Butterfly Bush blight
This is probably a fungal leaf spot, very common this year considering how wet a spring we have had. Or, it could be a bacterial leaf spot.
To minimize leaf spot diseases always water from below the leaf canopy. Redirect lawn sprinklers to keep the shrub leaves dry, or water lawns only early in the day so leaves dry quickly and are not wet overnight. Keep fallen leaves raked up. You may clip off the most damaged leaves and discard them. Only work on the plant when it is dry. In fall thoroughly rake up fallen leaves so fungi are reduced in the area.
Fungicides will reduce the occurrence of these problems, especially when used during wet periods in early spring. Copper-based products control bacterial diseases. Here are references listing garden fungicides-
The mention of brands or retailers is for example and not an endorsement of any retailer or product; there are others available. Please read and follow all directions and precautions on any chemicals you use.
You can submit sample leaves and pictures to MSU Plant Diagnostic lab for an exact diagnosis of which one of several diseases this may be. See https://pestid.msu.edu for instructions and the fee schedule.
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I wanted to add that I apologize for the long delay in your receiving an answer. If you need anything else please contact us here, or call MSU Extension Garden Hotline, a free service, which is open during business hours on weekdays- 888-678-3464 Best regards, Laura