Cedar-apple Rust

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Cedar-apple Rust infects apple trees and junipers. In the Rose family it infects prairie crabapple and paradise apple, rarely hawthorn. In the Cypress family in infects chiefly eastern redcedar but occasionally also creeping juniper and common juniper. These are the only species affected in Minnesota. Other apple (Malus) and juniper (Juniperus) species are affected in other areas.

On apple and crabapple trees, infections begin in the early spring. Bright yellow to orange spots (lesions), often with a red border, appear on the leaves in April and May. As the fungus matures, raised black spots form in the center of the spot on the upper leaf surface, and brownish threads or finger-like tubes, less than 1 ⁄10″ long, form on the lower surface. Eventually, the tubes open to release yellow to orange spores. Rarely, the fungus infects fruit, forming large green to brown spots with black dots on the fruit surface.

On eastern redcedar, infections first appear in July and August. Abnormal, greenish-brown growths (galls) appear at the base of a needle or in a crack or crevice of a twig or small branch. As it matures it becomes pitted, more or less circular, and 1″ or more in diameter. The following year, a short, pimple-like bump appears in each depression. The gall overwinters a second time in this form. In the second spring, eighteen months after infection, the bumps elongate, forming finger-like projections. During the first warm rain, sporulation occurs. The projections become bright orange, gelatinous, and ½″ to ¾″ long. This stage lasts for only about two days but may occur several times. The projections dry up, shrink, and become dark brown. The gall will remain on the tree for several years but will not produce more spores.

Anywhere eastern redcedar grows in close proximity to apples or crabapples, especially near apple orchards.

Evergreen hosts: chiefly eastern redcedar; occasionally also creeping juniper and common juniper.

Deciduous hosts: prairie crabapple and paradise apple; rarely hawthorn

April and May

Sources

4, 7, 24, 26, 30.

Common

Until very recently, the genus Gymnosporangium was placed in the family Pucciniaceae. Studies showed that the genus did not belong to Pucciniaceae orto any other existing rust family. In an article published 12/31/2020, it was proposed that a new family Gymnosporangiaceae be created to accommodate the genus. At that time the order Pucciniales (rust fungi) did not have any suborders. In an article published on 7/2/2021 a framework of seven suborders was proposed containing 80% of all rust species.

Cedar-apple rust gall on eastern red cedar, Freeborn County, MN, 2016

Location: Freeborn County, MN

Cedar-apple rust gall on eastern red cedar