Jim Chatfield and I visited the USDA Forest Service, Northeastern Research Station, in Delaware, OH, on Monday where they are waging a battle to eliminate Elm Yellows (EY) from their American Elm Restoration Project research plots. I last saw the disease in southwest Ohio in 2013.
The problem is that American elms (Ulmus americana) that are resistant to Dutch Elm Disease (DED) have no known resistance to Elm Yellows (EY). DED is caused by a fungal infection; EY is caused by a phytoplasma which is a type of bacterium. DED fungi plug the xylem. The EY phytoplasma destroys the phloem; this is true for all phytoplasmas.
Based on historical evidence, it is suspected the EY may have been around as long as DED. However, EY has not followed the same trajectory as DED relative to presenting a widespread threat to elms. Thus far, the disease has appeared as sporadic localized outbreaks. Still, with the widespread re-introduction of American elms into urban landscapes, this old killer has re-emerged to present a serious threat.
There are no effective treatments for EY and it appears that the phytoplasma can infect trees regardless of overall tree health. Once infected, trees will rapidly decline and die; often within the same year of infection. The most effective method for preventing the spread of EY is to quickly remove and destroy infected trees. Isolating infected trees by ditching will cut root-to-root grafts and may help to prevent phytoplasma transmission. As with all phytoplasma diseases, EY is spread by insects that use their piercing-sucking mouthparts to feed on the phloem such as leafhoppers, planthoppers, treehoppers, and froghoppers (= spittlebugs). However, using insecticides to control these vectors is problematic. The exact list of EY insect vectors is not yet known thus insecticide applications cannot be fine-tuned to match varying insect life cycles including differences in numbers of insect generations.
Given the dire prognosis and drastic response required for control of EY, it is essential to monitor for EY in American elms. If EY is diagnosed in the field, it is important to confirm the diagnosis by sending samples to a lab.
Diagnosing Elm Yellows:
Rapid Yellowing of Leaves: the name "elm yellows" clearly describes the color of the leaves on infected trees. Leaves start the season appearing normal in color and size; however, they may be undersized if infection occurred last season. Sometime in mid-to-late summer, the entire canopy rapidly turns an intense shade of yellow from lemon yellow to golden-yellow. The color-change occurs quickly - often within a few weeks - without the leaves first wilting.
Early Defoliation: Infected trees will defoliate with the yellowed leaves dropping in late-summer to early fall much sooner than normal. Leaves do not show any marginal browning or wilting prior to dropping.
Phloem Discoloration: the old name for the disease was "elm phloem necrosis" which captures the essence of the infection. The phytoplasma targets and destroys the phloem with the inner phloem becoming yellowish-brown to caramel colored and the staining extending to the surface of the xylem. The discoloration is generally confined to the lower portion of the trunk and the lower branches. This is because the phytoplasma first migrates to the roots causing a rapid and substantial dieback of the fine feeder roots, then the main roots. In essence, there is a "bottom up" pattern to the infection.
Carefully slicing away the bark on the lower trunk or lower branches using a sharp pocket knife or a draw-knife will expose the off-colored infected phloem tissue. On a cautionary note: elm phloem tissue will naturally become discolored by oxidation when exposed to the air. The oxidation occurs in minutes, and mimics the discoloration caused by EY, so samples showing phloem necrosis that is suspected to be caused by EY must be fresh!
Wintergreen Odor: Methyl salicylate (oil of wintergreen) is produced in the phloem tissue that is colonized by the EY phytoplasma. Cut a section of bark to the white wood near the base of the main stem and place the sample in a sealed jar. While the wintergreen scent is usually very faint at first, it becomes easily detectable after the sample has been held in the jar for about 1 - 2 hrs.
Quick Death of Trees: Trees that appear perfectly healthy with normal twig elongation and leaf expansion early in the growing season are often dead by the end of the season.
CONFIRMATION: Although the symptoms listed above provide strong evidence of EY, the only sure-fire way to know that a tree is infected (and infectious!) is to send samples to a diagnostic clinic capable of performing the appropriate tests for phytoplasmas. The organism cannot be cultured, but can be detected using Enzyme-Linked Immunosorbent Assay (ELISA); and Polymerase Chain Reaction (PCR) (DNA analysis).
Eliminating Other Elm Problems:
Nutrient Deficiency: The chronology of EY leaf yellowing is different from leaf chlorosis caused by nutrient deficiencies. Typically, nutrient deficiency symptoms appear early in the season with new leaves appearing chlorotic. Early defoliation almost never occurs.
Black Leaf Spot: this fungal disease of elms produces irregular dark brown to black spots that may be slightly raised. Heavy infections may cause leaves to yellow and drop. However, yellowing does not occur prior to the development of the leaf spots.
Environmental Calamities: Everything from poor soil conditions to drought to over and under watering can cause American elms to collapse. However, leave wilting is usually a tell-tale pre-collapse symptom.
Vascular Diseases: Verticillium wilt has long been the nemesis of American elm trees that are under stress and resistance to DED does not mean that elms will not suffer from branch dieback from the disease. However, vascular wilts produce noticeable leaf wilting and browning because the diseases prevent water from reaching the leaves. EY produces no leaf wilting or browning prior to defoliation.