We were surprised to find some trees in all areas of the state that we surveyed to have the phytoplasma. Based on these results we know the ash yellows phytoplasma is wide spread in the region and in Colorado. However, this organism does not seem to be causing widespread dieback and decline in green ash. Thus, it seems green ash in the Great Plains is resistant to this pathogen. The phytoplasma probably has some impact on tree health if the tree is previously stressed from other factors.
Further research is needed to determine the impact of ash yellows on ash in the Great Plains.
There are several other causes of dieback and slow decline of green ash in our region including soil moisture fluctuations, weather impacts, wood borers, and occasionally verticillium wilt. We do need to be aware that the organism is widespread and common in the state's green ash. We also should use white ash judiciously since white ashes are more susceptible. We did not sample white ash for the phytoplasma, but this would be a prudent activity especially along the Front Range where white ash has become a common landscape tree. Current research on the relative resistance of ash cultivars and species is ongoing at several locations in the Great Plains so we will know what cultivars are better suited for resisting the pathogen.