''Witches brooms'' may form at the root flare or on the lower portion of the ash tree, although ash trees infected with yellows disease may die without developing this symptom.
Photo: https://ag.umass.edu

The ash tree's nemesis, the Emerald Ash Borer (Agrilus planipennis), was first identified in the U.S. in southeastern Michigan in 2002. The EAB has destroyed tens of millions of ash trees in 25 states in the Midwest and Northeast.

Survival for ash trees is tough, but it's getting tougher.

Wisconsin Dane County officials are now reporting an increasing new threat to the area's white and green ash trees: yellows disease. Yellows disease is classified as a hytoplasma, a specialized bacteria that requires a vector (sap-sucking insects like leaf hoppers) to transmit its pathogens to plants. The disease can manifest as the yellowing of leaves, as a yellow growth on the bark, or an abnormal nest-like "bushy" growth, also referred to as "witches' broom." Yellows disease is also known to affect lilacs.

The disease was first identified in Southern Wisconsin a few years ago, but wasn't considered a major threat until recently. Yellow disease does not get as much attention as EAB, because it is a slower killer of ash trees.

Dane County forester Adam Alves told WKOW in Madison that the disease takes 5 to 10 years to kill an ash tree, and that there is no known way to treat the disease. Alves reported that Token Creek Park in DeForest, Wisconsin has been the hardest hit local area, with the loss of 22 trees on one small plot.

The county's strategy is to remove as many ash trees as possible before the end of the year, and begin replanting a diverse number of tree species next spring. Moving forward, the county aims to minimize the impact of yellows disease by having no single tree species account for more than 5 percent of the total number of trees in the county.