Saturday, August 31, 2013

Elm Yellows - a Sporadic Yet Lethal Disease of Elm

Elm Yellows, a lethal systemic disease of elm, was confirmed on two American elm (Ulmus americana) specimens in Franklin County in August 2013.  The disease can occur in isolated areas across the eastern portion of the US and can quickly devastate large plantings of native elm.  Elm yellows occurs only occasionally in Kentucky.  In fact, only one other incidence has been reported in the commonwealth during the past 30 years (Jefferson Co., 1990).

Symptoms of elm yellows usually appear during summer months and include bright yellowing that resembles early senescence (Figure 1).  Leaves can change hues with a few weeks, with petioles turning downward (epinasty) (Figure 2).  Leaves eventually turn brown and can remain attached to branches for several weeks (Figure 3). 

Figure 1.  Foliar symptoms of elm yellows disease include bright yellowing of leaves during summer.

Mature trees develop disease symptoms approximately nine months following infection, while young trees may show symptoms in as little as three months.  Trees usually die within a year or two after symptoms develop.  There is no cure. 

The causal agent of elm yellows is a phytoplasma (bacterium-like prokaryote) called ‘Candidatus Phytoplasma ulmi’. The pathogen inhabits phloem tissue of elm, and as the pathogen builds up in tissue, it becomes a metabolic sink for photosynthetic products.  Phloem then degenerates downstream from these sinks, causing root mortality in fine roots and subsequently in larger ones.  As this process ensues, tree canopies begin to show yellowing symptoms as described above. 

Figure 2.  Petioles droop and turn downward as elm yellows disease advances.
Hosts of the elm yellows bacterium are limited to elm species, particularly native elm, including the American elm (U. americana) and winged elm (U. alata).  Chinese elm (U. parvifolia) is more tolerant of infection and often remains unaffected in areas where disease has killed native elm. 

Spread of the bacterium is believed to be caused by several species of leafhoppers and possibly spittlebugs, although the white-banded elm leaf hopper has been confirmed as the primary vector.  These insects inoculate trees during summer or early autumn as they feed. 
Figure 3.  Within a few weeks of symptom development, elm yellows causes leaves to turn brown.  Leaves may fall or remain attached to trees for several weeks.

Control of elm yellows is not possible and control of insects is not practical.  Infected trees should be removed as soon as possible to prevent spread of disease.  Confirmation of elm yellows requires a molecular diagnostic test.  Non-elm or tolerant elm species, including Asian species and hybrids, should be used as replacement plants.