Fungal growth on walnut
I have a couple of walnut trees that almost died a couple of years ago when we had a severely cold winter. They have some trunk damage on the Southside which makes me think that the Sun heated up the tree on that portion during a milder spell. I've attached a couple of photos. I'm wondering, should I try to remove the fungal growth that I see there now? And should I paint the tree on the Southside a lighter color to prevent part of it thawing out in the Milder spells of the winter?
You are right in suspecting the harsh winters of 2014 and 2015 as being a problem especially a cold tender tree like a walnut. Winter injury can be caused by a lot of things but usually not the warm period but what happens after the warm period.
Here is a link to the Michigan State University articles that where written following those cold events. http://msue.anr.msu.edu/resources/winter_injury_in_plants
I would think the main problem was absolute cold and that the Southwest injury was only a contributing factor. The vigorous growth from the base of the tree indicates to me that the trunk itself was damage at and above the snowline and you got vigorous growth from the undamaged portion below the injury.
Yes the white paint might have worked to reduce Southwest injury.
Unfortunately the fact that you now have mushrooms growing out of the tree indicates that wood rotting fungi are already in the tree attacking digesting the dead wood. There is no treatment to eradicate the fungi inside the tree. The tree will rot from the inside out. Your best bet is to remove the affected portion of the tree and focus on retraining the new sprouts from the base of the tree.
I'm still not sure what to do with my walnut trees. I do see that the one tree at least has hollowed out as the expert said it would two years ago. I still would like the trees to recover, but if that's not going to happen then perhaps I need to be told exactly to cut them down. I don't think there's an unaffected portion. Unfortunately, I forgot about the previous reply and trimmed some sprouts that were growing near to base. However, the new sprouts were growing from the damaged areas as well. I suppose I pruned them because the larger branches looked like they were doing well. The trees have grown taller, although without a central leader. Would an expert please take a look at the new photo and let me know your thoughts?
The loss of wood from decay has not affected the growth of the walnut but it has destroyed the inner wood that's role is to provide support. The tree can go on living for years since the inner wood is not involved with transport of water, minerals or sugars produced in leaves. I do believe it is likely to cause the tree to split at the point where the branches come together. I would suggest getting another tree started nearby for when this occurs. The other options of course is to remove this tree and replant. The wound at the crotch of the branches has too much dead wood which will eventually cause the limbs to break apart.