Small- to medium-sized trees serve a valuable purpose in the landscape and provide multiple benefits for property owners. One of the best is the crabapple. Many species and cultivars produce persistent fruit, which is visually appealing for humans and edible food for songbirds. Crabapples are available in many sizes and shapes, which landscape designers can integrate when tall and thin, short and wide, arching, screening, softening and curb appeal needs are identified. Often referred to as “four-season” trees, they have a great appeal in the landscape including winter interest, spring flowers, summer green leaves and fall color of various types. But just like most other plant groups, crabapples can be injured by diseases and other maladies that limit their effectiveness and positive attributes:

Prevention of maladies

  • Whenever the topic of diseases, insects or other maladies is brought up, we tend to jump right to some sort of control measure — a pesticide, a fertilizer, a spray additive — rather than taking a step back and thinking about not having it at all….or preventing it from occurring. There are at least five methods of problem prevention that relate to woody plants. The first is locating the tree in the best place in the landscape for success. Crabapples perform best in full sun, well-drained soils and surroundings with good air circulation. These conditions help prevent foliar and root diseases. As well, adequate space for branch development is important. Generally, crabapples require less space than larger specimens such as tuliptree and sycamore, yet still need (on average) 500 square feet of unimpeded space in the root zone to develop well.

    Fruiting structures of cedar apple rust on a crabapple.

    Fruiting structures of cedar apple rust on a crabapple.

  • Many crabapple cultivars are susceptible to pathogens such as apple scab, powdery mildew and fireblight. Others are moderately susceptible and others are resistant. It’s quite dramatic to observe two different cultivars growing next to one another, with one heavily infected and the other with healthy, normal leaves and branches. When choosing specific cultivars, it’s very helpful to check with local extension offices or land grant universities for information on cultivar resistance.
  • Many maladies that develop in years 2, 3 and 4 of a tree’s life can be directly related back to the planting process. The depth and width is one of the common responsible factors involved. Best results are realized when the depth of the root mass is used for calculating how deeply to dig the hole, as well as the width. The simple but effective formula for this calculation is to dig the hole no deeper than the original root mass and two to three times as wide. An important caveat when planting is to find the top root of the mass and carefully remove any recently added soil that was placed over the top during production. This method of planting results in a “planting area” rather than a “planting hole,” with adequate room for redirecting girdling roots that have developed over time.
  • Equally as important as proper planting procedures are the ones proved after the tree is in the ground. Good care after planting is essential, especially in the first 90 days following. The most important practices to pay attention to are checking and maintaining the mulch layer, watering and weed control. Follow-up visits to the site to ensure that 2 to 3 inches of wood chips are in place over the root mass and that chips are not piled against the trunk are very helpful in tree establishment.  As well, probing the soil to determine the moisture content of the roots, and adding water as necessary to keep them moist, not soggy or dry, is an essential step. During these visits, taking a few minutes to pull the weeds and grasses that may have grown through the mulch is prudent to reduce competition for moisture and nutrients. A final preemptive practice is to take a good look at the limbs and remove crossing limbs or ones broken in the planting process.

Abiotic diseases

Maladies that are not associated with a particular biological organism and degrade the health of trees are known as abiotic.  Control of these are largely preventative as well:

White PVC collar to reduce damage from sunscald and critters.

White PVC collar to reduce damage from sunscald and critters.

  • Sunscald: Due to the normally thin bark of a crabapple tree, they are prone to sunscald. Most commonly occurring in winter, sunscald develops over time as the bark tissues are warmed by the winter sun during the daytime, then are harmed by the rapidly cooling temperatures in the evening and nighttime. When exposed to day after day of heating and cooling, its common to observe breaking, tearing and splitting of tender bark tissues, usually after the first or second winter. The south, west and southwest sides of the trunk are most often damaged due to the orientation of the sun’s rays. To prevent damage, white PVC columns are recommended to be installed in late fall and to be removed in early spring. Many apple orchardists simply paint the trunk white in an attempt to reduce damage, however, most property owners would prefer to avoid this type of discoloration.
  • Critter damage: Crabapple trees, especially newly planted ones, are susceptible to damage from four-legged critters such as voles, moles and mice. These unwanted biters seek nutrients and moisture and unfortunately, often find what they’re looking for. The type of injury that is caused creates desiccation from the trunk and future loss of movement of essential cell sap. The PVC columns described above are a good option for damage prevention, as are cylinders of hardware cloth and wire mesh. Placement of these devices should begin at the 4 inch below ground level for best results.

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    Stem girdling roots on this crabapple will eventually reduce internal transport of water and nutrients.

