Reports on Plant Diseases
RPD No. 660 - Elm Yellows or Phloem Necrosis
and Its Control
[ Symptoms ] [ Disease Cycle
] [ Spread] [ Control ]
Elm yellows (formerly called phloem necrosis) is a widespread and serious
systemic disease of American or white elm (Ulmus americana) caused
by an unnamed mycoplasma-like organism (MLO). The MLO is a minute bacterium
that lacks a cell wall and can assume a variety of shapes.
In addition to the American elm and its cultivars, natural infections
of elm yellows occurs in four other North American species of elms: winged
elm (Ulmus alata), cedar elm (U. Crassiflia), red or slippery
elm (U. Rubra), and September elm (U. Serotina). Red X Siberian
elm (U. Pumila) hybrids are also susceptible. The susceptibility
of rock elm (U. Thomasii) is uncertain. Other elm species have
been experimentally infected with the elm yellows MLO. Species of Asian
and European elms, and hybrids between them and native species, seem to
be highly tolerant or immune.
Elm yellows occurs throughout Illinois, being more common in the southern
two thirds of the state. At one time (1990) in Illinois, elm yellows killed
more elms than Dutch elm disease.
Elm yellows has also been found in most other states in central and southern
United States from 32 to 46 north latitude, and also in Pennsylvania,
New York, New Jersey, and Massachusetts. Apparently, the northward spread
of the disease is limited by cold winter temperatures. The disease is
not found where the minimum winter temperatures are below -15 F (-26 C).
The inability of the insect vector to overwinter could explain the northern
limitation of the disease.
The MLO is limited to diseased root and stem phloem sieve tubes which
are found in the innermost bark of susceptible elms. The pathogen proliferates
and moves passively in the phloem sieve cells and becomes systemic throughout
Figure 1. Elm
showing symptoms of elm yellows.
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Figure 2. Foreground,
tree defoliated by elm yellows: background, elm with brown leaves attached
(IL Natural History Survey photo).
The first noticeable foliar symptoms on American elms are usually seen
between mid-July and mid-September. The symptoms include a rapid, general
decline of susceptible elms which leads to the death of infected trees.
The leaves commonly droop, curl upward at the margins, turn yellowish
green then a bright yellow (Figure 1), finally brown, and drop off within
a few weeks (Figure 2). Death of branches then follows. In highly susceptible
elms the leaves rapidly wilt, wither, turn dark brown, and remain attached
for several weeks. Infected elms generally die within one growing season
after the onset of symptoms. Elms that exhibit leaf symptoms after early
August usually survive the winter and leaf out at the normal time in the
spring. These leaves wilt, turn yellow, and drop in late June or July.
Elms also may sometimes die overwinter without showing foliar symptoms.
American elms generally show symptoms throughout the entire crown at
the same time. Occasionally, one section of a tree will exhibit bright
yellow leaves while the rest temporarily remains green. This latter pattern
may cause the disease to be confused with Dutch elm disease. Dutch elm
disease (DED) tends to mask elm yellows in doubly-infected trees.
Cedar, winged, and September elms generally die over several years, branch
by branch. Red elms usually show symptoms for two years before dying. Witches'
brooms (dense clusters of new growth at the tips of branches that result from
the profuse sprouting of axial buds) form during the year that death occurs.
Red X Siberian elm hybrids and Chinese elms also produce witches' brooms, but
the trees do not die. The innermost bark and the cambial zone of infected elms
change from white to yellow, then to butterscotch or tan, sometimes with flecks
of dark brown, finally turning dark brown. The rest of the bark then dies. The
phloem sieve cells gradually become partially filled with callose (a hard, gummy
carbohydrate), distorted, then crushed, and die. Secondary phloem proliferates
for a short time, then is also killed. Surface wood may also be discolored due
to the diffusion of pigments from the phloem.
The inner bark has a faint odor of oil of wintergreen only in trees infected
with elm yellows. In red elm the odor is somewhat like caramel or maple syrup.
These odors may be detected most easily by enclosing several pieces of freshly
exposed inner bark of dying trees in a small clean jar or vial for five minutes,
than smelling the air inside. Detection is easiest in bark removed from the
base of the trunk or the buttress roots but is also possible in bark from small
branches (Figure 3). The odor disappears as the bark dies.
When trees become infected, the pathogen moves through the phloem to the fine
fibrous roots. The fine roots are killed before foliar symptoms occur. As the
larger roots die, foliar symptoms start to appear. General tree decline follows.
The MLO's are most abundant in petioles and twigs of witches' brooms, and are
more easily found in tolerant than rapidly killed trees. They occur in both
living discolored phloem and in apparently normal phloem of diseased trees.
Water stress does not appear to be the cause of foliar wilting, yellowing,
and death. Movement of water in the xylem of diseased elms is reduced several
weeks before foliar symptoms appear. Phloem degeneration is advanced when symptoms
appear in the crowns. The stomata remain partially or completely closed several
weeks before phloem discoloration is apparent.
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The most important means of spread from diseased to healthy elms is by
the feeding of the whitebanded elm leafhopper (Scaphoideus luteolus).
This insect overwinters as eggs on the corky bark of small, living elm
branches. The eggs hatch over a period of about three weeks, beginning
soon after the leaves unfold. Nymphs pass through five instars before
becoming adults, which requires 36 to 42 days. Adult leafhoppers are present
and deposit the overwintering eggs from early June until frost in the
fall. Nymphs and adults prefer the inner and lower portions of elm crowns,
but adults are dispersed throughout the crowns late in the season.
The leafhopper vectors become infected with the MLO by sucking
juices from the phloem of leaves and succulent shoots of diseased elms.
