JoAnne Skelly: Green ash decline worrying

I’m worried about my green ash trees (Fraxinus pennsylvanica). They have always been such hardy trees, tolerant of drought, cold, sun, wind and alkalinity. They have needed little fertilizer or care and, in the past, always seemed to thrive. This year the trees had a heavy infestation of aphids, common for many ashes, but uncommon on mine. I had thought the syrphid flies and birds were resolving the aphid issue, but the leaves are still curled on the lower half of the trees. And, their canopies seem sparse with few leaves overall and with dieback on many of the small lower branches.

Although aphids can be common, normally trees don’t suffer much. Ash trees can get diseases such as verticillium wilt or ash yellows. They can be hit by invasive emerald ash borers, Western ash borers or lilac borers. I haven’t seen the green emerald ash borer or evidence of their presence — holes in the trunk or frass (wood dust and feces). The emerald ash borer can kill a tree within three to five years of infestation.

There is also a vague problem called “general ash decline” that may be tied to borer activity, various diseases, environmental or cultural stresses. Symptoms start with initial thinning and/or yellowing of the leaves on random branches or throughout the whole canopy. Woodpecker injury may be evident, which is usually a sign of borer activity. There may be ‘D’-shaped holes that are the emergence holes for the emerald ash borer. Trees with ash decline seem to worsen with hot, dry weather.

When I researched how to resolve ash decline, the results were not encouraging: “no easy cure… fungicides are not beneficial, insecticides may help if a specific insect is identified” (University of Illinois Extension). Basically, the management strategy is to get the tree healthy with sufficient watering and fertilization and wait and see. Avoid fertilizer products that contain herbicides. Don’t nick the tree with lawn mowers or string cutters. Disinfect your pruning tools with isopropyl alcohol between cuts on an ash in case there is a disease.

I suspect my green ash trees were damaged in the winter flooding of 2017 when there was standing water on our property and saturated soils. Absorbing roots may have rotted and haven’t regenerated to the level needed to support the size of the canopies of our big trees. Last summer’s dieback seemed normal and the trees eventually leafed out fully. This year I’m concerned.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.