Gymnosporangium juniperi-virginianae Schw.
Apple-cedar rust occurs in practically all apple-growing sections of the central and eastern United States. Diseased fruits are only occasionally found on the market. Even then the rust is merely a slight blemish, since badly marked fruits are usually culled out before shipment. The varieties most often affected are York Imperial, Wealthy, Jonathan, Ben Davis, and Rome Beauty. The rust fungus belongs to a large group of fungi that must complete the different stages of their life cycle on different hosts. The most common second, or alternate, host of apple-cedar rust is red cedar (Juniperus virginiana L.), on which the fungus produces the familiar rough, brownish galls known as cedar apples. From these it spreads in the spring to apple leaves and young fruits, but it must pass again to a red cedar if it is to continue its development.
Infection with the rust fungus takes place when the fruits are young. It either causes the fruits to drop while still undeveloped or stunts their development. They often become noticeably flattened or otherwise malformed.
Apple-cedar rust usually appears on the calyx end of an apple as grayish-yellow to yellow spots (see photo). These vary in diameter from about 1/8 to 3/4 inch and extend into the flesh for 1/4 to 1/2 inch or even to the core. The surface of the spots may be smooth, or it may be roughened with the spore-producing bodies (aecia) of the fungus, which are in the form of pimples or of open, cup-shaped receptacles with flaring, papery edges. None of these measure more than 1/6 inch in diameter. The flesh beneath rust spots is woody and usually greenish, though in the Winesap and the Ben Davis the green is sometimes intermixed with a pronounced brown.
The disease can be controlled by spraying or by removing all red cedars in the neighborhood of an orchard. The control program followed should be that recommended by the State agricultural extension service or experiment station.