Three related rust diseases occur on apple trees in Kentucky: cedar-apple rust, cedar-hawthorn rust, and cedarquince rust. Crabapple, hawthorn, mountain ash, pear, and serviceberry are also susceptible to these diseases. All three rusts are caused by different species of the fungus Gymnosporangium, each of which must spend a phase of its life cycle as a parasite on Juniperus species such as native red cedars or ornamental junipers.
Although cedar rusts can cause unsightly growths on Juniperus, they do not usually cause serious damage to these plants. Rust diseases can cause serious losses on apples, however, as a result of both fruit and leaf infections. Infected fruits can drop prematurely or have a reduced commercial value if they remain on the tree through harvest. Leaf infections often result in premature leaf loss, which reduces the size and quality of the current season’s fruit crop, weakens the tree, and may cause a reduction in bloom the following year. Trees allowed to become heavily infected for several years become stunted, are increasingly susceptible to winter injury, and might eventually fail to produce fruit. In Kentucky, hawthorns and crabapple twigs infected by the cedar-quince fungus can become swollen and die.
Hartman, John R., "Rust Diseases of Apple" (1996). Agriculture and Natural Resources Publications. Paper 78.