Ithaca Journal Article: Colorful Crabapples

Colorful Crabapples
By Mary Hirshfeld

Very few flowering trees can compete with the lavish spring flower show presented by flowering crabapples (genus Malus). There is nothing quite like them, and to see one in bloom is to want one. However, along with these popular small trees come a host of disease and insect problems that can drastically reduce their aesthetic value and threaten their ability to grow and thrive in the landscape. Plant breeders are acutely aware of the disease problems that plague crabs and have been working to select individuals that are resistant to the three most troublesome diseases: apple scab, fire blight, and mildew.

Apple scab is the most common disease in our area and, although rarely life threatening, it can weaken trees by causing partial or complete early defoliation. Like most fungal diseases, warm, wet weather accelerates its spread, so a cool, dry spring will result in lower disease levels than a warm, moist one. Apple scab lesions first appear as dull, graying areas near the mid-ribs of the leaves, and gradually expand to form brown, circular spots. The fruits are also affected-dull brown spots can mask their normally bright red or yellow color. Heavily infested leaves will often drop by July, weakening the tree and leaving a skeleton robbed of all its seasonal ornamental value. The degree of scab exhibited will vary from year to year; in years when the weather is exceptionally conducive to the development of the disease, even resistant cultivars will exhibit some lesions, although they will rarely drop their leaves. The dormant stage of the fungus overwinters on fallen leaves, so a good annual fall raking will help reduce the inoculum available to infect leaves the following spring.

Fire blight can be an exceptionally destructive disease. A bacterium that survives from year to year in cankers on infected trees, it spreads at bloom time and, like scab, is likely to be more evident in warm, wet springs. Affected twigs and branches quickly brown; the leaves shrivel up and appear to be burned to a crisp. Cankers then develop at the base of infected shoots, providing a repository for the bacterium, which can then spread to infect additional shoots. To control the disease, plants can be sprayed with Streptomycin at bloom time. Diseased shoots should be pruned out and pruning shears disinfected with Clorox between cuts.

Powdery mildew is very common on crabapples during our typical warm, humid Ithaca summers. This fungal disease paints the leaves and fruits with an unappealing grayish powdery coating, giving the plant a dingy dirty-laundry look. Siting crabs in locations with adequate air movement is important in keeping this infection to a minimum.

Cedar-apple rust has not yet caused real problems in the crab collection at Plantations, although the classic orange bristly lesions have appeared in small quantities on hawthorns. The fungus alternates between crabs or hawthorns and junipers, completing a distinct portion of its life cycle upon each host. Ensuring that the alternate host is not planted nearby will reduce the risk of infection in your crabapples. Severe infection can lead to early leaf drop, resulting in reduced growth.

In addition to this litany of diseases, crabs are also assaulted by aphids, spider mites, Japanese beetles, webworms, deer, voles, rabbits, and mice. Yet aphids and mites can be controlled with horticultural soaps and oils, Japanese beetles and webworms can be picked off and squashed, and animals can be deterred with fencing and repellants. Disease problems are best avoided by selecting cultivars with good disease resistance in your region.

Luckily, good information on disease resistance, along with aesthetic assessments of the numerous crab selections currently on the market is available. Distressed by the number of defoliated and blighted crabs appearing in the landscape, crab enthusiasts banded together to form the International Ornamental Crabapple Society (IOCS), currently headquartered at the Holden Arboretum in Kirtland, Ohio. The IOCS coordinates the National Crabapple Evaluation Program (NCEP), a network of public gardens with extensive crabapple collections, and staff who are willing to assess them annually for their disease resistance and aesthetic qualities.

Cornell Plantations is a NCEP site for the Northeast. Our staff annually conducts assessments of the landscape performance and disease resistance of the 87 crab cultivars and species in our collections. Periodically, cooperators summarize and report their findings in Malus, the IOCS quarterly journal. Perusal of several years worth of performance summaries shows surprising differences in cultivar performance from region to region. For example, two cultivars on the "don't ever recommend" list for the Northeastern and Midwestern states are 'Hopa' and 'Radiant'. Stunning though they are in flower, both are reliably defoliated by apple scab infection by July of each year. However, in Alabama, where scab is not a problem and fire blight is severe, 'Radiant' is the top performer due to its high resistance to fire blight. 'Harvest Gold', which has been outstanding here at Plantations, has proven to be highly scab susceptible in Wisconsin.