  • Girdling roots: Like many other tree species, crabapples are prone to damage from stem girdling roots. This injury shows up in the form of roots that radiate from the original root mass in an encircling orientation rather than directly horizontally, like the spokes of a bicycle wheel. The consequence of this malady is that as the roots and trunk expand in size in the establishment process, they will impinge or push on each other, causing a restriction of movement of water and nutrients in the cambium and sapwood. Fortunately, girdling roots can be prevented by removing all plastic, burlap and wire, identifying roots that are beginning to encircle and gently teasing them out and reorienting them horizontally in the planting area.
  • Mechanical injury: String trimmers, lawn mowers, cars, skid loaders and fork lifts are the most frequent sources of mechanical injury. The effects are similar to that of critter damage and sunscald, simply caused by two-legged human operators rather than the sun or four-legged animals. The most effective preemptive solution is to thoroughly train maintenance staff, especially 15-year-old juvenile delinquents, to be aware of the tender bark and the possible damage that they can inflict.

Biotic diseases

  • Apple scab: Apple scab is favored by cool, moist spring weather, particularly when tree leaves remain wet for long periods of time. Initial symptoms appear as olive-brown round spots on lower leaf surfaces. As the disease progresses, the spots change to dark brown or black and take on a feathery appearance. In the latter stages, spots develop on the upper leaf surfaces as well. Once several spots develop on leaf surfaces, leaves begin to fall, causing the tree to become thin. Severe infections can render a tree 60- to 70-percent leafless by mid-summer. Not only is this unsightly, it deprives the tree of carbohydrate and sugar production, causing it to be weakened. In some situations, the infection will spread to crabapple fruits as well, causing them to be disfigured and/or fall from the tree. Control of apple scab begins with the selection of a disease-resistant cultivar. A consistent fungicide spray program can also be helpful in suppressing the effects of apple scab, particularly on susceptible cultivars, applied at seven- to 14-day intervals from prebloom through rainy periods of the growing season. Be sure to read and follow all label directions when using these products.
  • Cedar apple rust: The conditions that lead to the development of cedar apple rust (CAR) are similar to those of apple scab. Initial symptoms of CAR are small yellow to orange slightly-raised spots on upper leaf surfaces. The number and size of the spots is somewhat dependent on the degree of resistance to the fungus; trees that possess a moderate to high degree of resistance usually express smaller and fewer spots. CAR is a unique malady to tree care in that it’s a “two host disease” After initial infection on crabapple leaves, the spots grow and develop into the inner tissues of the leaf, as well as beneath. On lower leaf surfaces, fungal fruiting bodies develop and during periods of cool wet weather, erupt and spread spores to nearby cedar and juniper trees and shrubs. As a result of this disease transmission, small, tan to brown corky growths occur, which are usually not noticeable by most customers. In the year following, however, a rapid change in size and shape occurs. Long bright orange gelatinous stands are produced from the growths, which contain spores of their own that are then spread back to the crabapple, starting the cycle over again. The fruiting bodies that occur on the cedar and junipers are quite striking and thought by most to be ugly; yet a small percentage of the populous actually consider them to be attractive. At any rate, CAR rarely causes damage to cedars. Control of CAR is similar to that of apple scab. Attempts to reduce infection by removing cedars and junipers from the landscape are usually not effective, as the spores that cause CAR can travel up to 5 miles in the wind.

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    Thin foliage as a result of cedar apple rust and apple scab infection.

  • Powdery mildew: Unlike apple scab and CAR, powdery mildew doesn’t require moisture on leaf surfaces to develop. Cool, cloudy days and stagnant air are the key conditions that trigger infection. Powdery mildew creates a whitish cast to tree leaves. Both upper and lower leaf surfaces can be affected. Usually, the leaves appear as they have been dusted with flour or ground limestone. As symptoms progress, defoliation becomes greater, causing the tree to become weak from malnourishment. In many situations, trees infected powdery mildew are also infected with apple scab or CAR, which causes more damage to occur. Powdery mildew is best controlled by proper siting and following appropriate pruning practices that allow adequate air flow through the tree canopy. Fungicides are a third step in the protocol.
  • Fireblight: Fireblight has both leaf and stem tissue symptoms. Once infected, leaves turn a grey to blackish color, and turn limp. The stems, especially the terminal ends, take on the same color and usually exhibit a bend or droop, resembling a shepherd’s crook. These symptoms appear quite rapidly in spring, usually within a week or less. The disease is spread through spores that are released from stem cankers, oval-ish slightly sunken areas that look to be dead. Controlling fireblight is not easy, or any disease that is associated with a fungal canker. Other than streptomycin, limited numbers of registered fungicides are available, so one must rely on good cultural practices such as proper tree placement, separation of turf and ornamentals and avoidance of mechanical damage to the trunk and stems. Once infected, removing cankers through pruning is recommended to reduce the source of inoculum. The dormant seasons are best for canker removal. To reduce the risk of spreading the disease through pruning (from tree to tree or throughout the same tree), dip your shears in a mild bleach solution — one part bleach to nine parts water.