What is killing the box elders? (https://tonytomeo.wordpress.com/2017/10/04/what-is-killing-the-box-elders/) I still do not know. I know that does not sound like much of an update. All I can share is some pictures of secondary symptoms observed now that the affected trees are deteriorating.
The two pictures above, although not relevant to any symptoms, are important to our Community because the historic Felton Covered Bridge just got a new roof! (https://tonytomeo.wordpress.com/2017/12/02/felton-covered-bridge/) Dead box elders that are already starting to destabilize and collapse are now leaning onto the edge of the new shingles! They really need to be removed before winter storms move them around, and they dislodge any shingles.
The first of the two pictures below show basidiocarps associated with fungal decay of the trunks and roots. This decay destabilized and compromised the structural integrity of the necrotic trees with remarkably efficiency. The trees died only last spring, and are already collapsing! The second of the two pictures below simply shows decaying bark peeling from a necrotic trunk.
This all happened so suddenly. Like SODS (Sudden Oak Death Syndrome) it is likely to be misdiagnosed a few times before the primary pathogen is actually identified. Also like SODS, it may be ignored as an isolated situation affecting a few trees that are not very popular anyway. When SODS first started killing a few of the unpopular tanoaks, it did not seem like much of a problem. It did not get much attention until it started taking out majestic coast live oaks that had been healthy for centuries.
The two pictures below show a cross section of a necrotic trunk that needed to be cleared from a bridle trail. Galleries have been excavated by the larvae of unidentified boring insects. The second picture is merely a closeup of the first.