Julaine,

Thanks for contacting U of MN Extension with your question. It is indeed a beautiful Oak tree so I hope you can save it. We can use some more information to narrow the possible causes down. This information can be found either in the leaves or in the branches. Based on the pictures, here are some possibilities and some ways you may be able to identify the problem:

1) Oak Wilt: Look for browning of the leaf that generally (but not always) begins at the tip and margins and progresses towards the midrib and base. Oak wilt is characterized by very slowly progressing symptoms over 2 — 5 years. A dark brown to black discoloration on the wood surface may be found when the bark is peeled back from a branch with wilting leaves. Walled-off fungal infections (seen as rings or narrow discolored wood) may also be observed in the cross-section of an infected branch. See this page for more information: https://extension.umn.edu/plant-diseases/oak-wilt-minnesota#symptoms-of-oak-wilt-1258961. Important: If Oak wilt is the problem don’t prune the tree through July to avoid spreading the disease. Don't wound, prune or fell oaks in oak wilt counties during this time period. Immediately cover unavoidable wounds with paint or shellac.

2) Bur Oak Blight (BOB): It can be readily confused with oak wilt but starts in the lower crowns of trees. Like oak wilt, leaves have dead, brown areas, and many prematurely fall. However, with BOB, the splotches on the leaves have better defined edges, and often are shaped like a wedge. On the underside of the leaf, BOB also can have black spotting on the veins, especially early on when they are just starting to show symptoms. Another sign it is BOB rather than oak wilt is seeing more leaf loss on the inside and lower parts of the canopy rather than the edges. Unfortunately, it is possible that an oak can have both oak wilt and blight. See this page for more information: https://extension.umn.edu/yard-and-garden-news/late-summer-bur-oak-blight

3) Two Lined Chestnut Borer: Foliage at the top of the tree canopy starts to die back first and work its way down the tree. As the tree continues to decline, dieback extends down into major branches and eventually into the main stem. Twolined chestnut borer galleries can be seen when the bark is removed. See this page for more information: https://extension.umn.edu/tree-and-shrub-insects/metallic-wood-boring-beetles#trees-they-attack-1785410

It may be hard for you to discern the distinctions between these options, and you might have more than one thing going on as the tree has become weakened. Given that the tree is very valuable to you and you want to save it if possible, I would recommend having it examined by a certified arborist — not just by someone who prunes trees. Here is a page that will give you some suggestions in finding an arborist: https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional. I would do this as soon as possible. Until you find out what’s going on, I’d also avoid any further pruning.

Good luck. I hope you can save your tree! It’s a beauty!