The white on this branch of Elderberry is a fungus called Elder Whitewash (Hyphodontia sambuci). It is little researched, and might be put incertae sedis in the basidiomycetes.
In taxonomy, the term incertae sedis (abbreviated inc. sed.) is used for a taxon when its broader relationship to other taxa is unknown. The first person who used the term in botany was probably Antoine-Laurent de Jussieu. In his book Genera Plantarum of 1789 he listed a few plantae incertae sedis.
Reasons why a taxon might be classified as incertae sedis include:
The description is not detailed enough to permit a clear classification.
A scientific publication about a taxon does not focus on its classification. The taxon is labeled incertae sedis, because the authors of the publication do not want to guess where the taxon belongs.
Different researchers may have different opinions of where a taxon should be classed. Until the conflicts are resolved, the taxon is classed as incertae sedis.
