ash tree disease
Emerald Ash Borer (EAB), a small, iridescent green beetle from Asia, has killed many ash trees in the Midwest and upper Midwest. It has been detected in several ash trees in Boulder, but so far has been confined to Boulder.
Until EAB has been found within 5 miles of your location in Denver, there is no need for or benefit to preventive treatments applied to your ash trees. If or when EAB arrives in Denver, you will need to make a decision on whether to preventively treat your ash trees, basing that decision on the tree's health and value in your landscape.
Learn more about EAB and the threat it poses to ash trees in NE Colorado at
www.eabcolorado.gov and see
http://www.ext.colostate.edu/pubs/insect/emeraldashborer.html There is another FAR more common borer of ash trees called Lilac-Ash borer (LAB) ; the adult is a moth. LAB Is very common in the Denver area and has attacked MANY ash trees, most of these have been drought-stressed. Generally, ash trees that are healthy, growing well and not under drought stress or other stresses are much less of a target for LAB.
Much more info at