Each year, at the Plant Clinic, we receive many requests for testing for this fungus. Unfortunately, we are still able to make many positive isolations. The fungus works much like the other vascular pathogens, causing plugging of the vascular tissues and resultant wilting and death of foliage. American elms are very susceptible to this pathogen. Although Chinese elm and Siberian elm are known to be more resistant, infection of these species can occur as well.
Work is still under way to develop resistant elms.
So far, this work has produced the more resistant Sapporo Autumn Gold, American Liberty, and Urban elms. Watch the elm for yellowing of the leaves, followed by wilting and browning. Look for vascular discoloration to help with diagnosis of this disease. Peel back the bark of a symptomatic branch to reveal the brown streaks in the otherwise tan outer sapwood. We generally select branches that are about as thick as a thumb with wilted leaves.