Not since Dutch elm disease are so many threats converging on Iowa's trees. Dutch elm disease was devastating to Iowa's landscape wrecking widespread destruction and great cost.
I remember as a young man seeing those large stately trees begin to die from the disease. My great-grandfather's 40-acre timber pasture was almost entirely American elm and in a short time my uncle was forced to bring in the bulldozer and clear the site, but that was one of what seemed like at the time only one disease that affected trees. That is sure not the case today.
I've written about emerald ash borer in a previous column and the devastation that is expected when it reaches us. Emerald ash borer (EAB) is a small green invasive wood boring beetle that attacks and kills ash trees. The adults live on the outside of ash trees feeding on the leaves during the summer months. The larvae look similar to white grubs and feed on the living plant tissue underneath the bark of ash trees. The trees are killed by the tunneling activity of the larvae under the tree's bark, which disrupts the vascular flow. EAB is a highly invasive forest pest that has the potential to kill nearly 100% of the native ash trees of any size, age, or stage of health where it is present. We are told that over 50 million ash trees outside of Iowa have been killed where EAB is present. Much of Iowa's forest land is populated with ash trees, and Iowa's community street trees are heavily planted with ash cultivators. The US Forest Service 2008 inventory indicates that there are 52 million woodland ash trees and 3.1 million urban ash trees in Iowa. Trees attacked by EAB can die within two years. Once EAB killed trees are discovered in a community nearly all ash trees in that community will be dead in five to six years, but if that was the only disease in town to worry about that would be bad enough. Sadly, that is not the case. Far from it. Here are a few others:
Bur oak blight (BOB) is another. BOB is a newly named disease that can cause severe defoliation, leading to mortality of branches or entire trees. Bur oak blight is caused by an undescribed species of the fungus - tubakia. Based on reports of BOB to the Iowa State Plant Insect and Disease Clinic in 2010, 27 counties in Iowa reported the presence of the disease. However, the disease has been observed by DNR foresters across the state.
Iowa's spruce trees are being attacked by needle cast disease. This fungus is causing second-year needles to turn purple, or brown and fall off. The fungus appears as small black growths in pores on the underside of infected needles. Healthy needles will have white pores.
And there is pine wilt. scotch pines are most impacted but several other pine species are impacted as well. Pine wilt is spread by the sawyer beetle but caused by the pinewood nematode. The nematodes leave the beetle and feed within the pine tree damaging the living tissue beneath the bark.
And if that's not enough, here's one to worry about and watch for - thousand canker disease (TCD). This is a disease of walnut trees and while prevalent in western states it has yet to reach Iowa. The deaths are caused by a walnut twig beetle that carries a fungus which is spread as the beetle tunnels through tree tissues. The introduction of TCD into Iowa would have disastrous effects economically to the wood industry in the state. Some experts believe that TCD has the potential to decimate black walnut in the same way Dutch elm disease destroyed our American elms.
I wish that there was a magic solution to these and other problems but there is not. The best advice that I can offer is maintain a diversity of species in your woodlands or backyards. As we manage your parks and wildlife areas that is the approach we must take.