In the past week or so, crabapple trees have stolen the spring flower show among our blooming landscape plants. These beautiful trees produce abundant flowers for up to 4 weeks each spring, with one of the best, most showy displays of any ornamental tree species. They pick up where the magnolia trees left off, often flowering in close succession. The beautiful, tiny flowers are produced from branch tips to the tree’s interior, creating a canopy of blooms.
Crabapples are one of the most widely-planted ornamental trees in the US. They freely hybridize, which has resulted in an extremely large number of commercially available cultivars. Although exact numbers are open to debate, there are over 500 different crabapples in cultivation across the US. They come in a large assortment of growth forms from weeping to upright and narrow. Many have been bred for a wide variety of other characteristics such as varying leaf or flower colors or, most importantly, disease resistance.
Anyone who has ever inherited an older, already-planted crabapple can attest to their poor appearance in late summer. They start the year looking healthy with a robust array of flowers and slowly look more and more decrepit, almost leafless in some cases, by summer’s end. Sadly, no amount of watering or TLC can bring them out of this annual spiral of declining health once it has begun. However, they do happily leaf out and flower in the coming year, which is a testament to their resiliency.
Apple scab is the pathogen primarily responsible for the poor appearance of crabapples in the late summer. Although they are plagued by other ailments (fire blight, powdery mildew, cedar-apple rust), apple scab seems to be the more prevalent pathogen across the board. It is caused by a fungus that infects both leaves and fruit. Primary infection occurs early in spring and slowly spreads as the season progresses, which is why infected trees start out looking great and slowly loose leaves until they are often nearly barren by late summer.
Through careful selection for resistance, plant breeders have been able to produce disease resistant or immune cultivars. Accordingly, the current best practice for apple scab control is planting disease resistant varieties of crab apple. Resistant varieties are widely available, although some susceptible plants are still on the market, so it does pay to do your homework. Currently, there are enough resistant crabapples available to meet the needs of anyone seeking other specific characteristics such as leaf or flower color or specific growth habits.
If you are like me and have inherited a crabapple with little apple scab resistance, there are control options available depending on the level of investment you are willing to allocate. Fungicidal sprays are available to control apple scab prior to leaf infection. These sprays should commence each spring when shortly are leaves emerge. A series of spray applications are needed between leaf emergence and full leaf maturity, which occurs around early or mid-June.
For smaller trees, this spray application can often be done with equipment readily available to homeowners, but larger trees will require a hired professional. Homeowners should be wary to avoid fungicides on the market which contain insecticides as well. Since flowering overlaps the spray period, many pollinators may be present on your crabapple and the unneeded insecticide can have detrimental effects on pollinators populations.
If spraying seems too labor intensive or expensive, there are other options to reduce the level of apple scab infection, but rarely provide total control. Since the fungus overwinters in infected leaves and fruit, simply removing all debris during the winter time can limit the infection source, although other crabapples (or apple trees) in your neighborhood may provide a significant amount of airborne spores to re-infect your trees. Regular pruning to open up more airflow can allow leaves to dry quicker and limit infection from the moisture-loving apple scab fungus. Additionally, basic plant health care practices, such as mulching and watering, can boost your crabapples overall health and provide more resiliency.