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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