and fruit of apple trees. Quince rust uses ERC, RMJ, Creeping Juniper (CJ), and Bush Juniper as its hosts and attacks fruits, especially of hawthorns. Hawthorn rust uses ERC and RMJ as hosts and attacks the leaves of hawthorns, apples, mountain ash and pear trees. Juneberry rust uses ERC, RMJ and CJ as hosts and attacks fruit, stems and leaves of juneberry, quince, apple and mountain ash.

Cedar-apple rust develops as small, yellow-to-orange spots on the upper leaf surface of both apple leaves and fruits shortly after bloom. Black dots soon appear in these spots. The infected spots are often thickened or blistered. In mid-summer, tiny orange-colored tubes form on the lower leaf surface, opposite the spots on the upper surface. Heavy infection can result in severe defoliation. Spots on the fruits are similar, except that the tubes are not always formed.

Cedar-apple rust on cedars forms red-brown galls over a period of nearly two years. In the spring the mature galls (“cedar apples”) produce orange gelatinous tendrils (“horns”) during moist weather. The spores formed on these tendrils infect apple leaves and fruits. The quince rust galls are elongate, perennial and may live for several years, producing new crops of spores each spring.

Theoretically, fungal diseases can be controlled by removing cedars within two miles of apple and crabapple trees. Practically, the use of fungicides can be very effective. Application of myclobutanil (Nova or Rally) or fenarimol (Rubigan) periodically, starting when the flower buds show pink and at 7-10 day intervals to a maximum of three sprays, or until cool wet weather (spring or early summer) is past. This will protect the emerging leaves and developing fruits. If possible select the more cedar-apple rust resistant varieties such as Dakota, Haralson, Mandan, or Red Duchess.

Sulfur, registered for scab control, may help to suppress rust development on apple trees grown by the homeowner. Native crabapples are susceptible to cedar-apple rust; Asiatic crabapple varieties are generally resistant. Dolgo, Centennial, and Manchurian crabapples are resistant to cedar-apple rust. The reaction to other rusts is unknown.

My sources for this news release were the USDA Forest Service and NDSU Extension Service. If you would like more information about “Rust diseases of trees and their control,” call Bob Drown at the Conservation Office at 605-244-5222, Extension 109, or by email @ robert.drown@sd.nacdnet.net.