Recently I've seen my Elderberry trees develop some yellow leaves and I don't know what the problem could be or how to fix it, so I'll leave a picture if anybody can help me.
I must say that while this plant has a leaf like Sambucus the bark does not look right. It should be brown and corky with light speckles on old wood, bright green on new wood with longitudinal ridges, not smooth dull green as in picture. A distinctive characteristic of Sambucus is the hollow stems. Cut a piece off and examine inside the stem for a hollow space except at the nodes where it tends to fill in.
The yellow resembles magnesium deficiency. Epsom salts applied as a foliar spray in a test area, if it produces a quick response, might be a test for this.