The Morton Arb

Selection guide

Use the pulldowns below to find information about trees or shrubs

or

or

or

plant_clinic_hours

Container gardens & Spring bedding at The Morton Arboretum

Spring Creations
Download a list of plants that can be found in our Container Gardens and Display Beds.

Use our Interactive Collections Map and our Plant Collections Map to identify where plants are located on our grounds.

Register now for Summer 2012 education programs at The Morton Arboretum

Search tree & plant advice

Tagged as: Apple Scab

Apple Scab Resistance—Pursuing a Moving Target

By Susan J. Wiegrefe, Ph. D.
Morton Arboretum Tree Breeder

It's not just your imagination—apples and crabapples have been infected with apple scab more than usual in the recent years. The fungal disease affects both leaves and fruits, causing early defoliation and fruit drop when the infection is severe. The severity of the infection is determined by a number of factors: the level of genetic resistance of the apple tree, the weather conditions (temperature and rainfall) at the time of leaf expansion and throughout the season, and the amount of fungal spores present during the growing season. Although recent springs have been characterized by cool, overly wet weather during the leaf expansion time - conditions that promote disease (recall the mild, wet spring of 1995 that resulted in unofficially the worst year of scab on record) - the increase in diseased trees may not be entirely attributed to weather conditions. Astute observations by crabapple enthusiasts suggest that the fungus itself may be becoming more virulent and capable of overcoming resistance and fungicide toxicity.

The information supporting this conclusion is from an evaluation program begun in 1983, in which cooperators (including The Morton Arboretum) in 21 states each planted 50 of the best (most popular) cultivars then available. This program, called the National Crabapple Evaluation
Program (NCEP), monitored apple scab infection levels among other horticultural traits for a minimum of 5 years with some plantations continuing their observations to the present. In 1989, the National Crabapple Introduction Program (NCIP) was begun to expand that evaluation process to newly introduced and potential crabapple introductions. Throughout the course of the observations it was determined that some cultivars that had originally been disease free and were considered tolerant or resistant to the disease were found to have increasingly severe infections.

One explanation for this pattern is that a more virulent form of the fungus has been spread to a larger area of the country on nursery stock.

Members of the National Ornamental Crabapple Association are pursuing funding for study of the genetic interactions between the apple scab fungus and crabapples. This will ultimately aid in the breeding and selection of trees with resistance to the presumed newer strain that possesses greater virulence. Meanwhile, it is important to keep up on tried and true control measures and to have an up-to-date list of disease resistant cultivars for your region. The Morton Arboretum is in the final stages of developing just such a list for the Chicagoland area. It will be available later this year.

Print

Related Articles