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).
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| 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.
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Figure 2. Petioles
droop and turn downward as elm yellows disease advances.
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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.
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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.
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