Issue 1, April 16, 2012
Do You Recommend a One-Time Fungicide Injection Treatment for Apple Scab?
Crabapple infected with scab, Picture taken by Nancy Pataky
Apple scab is a well-known and easy disease to identify disease; however if you need some help, refer to the following report on plant disease: http://web.aces.uiuc.edu/vista/pdf_pubs/803.pdf
The apple scab fungus infects under a wide range of temperatures but requires a wetting period to become established on a tree. Usually, Midwestern weather in the spring provides just what the scab fungus needs. The minimum wetting period on the leaves is only about 6 hours if temperatures stay near the optimal 60 degrees F. If temperatures are cooler, the wetting period must be longer for infection to occur. In a normal spring, scab symptoms might start to show on the leaves from 8 to 18 days after infection. Under cool, dry conditions, this incubation period might be longer.
The first step is to promote basic IPM (Integrated Pest Management) and we do this by recommending that you plant resistant crabapple varieties. Here is a link to "Recommended Crabapples for Illinois Landscapes" (Adobe PDF).
If your ornamental crabapple is considered to be highly susceptible to apple scab, you may even consider replacing the tree.
Raking up the diseased leaves that have fallen below the tree will help a bit, but most of the time, if the tree is susceptible, it may not be able to escape apple scab infection.
When we give apple scab recommendations, we always give recommendations from the University of Illinois Home, Yard, and Garden Pest Guide as well as the University of Illinois Commercial Landscape and Turfgrass Pest Management Handbook. Various protectant fungicides are listed and applications should begin when leaves to emerge from buds (1/4 inches green) and continue at labeled intervals until 2 weeks after petal fall.
In the recent research paper, "Evaluation of microcapsule trunk injections for the control of apple scab and powdery mildew," by G.C. Percival & S. Boyle, they had good results with fungicides injected into crabapple for control of apple scab; however, none of these fungicides are registered in Illinois. With injections, we worry about repeated injury to the tree and the possibility of other plant pathogens being able to enter through these wounds. In this research, they found that the trees healed or callused fairly quickly, which is good to avoid problems. But, this may not always be the case. Also, what are the long term effects to trees from injections?
At this time, the only injectable fungicide registered in Illinois for ornamental crabapple for leaf diseases is Alamo (propiconazole) and it is applied by macro injection. This is not an option for apple tree or food crops. The University may recommend this type of systemic fungicide application for crabapples infected with apple scab in "sensitive areas" or areas where fungicide drift could be an issue in the environment. For example, a sensitive area may be crabapple trees near a pond with fish. But, most of the time, if basic pesticide safety is practiced or fungicide applications are made during reduced wind speeds, drift should not be an issue.
Therefore, in most instances, injections to prevent apple scab or other leaf disease are not recommended.
(Stephanie Porter and Travis Cleveland)