Just heard back from UC Davis Plant Disease Diagnostic Clinic: their assessment is that the yellowing is due to an abiotic condition. Possible triggers include:
- too much fertilizer, specifically too much of some specific nutrient (though they didn’t suggest which)
- root unhappiness due to poor drainage or too much water
In most cases the affected growth was near the top of the tree. The pathologist suggested that the conditions of the roots while these shoots were emerging help determine the condition of the foliage. The most vigorous shoots rely on healthy roots more than slowly growing shoots during growth spurts so they were good candidates for showing stress.
The tree in the photo is in dire need of repotting so it’s not surprising that it has the most yellow foliage.
As the clinic doesn’t think it’s a disease, they didn’t worry about the condition spreading to other trees. They suggested repotting the affected trees and experimenting with soil and fertilizer used going forward. We’ll see how it goes!