Should I do something about these? Does it signify something bad?
Fungi almost exclusively only feed on non-living material. The one you have shown is a slow-growing, woody, example indicating that the trunk of the tree has been at least unwell for some time.– PekkaOct 13, 2016 at 10:07
I purchased the home 4 years ago. They are the most healthy crab apple trees in the neighborhood, but I've been spraying for frog eye spot the entire time. They keep their leaves longer than any crab apple around but the disease is still fighting back.– Evil ElfOct 13, 2016 at 11:14
Yes, it does signify something bad, if the fungus is actually attached to the trunk of the tree (which it looks to be) and not simply appearing on woody parts of the mulch. This is one of the bracket fungi, and they occur on trees which are dying, or will soon be dead. If it's somewhere it might hit something or someone when it rots through and falls, it might be wise to have it removed now before it does that.
Out of curiosity, is this the same crab apple tree you asked about in March, which had a problem on one of its branches?
No, different crab apple. I have three nicely sized ones in the corner of a suburban corner not. They look very nice but I fear I may lose them over the next ten years, if not sooner. They were probably planted in the early 80's.– Evil ElfOct 13, 2016 at 12:43
Not necessarily - they have an anticipated life span between 30 and 70 years,so they could go on a fair bit longer, hopefully– BambooOct 13, 2016 at 16:12