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Apple Trees

Cedar-Apple Rust Galls

Galls are abnormal plant growths caused by various agents including insects, mites, nematodes, fungi, bacteria and viruses.  During the summer spores of a particular fungus cause the formation of brown Cedar-Apple Rust galls (Gymnosporangium juniperivirginianae) on Eastern Red Cedar trees. Members of the fungal family Pucciniaceae are known as rusts because the color of many is orange or reddish at some point in their life cycle.

This fungus requires two hosts, Eastern Red Cedar and primarily apples or crabapples, to complete its life cycle.  The two host trees are usually located within a mile of each other. When the Cedar-Apple Rust galls on cedar trees get wet from spring rains, orange, spore-filled fingers or horns, called telia, emerge from pores in the gall. As the horns absorb water, they become jelly-like and swollen (see inset). When the jelly dries, the spores are carried by the wind to apple trees, where they cause a brownish mottling on apples, referred to as Cedar-Apple Rust, which makes apples difficult for growers to sell, even though it doesn’t affect the flavor or texture of infected apples. The rust produces spores on the underside of apple leaves in late summer, which, if they land on Eastern Red Cedar trees, cause galls to form, thereby continuing the cycle. 

Spores produced on apple trees do not infect apple trees, only cedar; spores produced on cedar trees infect only apple trees. (Photo: Brown winter form of Cedar-Apple Rust gall & (inset) orange spring form of Cedar-Apple Rust gall. Blue “fruit” on Eastern Red Cedar branch is actually a cedar cone.)

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Cedar-apple Rust Galls Maturing

6-23-17 orange cedar gallsIf you have both Eastern Red Cedars (Juniperus virginiana) and apple (Malus spp.) trees, you may be privy to the show of a lifetime on your cedar tree one of these days. There is a fungus, specifically a rust fungus, that needs two hosts, Eastern Red Cedar and apple trees, to complete its life cycle. In order to survive, the fungus must “move” from one host species to another.

If contaminated by the cedar-apple rust, an Eastern Red Cedar tree will have small, woody brown galls on its twigs for the better part of a year. Following a warm spring/early summer rain, these brown galls transform into orange, gelatinous growths the size of a golf ball, adorned with “telial horns” that point in all directions. The function of these horns is to disperse spores. If the spores happen to land on the leaf of an apple or crabapple tree, and conditions are just right, galls will result. These galls look very different from the cedar’s galls. Small, yellow spots on the upper surface of the leaves appear after affected apple trees bloom. The spots gradually enlarge and become yellow-orange-red. Small, raised, black dots form in the center of the leaf spots on the upper surface of the leaves as the leaf spots mature. (Apple trees may defoliate early or spots may develop on the surface of the apple as a result of this rust.) Very short, finger-like, fungal tubes protrude from the lower surface of the leaf directly below the spot which release yellow to orange powdery spores. If the wind carries them to an Eastern Red Cedar, the cycle continues. The complete cycle of cedar-apple rust takes 24 months to complete and requires infection of two different hosts.

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