It’s Relative, Not Absolute!
By Guy K. Ames, NCAT Horticulture Specialist
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.” For example, I have never in my 40+ years of growing seen a fleck of rust on 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.
Deciphering Disease Resistance
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, disease development, especially spread and infection, depends on weather conditions. So 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 the tree they bought as fire blight resistant starts to show some blight in a year like this, they might even feel cheated.
Examples of Varying Resistance Levels
To illustrate these principles, here are some photographs of leaves from different varieties with different levels of resistance to cedar apple rust. I took the first three 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.
Determining Disease Resistance Levels
In conclusion, disease resistance in apples is complicated and often misunderstood. It can be very important to the commercial-scale grower and home grower alike, though. 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 Guide.