I had a call this week about mature Modesto ash trees with more than half the branches dying back. The worried caller told me there were brown spots on the trunks, which made me think of borers. The limbs had started dying in one tree and then progressed to two other ash trees nearby. All the trees are in a lawn and receive regular lawn watering.
I often am a detective when I try to solve a plant problem. I looked up Modesto ash, Fraxinus velutina “Modesto” in Sunset Western Garden Book, to find out more about it. It is hardy to minus-10 degrees Fahrenheit, although other sources say hardiness ranges from 0-5 degrees. It is very susceptible to verticilium wilt and ash decline syndrome. Verticilium wilt, a fungal disease, kills plant cells that move water up into the branches. When water can’t get through, branches die. Ash decline, a bacterial disease, causes leaf yellowing, gradual decline and eventual limb and tree death. The Modesto ash is also susceptible to borers, which cause considerable dieback in the tops of trees and shorten the life of the tree.
Plant problems, such as limb dieback, are often the result of improper cultural practices that stress plants, making them susceptible to damaging diseases and insects. What cultural practices might have weakened these trees? They are in a lawn. Lawn watering is not optimum for tree health because the grass competes with the trees for water. In addition, lawn irrigation systems are often turned off in October and not turned on again until April. Trees, even dormant trees, must be watered at least once a month through the winter unless there is significant rain or snow. Drought, particularly winter drought over a number of years, could be a contributing factor to the dieback issue.
For the past few winters, temperatures have dropped below minus-10 degrees in outlying valleys where the trees are. Perhaps cold damage was another stress factor. Other potential stress factors to consider include improper fertilization or pruning practices; string cutter or lawn mower damage to the base of trees and pesticide use. In combination or alone, these may have weakened the trees and reduced their resistance to diseases and borers.
Unfortunately, there are no chemical treatments for the two diseases or borers. The trees may or may not survive. My advice is to remove grass in a 6-foot ring around each tree and water the trees deeply throughout the year. Then, wait and see what regrows next year.
JoAnne Skelly is the Carson City/Storey County Extension educator for the University of Nevada Cooperative Extension.