One of the most common ornamental trees in the U.S. is the crabapple. They can be found in most any urban and residential landscape. In general, they are easy to grow, provide beauty in four seasons and are not considered to be high-maintenance plants. However, in some locations they can become infected with foliar and systemic diseases that limit their appeal and function in the landscape. As with many other plant species, some cultivars are disease-resistant, while others are quite susceptible.
Many foliar diseases can plague crabapples; below are the three most common:
Apple scab: Apple scab is favored by cool, moist spring weather, particularly when tree leaves remain wet for long periods of time. Under these conditions, fungal spores are blown by the wind and splash from leaf litter and tree to tree to spread the infection. Initial symptoms appear as olive brown, round spots on lower leaf surfaces. The spots are usually the size of a standard pencil eraser. 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 midsummer. Not only is this unsightly, it deprives the tree of carbohydrate and sugar production, causing it to weaken. 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. Location of the tree is also important. Crabapples should be placed in full sun, and where moderate breezes can help to keep leaves dry. A consistent fungicide spray program can also be helpful in suppressing the effects of apple scab, particularly on susceptible cultivars. Examples of these include thiophanate-methyl, myclobutanil, mancozeb and propaconizole. These are best applied at seven to 14-day intervals from pre-bloom 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, ranging from pinhead to eraser in size. 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 is a “two-host disease,” much like the common chicken and egg anecdote. 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 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 that 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. 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 can travel up to 5 miles in the wind.
Powdery mildew: Unlike apple scab and CAR, powdery mildew does not 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 if 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 with powdery mildew are also infected with apple scab or CAR, which causes more damage to occur.
Powdery mildew is best controlled by proper sitting and following appropriate pruning practices that allow adequate air- flow through the tree canopy. Fungicides are a third step in the protocol.
Several diseases invade the woody tissues of crabapples, as well.
Fire blight: Fire blight 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, slightly sunken areas that look to be dead.
Controlling fire blight is not easy, or any disease 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.
Nectria canker: Nectria canker attacks several species including deciduous shade trees such as walnut, honeylocust, maple and linden, as well as crabapple and pear. The fungus lives between the bark and heartwood, killing the cambium and sapwood. Cankers develop commonly between nodes or points of attachment for new lateral stems. They can occur on all sides of the tree, and spread to small and large tissues alike. Nectria sometimes produces target-shaped cankers around bud scars, wounds and branch stubs. Equally as often, it produces oval, elongated areas of sunken, withered bark and sapwood, with exposed heartwood.
Cultural management strategies are most effective for controlling canker diseases. Pruning out cankers can remove the source of canker fungi from the tree, reducing the risk of new infections. Because spread of canker pathogens is favored by rainfall, pruning should be done in dry weather. In general, it’s best to make pruning cuts at least 4 to 5 inches outside the edges of the canker. Because cankers are more common on trees that are weak, use proper techniques for transplanting, mulching and watering. Excessive nitrogen applications should be avoided, since they can produce succulent growth that causes trees to become more susceptible.
Sunscald: Sunscald isn’t a disease in the traditional sense, yet it has a similar appearance, somewhat like nectria canker. Confusion between the two, and other fungal cankers, is common. In most cases, sunscald is caused by fluctuations in temperature in winter. In many parts of the U.S., daytime temperatures reach 40 to 60 degrees in winter. Sunscald occurs when warm temperatures are followed by sudden temperature decreases; 55 degrees one day and 25 the next.
The signature symptoms of sunscald are oval to elongated-oval sections of a tree trunk where the bark is loose. As the malady progresses, more and more tissue sloughs off; the bark, then the sapwood and eventually the heartwood is exposed. In almost all cases, this occurs on the south and southwest side of the tree, where temperature fluctuation is greatest. Fungal and bacterial cankers can develop throughout the tree, regardless of side. Trees with thin bark, such as sugar maple, Norway maple and crabapple, are more prone to damage than other species The lack of wind protection or shade from other trees usually increases the susceptibility.
Sunscald is not a fungal or bacterial problem, therefore treatment with pesticides is futile. Sunscald is best avoided by selecting trees that are resistant. If thin barked trees are chosen, place them where they are not exposed to afternoon sun in winter. The damage to existing trees with sunscald can be lessened by placing a white board next to the tree trunk on the south/southwest side. A simple piece of scrap lumber will suffice; two old boards nailed together in a V shape, and leaned against the trunk will go a long way to prevent future damage. A rag should be used between the board and the tree trunk to avoid rubbing.
There is great benefit to selecting crabapple cultivars that are less likely to become infected with various diseases.
Nationwide, botanic gardens, state/local extension offices and the International Crabapple Society are good sources of information about the disease resistance of specific cultivars. A Web search for “crabapple cultivar selections” yields a multitude of study reviews, including ones from universities in Wisconsin, New York, Ohio, North Carolina and Kansas. Look for information about trials that have been conducted near your location. Crabapples, as well as most other plants, tend to perform differently from state to state; the closer the trial is to your area, the better.
The author is a horticulturist and certified arborist located in Omaha, Neb.