At the head of everyone's list is the graceful, weeping, pink flowered 'Louisa', which annually receives top ratings both for its aesthetic quality and disease resistance. In spring, 'Louisa' is a delicate umbrella of cascading branches sheathed in pale pink flowers. Its fruit is gold with a rose cheek and, although quite attractive in late summer, it does not persist. 'Molten Lava' is another winning weeper, similar to 'Louisa' in its grace but slightly wider than round, reaching 12 feet in height and 15 feet in width. This selection derives its name from the stunning fall display of bright red fruits that cascade down the weeping branches like a lava flow. White flowers open from bright carmine-pink buds, and the unusual form and yellowish bark provide winter interest. Unfortunately, one of the most widespread crab weepers remains 'Red Jade', a stiffly formal tree that struggles with scab on its fruit and foliage, and should perhaps give way to 'Louisa' and 'Molten Lava'.

Another high quality crab with an unusual growth habit is 'Adirondack', a slender, columnar cultivar originally selected by Donald Egolf at the U.S. National Arboretum, as one of 500 open-pollinated seedlings of M. halliana 'Koehne'. It is a lovely, small tree that reaches 12 feet in height and half that in width. Its bright pink buds open to white flowers, and are followed by vivid red fruits that persist through December. Prior to its introduction, 'Adirondack' was inoculated with fire blight to determine its resistance to the disease. Initially thought to be completely disease free, it has shown very minor apple scab susceptibility in NCEP plots in Wisconsin and Ohio. Still, it remains one of the top picks for overall disease resistance and outstanding ornamental quality. In fact, so well has 'Adirondack' performed in the Northeast, it was recently awarded the Pennsylvania Horticultural Society's coveted Gold Medal Plant Award, given to "little-known and underused woody plants of exceptional merit."

A more traditionally shaped crab that can reach 20 feet in height and width is 'Prairifire'. Selected by D. F. Dayton of the Horticulture Department at the University of Illinois in 1982, this cultivar has skyrocketed to the top of crabapple popularity charts throughout the Northeast and Midwest. The primary reason for its popularity? 'Prairifire' is one of a very few disease-resistant crabs with reddish-purple-flowers (most display white flowers from bright pink buds). Late summer brings deep purplish-red, ½" diameter fruits that are too dark to be really showy.

'Ormiston Roy' is an old cultivar that still tops the crab charts for its disease resistance, and floral and fruit display. Introduced in 1954 and named to honor William Ormiston Roy, a Canadian landscape architect, it is worth growing simply for its graceful, low-branching, wide-headed habit. Lovely in form, this small, spreading tree can reach 20 feet in height, although at Plantations, it has slowed down at 10 feet in height while continuing to expand in width. Its white flowers open from pink buds, and are incredibly profuse. But this selection really stands out in the winter landscape, when it is covered with yellow-orange fruit that darkens and persists well into the winter.

Finally 'Schmidtcutleaf', trademarked as "Golden Raindrops", is a selection of the Tibetan crabapple (Malus transitoria) with unusual, lobed, finely textured foliage. The species itself being a rather drab disease magnet, this selection offers improved ornamental qualities and disease resistance. A strong grower that can reach 20 feet in height and 15 feet in width, it is not a crab for confined gardens-but where space allows, its delicately cut foliage, small white flowers, and cascades of tiny golden fruits make it an excellent choice. And, although 'Golden Raindrops' doesn't pack quite the same spring floral wallop as 'Prairifire' or 'Adirondack', it's graceful habit and more subtle ornamental qualities allow it to harmonize smoothly with native companion trees such as shadblow (Amelanchier) or silverbell (Halesia).

If you decide to add a crabapple to your garden, site it in full sun and provide it with well-drained soil. To minimize disease incidence, plant your crab where it will receive good air circulation and early morning sun to dry the leaves of dew, and ensure quick drying after rains. Crabs are also cherished by gourmandizing deer, so plan on enclosing your crab in a protective fence until it is large enough to fend for itself.