Some trees and shrubs have a bad reputation for being susceptible to diseases: lilacs to powdery
mildew, junipers to juniper tip blight, flowering crabapples to apple scab, elms to Dutch elm disease.
“If you choose carefully, you can still have these plants,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. “You just have to seek out disease-resistant varieties.”
Flowering crab apples, for example, are useful small ornamental trees that offer lovely red, pink or white flowers in late spring and autumn fruits that feed wildlife. Dozens of cultivars, or cultivated varieties, of crab apple trees grow at The Morton Arboretum. Yet many homeowners avoid them because older varieties are prone to a number of diseases, including apple scab, cedar apple rust, fire blight and powdery mildew.
“Newer cultivars have been bred to be resistant to one or more of these diseases,” Yiesla said. A number of them can be found if you search for “crab apple” on the Arboretum website at mortonarb.org/search-trees-and-plants.
For example, the Prairifire crab apple (Malus ‘Prairifire’), a 20-foot-tall tree with red flowers, and the Adirondack crab apple (Malus ‘Adirondack’), a narrow, upright cultivar with white blooms, both have excellent resistance to apple scab, fire blight, rust and powdery mildew.
There are also juniper cultivars, such as Iowa, that are resistant to juniper tip blight, as well as lilacs such as Charles Joly and Old Glory that have good resistance to powdery mildew.
“Many of these diseases have been around for a long time, so the word has gotten around to avoid the plants that are affected by them,” Yiesla said. “But plant breeders have also had time to develop resistant cultivars.”
Not all new cultivars are available in every region. Once you have figured out what to ask for in terms of disease resistance, check with local nurseries to see which cultivars they have.
Even elm trees, notorious victims of Dutch elm disease, aren’t out of the question. Although the native species of American elm (Ulmus Americana) is extremely susceptible, some cultivars of the species have been selected with some resistance. They are not available everywhere, so it is also worth considering other Asian elm species with natural resistance.
Two widely planted elm cultivars that were developed at the Arboretum are the Triumph™ elm (Ulmus ‘Morton Glossy’) and the Accolade™ elm (Ulmus davidiana var. japonica ‘Morton’).
Another approach is to choose a species that is naturally problem-free. “Some kinds of trees just don’t seem to be bothered by pests and diseases very much, if at all,” Yiesla said.
For example, there’s a good reason that sites along streets are often planted with hackberry (Celtis occidentalis), a handsome native tree; gingko (Ginkgo biloba), an Asian species known for its golden-yellow fall color; and bald cypress (Taxodium distichum), a deciduous conifer. “These species are all tough, resilient, easy-care plants,” she said.
Disease resistance is one of the many reasons it’s worthwhile to invest some time in researching a plant before you buy it. “It will pay off in the long run,” Yiesla said, “because a disease-resistant tree is a lower-maintenance tree.”
For tree and plant advice, see the online resources of The Morton Arboretum at mortonarb.org/plant-care, or submit your questions online at mortonarb.org/plant-clinic or by email to plantclinic@mortonarb.org. Beth Botts is a staff writer at the Arboretum.
Comments are closed.