Thanks for the question. It is like several other hydrangea questions that we’ve received this 2024 season. Brown leaves and flowers on an Incrediball Hydrangea can be caused by several factors, including environmental stress and disease issues. Here are some possibilities and their treatments:

1). Both underwatering and overwatering can cause browning of leaves and flowers. Underwatering can cause the plant to dry out, while overwatering can lead to root rot. Based upon the information you provided, I don’t think there are any watering causes for the brown leaves and flowers. I mention this just so you are aware of the possibility.

2). Incrediball Hydrangeas prefer partial shade. Too much direct sunlight, especially during the hottest part of the day, can scorch the leaves and flowers, causing them to turn brown. You did not provide any information about the plant’s location. You are in the best position to determine whether intense sunlight might have caused this problem. However since adjacent hydrangea are unaffected, intense sunlight may not an issue.

3). Fungal and bacterial infections can cause brown spotting and/or brown patchy areas on leaves and flowers. In this 2024 growing season, these types of diseases have been quite common in Minnesota in several different plants. From the pictures you provided, I don’t see much in the way of leaf spots. Rather there are apparent dead areas on leaves. Based upon this, I think that this is the most likely possibility is Anthracnose. This is a disease caused by several different fungi in the genus Gloeosporium. The best way to contend with this is removal and discarding of affected leaves and flowers coupled with avoiding too much moisture sitting on the leaves. As fall approaches, certainly remove as much leaf, flower, and plant debris from under and around the plant. Prune off all affected flowers. It generally is not fatal to the hydrangea though intense and prolong infections can weaken the plant. See the following for further information and remedial procedures:

https://pddc.wisc.edu/wp-content/blogs.dir/39/files/Fact_Sheets/FC_PDF/Anthracnose.pdf

https://www.hgtv.com/outdoors/flowers-and-plants/trees-and-shrubs/a-hydrangea-problems-primer

https://pddc.wisc.edu/2015/06/30/anthracnose/

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs

https://gardenerspath.com/how-to/disease-and-pests/hydrangea-diseases/ - :~:text=The first symptoms are water,, humid, and rainy weather.

Application of a fungicide in early spring (April 2025) would be good precautionary procedure.  Applying a fungicide now (late August 2024) would be ineffective.   Be sure that the product label for any fungicide indicates that it is effective against anthracnose.

A cautionary note. Besides anthracnose, there are other possibilities. Based upon the information and pictures you provided, I think it is the most likely. However if there is a reappearance of this condition in 2025, consider submitting plant samples to a diagnostic disease laboratory for an analysis. This would be a definitive way of getting to the bottom of things. Both Wisconsin and Minnesota have excellent labs to do this. Before submitting anything to either of them, call first as to what type of samples should be submitted. Both have a nominal charge for their analysis. See

https://pdc.umn.edu/

https://pddc.wisc.edu/clinic-overview/

Good luck. Thanks for consulting us.