DISEASE RESISTANCE AMONG APPLE VARIETIES IS RELATIVE, NOT ABSOLUTE!
The concept of disease resistance among apple varieties seems to be often misunderstood. Rarely is resistance so strong that it is absolute or immune. There are some bona fide scab-immune apple varieties (e.g., Williams Pride, Liberty, Pristine, and Enterprise), and there are some cedar apple rust-resistant varieties that might as well be called “immune” since I have never in my 40+ years of growing seen a fleck of rust on them (e.g, Williams Pride, Arkansas Black, King David, and Stayman Winesap). But there are also medium levels of resistance that provide significant protection, and, of course, there are varying levels of susceptibility, some possibly fatal (especially when it comes to fire blight).
In short, disease resistance exists on a spectrum, a continuum from very resistant to very susceptible. To complicate matters, resistance to one disease does not imply resistance to any other disease. For instance, GoldRush™, developed by Purdue University for immunity to scab, is very susceptible to rust to the degree that it can suffer defoliating, debilitating infections!
Furthermore, since disease development, especially spread and infection, depends on weather conditions, varieties will express their relative level of resistance differently in different years. For instance, Williams Pride is resistant to fire blight, but in a very wet year like this one, I can find multiple blight strikes on new growth in my Williams Pride trees. However, those strikes do not extend into older wood and so do not threaten the life of the tree. But for a blight-susceptible variety like Gala, a year like this one could be fatal!
To the would-be orchardist, especially at the home-grower level, descriptions by mail-order nurseries which might simply label a variety “disease resistant” without being more specific can be very misleading. And if their tree they bought as fire blight resistant starts to show some blight in a year like this, they might even feel cheated.
To illustrate these principles, here are some photographs of leaves from different varieties with different levels of resistance to cedar apple rust. The first three were taken in my nursery all on the same date and within a 25-foot radius of one another. This detail is important because it removes from consideration climatic or micro-climatic variables as well as variations due to weather in different years. In other words, these three varieties were all subject to the same conditions with only genetics left to explain the different reactions.
These leaves devoid of any cedar rust lesions belong to the variety Royal Limbertwig.
This leaf belongs to an Arkansas heirloom variety named Oliver Red (aka, Oliver and Senator). The rust lesions are small and (not shown) new leaves further up the young trunk show no lesions at all. From years of growing this variety, I would tag this variety as “moderately resistant,” which to me means that though I can find rust spots on some leaves, the performance of the tree seems otherwise unaffected.
These leaves belong to a seedling rootstock, and not a variety, per se, but it is a good representative of a reaction that I’d call “susceptible.” Note that some of the spots have coalesced into bigger lesions, and that the overall color of the leaves is not as deep and vibrant as the leaves in the previous two photos. If you could feel this leaf you’d note that the lesions are slightly raised and bumpy. The photosynthetic capacity of such leaves is compromised. Though I no longer sell any rust-susceptible varieties through my nursery or grow them in my orchard, my experience with such varieties is that rust-susceptible varieties will gradually decline and never equal the performance of resistant varieties in my orchard. Almost all yellow apples, like Golden Delicious, exhibit at least this level of susceptibility; Pristine is about the only exception to this general rule. Organic growers in the eastern U.S. should probably avoid such varieties or be prepared to spray lime-sulfur on the apple trees at the proper time(s) in spring.
In conclusion, disease resistance in apples is complicated and often misunderstood, but it can be very important to the commercial-scale grower and home grower alike. For organic growers, some level of genetic disease resistance is practically indispensable especially in the eastern half of the United States. To ascertain that a variety has the level of resistance to the diseases common in your region, email me here: firstname.lastname@example.org and/or consult the appendix on disease resistant varieties in ATTRA’s Apples: Organic Production.