(Gymnosporangium juniper-virginianae), and several closely related rust diseases, require two hosts to complete the life cycle of the disease. For cedar-apple rust, that means that both an apple or crabapple and eastern red cedar (or other juniper) must be present within a distance that allows the spores to travel from one plant to the other.  Cedar-hawthorne rust and cedar-quince rust disease affect  other members of the family rosaceae, which includes pears, plums and cherries .

The first symptoms noticeable on an apple tree are yellow leaf spots that appear shortly after bloom.  A wet spring, particularly just after leaf buds break, will increase the severity and spread of the infection. Sometimes oozing, orange bumps will form under the leaf spots and then become black.  Later in the season, clusters of thread-like fruiting bodies grow from the underside of the leaf spots, as well as twigs and fruit.  Spores that infect the needles of the alternate (cedar) host are transmitted in warm, damp weather. 

While the cedar host may not be present on the same property (spores can travel for up to 5 miles), the infection on that plant can be identified by the emergence of a gall six months after spores reach it.  About 18 months later, brown telial horns emerge from depressions on the gall, elongate and become bright orange and gelatinous in the spring rain.  They dry up after releasing spores.  Leaves may fall from the tree, especially in dry weather.

This disease rarely kills its hosts but it can make them unsightly and gradually weaken them in succeeding years of early leaf fall.  While growing the two plants in close proximity should be avoided, removal of the cedar host may not be an effective solution because the spores can travel well beyond the confines of a single property.

An important control measure is the placement of trees in an area where sun and air circulation help control moisture. Use of fungicides on apples and crabapples can be effective if timed with the release of spores by the cedar host while the gall is orange and gelatinous.  Reapplication may be necessary repeatedly if the weather is rainy but the risk to the apple tree passes about a month after bloom.

The blackened tip of the leaf in your photo looks less like a disease than a symptom of environmental stress -- fluctuations in temperature, problems with drainage or water availability -- in a young tree. Evaluate the growing conditions of the trees and check the trunks for any mechanical or frost damage that may have occurred.