European and US elm trees, already endangered by Dutch elm disease, are now facing a new threat. Scientists have discovered that elms are being killed by a rapidly spreading bacterial infection for which there is no known cure. The disease, called Elm Yellows, is caused by bacteria spread by insects known as leaf-hoppers. Chomping insects pass the bacteria into leaves through their saliva. The infection then migrates to the trunk and strangles the tree's circulatory system, killing the plants in a similar way to Dutch elm disease.