Apple trees host so many common tree diseases that growing susceptible varieties could require spraying at two week intervals throughout the season. Many homeowners find that too labor intensive to be practical. Though choices of highly resistant apple trees are limited, choosing disease resistant apple varieties protects any orchard from the major apple blights.
No apple cultivar is completely disease resistant. Development of resistant cultivars focuses on four major apple diseases. Scab affects the fruit, scarring and deforming the apple. Fire blight--a bacterial infection--attacks all parts of the tree. Powdery mildew causes short lived infections during wet weather but can seriously affect foliage and apple yield. Cedar-apple rust passes between apples and cedar trees and over several years of infection may kill an apple tree. Resistant cultivars show different levels of immunity to these four major blights.
Commercial apple varieties propagate from grafted scions of one individual tree selected for the quality of its fruit. Cross breeding a commercially valuable tree with a disease resistant tree may pass the resistant gene to the new seedling, but the new fruit could be worthless. Experimental programs take many years to produce resistant cultivars with commercially valuable fruit. Selecting resistant rootstocks as hosts for the scions of known varieties can increase the resistance of the bearing section of the apple tree. This gives a second approach to the development of disease resistant trees.
Apple cultivars that do well in one region of the country may not thrive in an area with different temperature extremes and weather patterns. Part of the choice of varieties for a home garden should include local suitability. Certain apple diseases may be endemic in some parts of the country and only slight problems in others. Matching cultivars to both growing conditions and prevalent diseases can be complicated. Check with local growers and nurseries for the best apples trees for your area.
The overall best choices in terms of disease resistance include Pristine--tart and suitable for cooking or eating out of hand--and William's Pride, a good keeper for winter storage. Redfree and Enterprise store well if refrigerated. Jonafree, similar to the Jonathan apple in taste, makes good pies and cider. Goldrush, a large crisp apple with a long storage life, and Liberty--a cultivar developed in the late 70's--also combine excellent taste with broad disease resistance. All are either immune or highly resistant to scab and are resistant to the other major blights.
Many factors affect the quality and amount of fruit as well as the health of the apple tree itself. Choosing resistant varieties helps maintain a healthy orchard but isn't a foolproof solution. Other fungal infections could still infect resistant trees through pruning cuts and storm damage. Gardeners themselves often spread disease from tree to tree by neglecting to sanitize tools. Trees stressed by insects or drought will always be more susceptible to illness.