Ash tree defoliation
My 25 year-old ash tree started dropping entire compound leaves for the first time in June. There is occasional browning at the edges but the vast majority of the dropped compound leaves appear completely normal. There is no honeydew. A tree company applied a systemic insecticide three weeks ago (~July 20) by pouring it on the ground around the base of the trunk. The tree continues to defoliate. It is 1/3 to 1/2 defoliated, principally on the north side. Other than losing so many leaves the tree appears completely normal. No evidence of bore holes or an unusual amount of insects on it. Thank you for your help!
Larimer County Colorado trees and shrubs
What was the tree company treating for when they applied the systemic insecticide? Were there other insects you were concerned about?
Sometimes trees drop leaves early for a number of reasons. They may be under stress (too much/too little water), they may have made too many leaves in the spring and need to shed some to "balance" their physiological systems, the tree may have been damaged by wind/hail/rain or the soils may be compacted, resulting in inefficient roots. There are also many other possible causes. My co-worker, Mary Small, of the Jefferson County Plant Diagnostic Clinic, wrote a great blog on this subject last summer: http://csuhort.blogspot.com/2013/08/the-mid-summer-leaf-drop-blues_1002.html
At this point, there's not much you can do. You can try to rule out possible causes for the leaf drop and continue good management practices with your tree. Give the tree regular water throughout the entire year (including fall and winter), consider aerating your turf (if the tree is in a lawn) and monitor for other health and disease issues. Also, check out the growth increments to see if they are increasing, decreasing or staying the same: http://www.ext.colostate.edu/mg/Gardennotes/102.pdf (see page 102-9).