What it means when plants are rated immune, resistant or susceptible to diseases
Disease resistance is a major selling point for plants, from trees to tomatoes. Gardeners know that choosing a disease-resistant plant can save trouble and sometimes save a crop.
But it’s important to understand what the term means. “It’s not a promise of perfection,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “A plant that is resistant isn’t necessarily immune.”
Distinctions are important when you’re reading plant labels or catalog descriptions. “The words have precise meanings,” Yiesla said.
The word “immune” means that a plant is not subject to attack by a particular disease or pest. There are no degrees of immunity; a plant is either immune or it isn’t. However, just because a plant is immune to one disease doesn’t mean it’s immune to another. “No plant is immune to everything,” Yiesla said.
The word “resistant” means a plant is generally able to limit the damage from a particular pest or pathogen. “It doesn’t mean the plant will be totally unaffected,” she said. “It does mean the plant will likely show less damage than a nonresistant plant.”
Plants may be resistant to one disease or pest but not another. The label may specify. For example, hybrid tomato plant labels have codes that indicate disease resistance: V for verticillium wilt, F for fusarium wilt, T for tobacco mosaic virus, A for Alternaria leaf spot, N for nematodes, St for gray leaf spot. A tomato cultivar, or cultivated variety, labeled VFN is resistant to some of these problems, but not all.
Plants can be bred to have increased resistance, but their resistance will still vary with conditions. In a wet year, even a disease-resistant rose bush may get black spot, a fungus disease. When insects of a particular type are particularly abundant, they may turn to eating plants they usually leave alone or that can usually fight them off.
“Susceptible” is the opposite of resistant; it means the plant can’t stop or limit the damage from a disease or pest. For example, catalpa trees, viburnum shrubs and strawberry plants are all susceptible to the fungal disease called verticillium wilt.
The word “tolerant” refers to a plant’s ability to handle stresses from the environment, not diseases or pests. For example, bur oaks are drought-tolerant, while bald-cypress trees are tolerant of wet soils.
When choosing a plant, research the diseases and pests that commonly affect the species and look for cultivars that are bred to have resistance to them. For example, newer cultivars of flowering crabapple trees are usually resistant to apple scab and fire blight, diseases that disfigure many older trees. Some lilac cultivars have been bred to resist powdery mildew.
Native tree species can often naturally resist local diseases that would damage plants imported from elsewhere.
The importance you place on disease resistance may vary by plant. For a large tree that will live a long time, a minor disfigurement such as maple leaf tar spot may not seem important. But if you are shopping for tomato seeds in winter or tomato transplants in springtime and you’re counting on a crop in summer, it may be very important to select disease-resistant plants.
Choosing plants bred for resistance can make gardening easier. But it’s important to have reasonable expectations, Yiesla said. “Expect that in years with wet springs, you’ll see more diseases even on resistant plants. Expect that by the end of any summer, most plants will have a few nibbled leaves,” she said. “No plant will ever be pristine all season long.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
