Category Archives: Disease

Elm Yellows—Methods for Controlling EY

Yellow, wilting leaves on an infected tree next to a non-infected tree. Photos courtesy Joe Boggs

By Joe Boggs and Francesca Peduto Hand, PhD

The most effective method for controlling EY is to quickly remove and destroy infected trees. This reduces localized reservoirs of the phytoplasma pathogen, making new infections less likely.

Thankfully, EY does not appear to be following the same trajectory as DED relative to presenting a widespread threat to elms. Thus far, the disease has appeared as sporadic localized outbreaks; however, given the dire prognosis and drastic response required for control of EY, a field diagnosis should never serve as the final word on elm yellows. Stem samples should be sent to a plant diagnostic clinic for confirmation of a suspected infection.

Read the full article: Elm Yellows: The Re-Emergence of an Old Tree Killer

Elm Yellows—Another Diagnostic Indicator

Left: Adding bark section to a jar to detect wintergreen scent. Right: Bark section held in a closed jar for one to two hours. Photos courtesy Joe Boggs

By Joe Boggs and Francesca Peduto Hand, PhD

An interesting chemical reaction that produces methyl salicylate (oil of wintergreen) occurs in the phloem tissue that is colonized by the EY phytoplasma. This provides another diagnostic indicator of EY on American elm. The so-called scratch-and-sniff method of detecting the wintergreen scent involves cutting a section of bark to the white wood near the base of the main stem and placing 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 approximately one to two hours.

Death from EY occurs quickly. In fact, 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. There are no effective treatments and susceptible trees can become infected regardless of their overall health.

Read the full article: Elm Yellows: The Re-Emergence of an Old Tree Killer

Elm Yellows—Diagnosing EY in the Field

Left: Cutting bark to expose the phloem. Right: Discoloured phloem. Photos courtesy Joe Boggs

By Joe Boggs and Francesca Peduto Hand, PhD

The name elm yellows clearly describes the colour of the leaves on infected trees. Typically, new leaves become fully expanded and deep green. However, sometime in mid-to-late summer, the entire canopy rapidly turns an intense shade of yellow. The colour-change occurs without the leaves first wilting; the yellowed leaves appear otherwise normal. The chronology of the 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. Eventually, infected trees will defoliate with the yellowed leaves dropping in late-summer to early fall.

The disease was once called ‘elm phloem necrosis,’ which captures the essence of the infection since the phytoplasma targets and destroys the phloem; the inner phloem becomes yellowish-brown to caramel coloured. The discolouration is generally confined to the lower portion of the trunk and the lower branches 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. The discoloured phloem can be exposed on the lower branches by carefully whittling away the bark. On a cautionary note, however, elm phloem tissue will naturally become discoloured by oxidation when exposed to the air. The oxidation occurs in minutes, and mimics the discolouration caused by EY, so samples showing phloem necrosis that is suspected to be caused by EY must be fresh.

Read the full article: Elm Yellows: The Re-Emergence of an Old Tree Killer

Elm Yellows—A Background on Elm Yellows

Leaf yellowing from elm yellows versus normal leaf colour. Photo courtesy Joe Boggs

By Joe Boggs and Francesca Peduto Hand, PhD

Elm yellows (EY) presents a similar problem; resistance to DED does not impart resistance to EY. While much has been learned about DED, the entire story behind EY remained elusive for many years. Even the discovery of the causal agent was a relatively recent event, and the taxonomy has yet to be resolved; for now the pathogen behind EY is referred to as ‘Candidatus Phytoplasma ulmi.’ Much remains unknown about this pathogen including its origins.

However, this does not mean the disease was not recognized early on during the urbanization of American elms. Symptoms that are now considered to be associated with EY were noted on American elms in the early 1900s.

EY is a tree-killing disease caused by a micro-organism belonging to a group of plant pathogens called phytoplasmas. These single-celled organisms were not known to exist prior to 1967 and until their discovery, EY was thought to be caused by a virus. Indeed, there are still many references in older literature to a virus causing EY. Even after its discovery, phytoplasmas remained enigmatic to microbiologists and plant pathologists. Early on, these organisms were found to share many characteristics with mycoplasmas, which belong to a class of bacteria known as Mollicutes. These extremely small bacteria lack a cell wall; ‘mollis-’ is Latin for ‘soft,’ and ‘-cutis’ means ‘skin’ (e.g. ‘cuticle’). So, phytoplasmas were once referred to as mycoplasma-like organisms (MLOs); however, mycoplasmas were most strongly associated with diseases that occur in animals such as atypical pneumonia (walking pneumonia) in humans.

Read the full article: Elm Yellows: The Re-Emergence of an Old Tree Killer

Elm Yellows—The Re-Emergence of an Old Tree Killer: A Brief History of American Elms and DED

American Elm at Spring Grove Cemetery and Arboretum, Cincinnati, Ohio Photo courtesy Joe Boggs

By Joe Boggs and Francesca Peduto Hand, PhD

Dutch Elm Disease (DED) was first diagnosed on American elms in Cleveland, Ohio, in the late 1920s. The disease first appeared in eastern Canada in the 1940s and gradually spread westward into Ontario (1967), Manitoba (1975), and Saskatchewan (1981). DED arrived in North America from Europe; however, it was not native to Europe.

DED was first detected in Europe in 1910 when native European elms began succumbing to a mysterious malady. The causal agent was not known until 1921 when a fungus (Ophiostoma ulmi) associated with the elm die-off was isolated and identified by two Dutch plant pathologists, Dr. Bea Schwarz and Dr. Christine Buisman, who were working in the Netherlands at the University of Utrecht; thus, the ‘Dutch’ in DED. It is now known the originally identified fungus, which produces a slow death, has been largely supplanted in North America and Europe by a closely related, but much more aggressive species, O. novo-ulmi. It is also generally accepted that DED originated in Asia, which accounts for the resistance of most Asian elms to the disease.

Research on elm resistance to DED started in Europe as soon as the causal agent of the disease was identified. Indeed, Buisman spent the remaining years of her short career researching DED, developing methods to test resistance to the fungus, and working with others to identify resistant trees. The first resistant elm selection was named (posthumously) in her honour in 1936; Ulmus ‘Christine Buisman’ is now referred to as Ulmus procera ‘Christine Buisman.’ One of the first elm cultivars found to be resistant to DED in North America was Ulmus Americana ‘Princeton.’ The cultivar was actually first selected in 1922 by Princeton Nurseries in Kingston, NJ, for its landscape value; its resistance to DED was not discovered until years later, however.

Read the full article: Elm Yellows: The Re-Emergence of an Old Tree Killer