The ornamental crabapple tree is one of the most popular flowering trees in Minnesota landscapes. Many homeowners were alarmed and disappointed last spring and summer when their crabapple trees were severely defoliated by apple scab disease. By mid-July, many of these trees were almost completely defoliated and the ground was littered with yellowish or reddish-colored leaves. The big question now is “What can I do to prevent this from happening again this year?” Good air movement discourages the development of fungus diseases. So, begin by doing some selective pruning to open up the canopy of the tree. This increases airflow and allows the leaves to dry more rapidly which inhibits the development of the apple scab fungus. March and early April is the recommended time to prune ornamental crabapples and other fruit trees. The disease infects new leaves in spring when the weather conditions are right. Apple scab has been especially prevalent the past two seasons because the disease is more infectious when we have spring with cool wet weather. Although it can be quite labor intensive, a chemical spray schedule will reduce the amount of infection that occurs. In order to be effective, any chemical control used must be timed to coat the leaves with the fungicide before infection can occur. The first fungicide application should be made when the new leaf growth is 1/2 inch in length (1/2 green tip stage). A second application must follow when the flower buds are showing color, but before they open (pink stage). A third application is made when 3/4 of the petals have fallen, followed by spray applications every seven to 10 days until the end of June. Fungicides labeled for apple scab control include Captan, Chlorothalonil (Daconil) and Thiophanate-methyl (Bonomyl or Cleary’s 3336). Do not use chlorothalonil on trees if you plan to eat the fruit. Be sure to read the label carefully before applying these fungicides and follow all precautions. If you have flowering crabapple trees that are susceptible to the disease and are infected nearly every year, it may be wise to consider replacing them with one or more of the new scab-resistant cultivars. Adams, Adirondack, Camelot, Centurion, Lancelot, Prairiefire, Sargent and Thunderchild have good resistance to apple scab and also to fireblight which is a bacterial disease that can be fatal to apple trees. Visit your nursery to learn the flower and fruit color, growth form and other characteristics of these cultivars.
