Classically defined, a disease-resistant plant is one that inherently or naturally resists infection from ubiquitous fungal, viral and bacterial agents. In most scenarios, two related plants, side by side, react differently to the infection; one develops lesions, degraded cambial tissues or rotten roots, while the other is either completely or almost completely unaffected. Crabapple trees infected with cedar apple rust or apple scab typify this group of plants. In other cases, a plant may be slightly infected with a disease, yet show no reduction in overall health, and the infection locations are merely an aesthetic distraction. Cedar trees infected with cedar apple rust are in this category.
When you examine the causes of pathogenic diseases carefully, three factors must be in place in the right proportion to cause infection. The classic disease triangle explains this well. One angle of the triangle represents the pathogen itself, and another is the weather conditions necessary for the growth and development of the fungus, virus or bacteria on or in the plant.
The third piece of the triangle is the host plant; it may be able to facilitate infection or may be resistant. The reason that this illustration is a triangle is that all three parts must be satisfied for infection to occur. If one of the three is not viable, then the tree or shrub does not become infected with the disease. For example, sycamore anthracnose is a common foliar disease that is favored by cool, wet springtime conditions. The tree is reasonably susceptible (host), previous areas of infection or cankers are the source infections (pathogen) that may be present on the tree or nearby trees and cool, wet springs (weather) create moist leaf conditions for the fungus to grow.
Disease-resistant vs. disease-susceptible plants
There are certain woody species that simply aren’t bothered by many pathogens. Red oak and ginkgo are examples. Sure, from time to time, oak wilt, powdery mildew and leaf spots have been observed, but for the most part, these are tree species that don’t seem to be good hosts for disease infections.
On the other hand, certain species are known for regular bouts with fungal or bacterial diseases. Poplars, lilacs and cockspur hawthorns routinely develop diseases such as cytospora canker, cedar hawthorn rust and bacterial shoot blight. Whether the tree or shrub species is well-known as a resistant or susceptible plant, it’s wise to know so that it can be minimized or considered for addition to the landscape.
Disease-resistant cultivars
The majority of plant species are a mixed bag; their cultivars are either disease susceptible, intermediate or mostly resistant. A good site to see examples of this is at an All America Rose Selection (AARS) garden. AARS gardens evaluate dozens, sometimes hundreds, of cultivars each year. The rose judges that conduct the evaluations score the individual cultivars on many factors (hardiness, form, novelty, fragrance, etc.), including disease resistance to several pathogens. Rust, black spot and anthracnose are some of the common diseases that are noted. Visiting these gardens, or at least visiting their Web site and looking at the cultivar ratings for diseases, can be quite helpful when it comes to choosing cultivars for your customers.