Question: Do you recommend a one time fungicide injection treatment for apple scab?

Answer:  We first want to promote basic IPM and recommend using resistant crabapple varieties.  Here is a link to "Recommended Crabapples for Illinois Landscapes" : http://extension.illinois.edu/IPLANT/plant_select/trees/Selecting_Crabapples.pdf

If the ornamental, crabapple is highly, susceptible to apple scab, you may even consider replacing the tree.

Raking up leaves 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.

I found a paper where they did some research on injections to prevent apple scab:
http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7348.2005.00019.x/pdf
They had good results with the fungicides applied in this research; 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 leaf disease is not recommended.
(Stephanie Porter and Travis Cleveland)