How did elm yellows spread to central Pennsylvania?
Elm yellows has devastated elm stands in isolated outbreaks in at least 22 states, mostly east of the Mississippi River. It is spread from tree to tree by a tiny insect, the elm leafhopper, whose only reported hosts are elm trees. There is some indication that once a tree becomes infected, the disease may spread to others via root contact, although research is not definitive on this question. It is impossible to say how the disease came to University Park, but storm winds depositing a cloud of leafhoppers on or near the campus is a likely possibility. University Park is a vulnerable target because it has the one of the largest stands of mature elms in the nation.