Three weeks after exposure, the insects are capable of transmitting the
pathogen to healthy elms by feeding on the midribs and large veins on
the underside of the leaves. Once infective, leafhoppers can transmit
the MLO for the remainder of their lives. Other insects, possibly meadow
spittlebug (Philaenus spumarius) and another leafhopper (Allygus
atomarius), serve as vectors for the pathogen.
3. Left, branch showing inner bark symptoms: right, branch from healthy
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The MLO overwinters in the open phloem sieve tubes within the roots of American
elms and perhaps in the shoots of witches' brooms of red elms. The insect vectors
can acquire the MLO from elm phloem approximately in mid-June when the first
flush of shoot growth is complete. The leafhoppers then undergo a three-week
incubation period with the pathogen, and transmit the disease from mid-July
through September. Symptoms generally do not appear on an elm the year of inoculation;
they appear at least three months after inoculation in very small elms; 9 to
10 months or longer in large trees. A diseased elm can be a source of inoculum
roughly seven weeks after it was inoculated. The full disease cycle takes one
year; secondary disease cycles are probably not important. Root grafts between
closely spaced healthy and diseased elms can spread the MLO but their importance
has not been established.
Outbreaks of the disease tend to be local, and spread is five to eight kilometers
(3 to 4.8 miles) per year. Spot outbreaks and single tree infections are likely
caused by long distance wind transport of the leafhopper vector. The MLO may
be endemic in certain areas for years before causing disease. Once an outbreak
occurs, most susceptible elms in a locality are killed.
The MLO exhibits a relatively high rate of infection of previously healthy
elms, suggesting that it may have been introduced into the United States from
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Saving a diseased elm tree is not possible. The control measures outlined below
are aimed at reducing the spread of disease.
- All dead and diseased elm trees should be promptly removed, then burned
or buried to minimize spread of the MLO and the Dutch elm disease fungus to
healthy elms. Trees dying from elm yellows and/or Dutch elm disease provide
a breeding ground for elm bark beetles which transmit the DED fungus.
- Where desired, plant Asian and European elms (e.g., Ulmus carpinifolia,
U. glabra, U. laevis, U. parvifolia, and U. pumila) which appear
to be highly resistant to elm yellows. However, DED can be damaging to European
elms. Individual American elms were found to be resistant to the MLO but were
later killed by the DED fungus.
Spraying healthy elms with an insecticide has been recommended for vector
suppression by some researchers; others suggest that this practice is ineffective
in minimizing disease. Where recommended, the first insecticide spray is
timed when maturation of the spring leaf cop has taken place. The second
spray should be applied when the second leaf crop appears, usually about
July 15. The third spray is applied one month later (mid-August). When applying
an insecticide, the registered arborist should carefully follow all precautions
and directions as printed on the container label. Applications may be made
using a hydraulic sprayer or a mist blower that can reach and adequately
cover the uppermost foliage of the tallest elms. Twenty-five to 30 gallons
of spray is required to spray a 50-foot elm with a hydraulic sprayer and
4 to 5 gallons is needed with a mist blower. Healthy elms need to be sprayed
each year if they are to be continuously protected. However, we do not endorse
the application of insecticides to control elm yellows, because the likelihood
of beneficial effects is not good and applications must be made annually.
- Breaking root grafts between diseased and healthy elms growing within 50
feet of each other may be beneficial although research data to support this
practice is lacking. Root grafts may be broken chemically or mechanically.
A. Apply metham-sodium or SMDC (sold as Vapam Soil Fumigant) in 3/4-inch
holes drilled or punched 6 inches apart and 16 to 24 inches deep. The holes
should form a single straight line midway between diseased and adjacent
healthy elms. Pour 1/2 to 3/4 cup of dilute solution (1 part Vapam to 3
parts of water) into each hole. Seal the holes by tamping the soil with
your heel. For best results, the soil temperature should be least 50 F (10
C). Vapam kills a small portion of the roots and thereby "isolates"
the infected tree. In a row or group of elms, where an immediately adjacent
tree is already infected through root grafts, a second fumigation line to
break root grafts is highly desirable. Trees showing foliar symptoms should
be cut down, removed, and burned or buried two days after fumigation. (Note:
Do not use Vapam within 8 to 10 feet of healthy trees and within 3 feet
of shrubs. The treatment will kill a small circle of turfgrass or other
vegetation around each hole. The turf can be reseeded or sodded in two or
three weeks. If left alone, the turf usually recovers in the next few months.
Handle Vapam with care. (Be sure to follow all label precautions and directions).
B Dig a slit trench at least 2 feet deep midway between diseased and adjacent
healthy elms when symptoms are first evident. (Note: The chemical and mechanical
barriers described above should extend beyond the drip line of adjoining
- Experimental injection of tetracycline and oxytetracycline antibiotics
into the trunks of diseased elms has resulted in symptom remission for several
months to 3 years. However, tetracycline therapy is not recommended for control
due to its high cost, advanced root and phloem degeneration before foliar
symptoms appear, lack of lasting symptom remission after discontinuation of
therapy (trees are not cured), tetracycline phytotoxicity, and the risk of
MLO strains developing resistance to the antibiotics. It is doubtful that
the federal EPA will ever clear the use of tetracycline antibiotics as a control
measure for elm yellows.
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For further information concerning diseases of woody ornamentals, contact
Nancy R. Pataky, Extension Specialist and Director of the Plant Disease Clinic,
Department of Crop Sciences, University of Illinois at Urbana.
University of Illinois Extension provides equal
opportunities in programs and employment.
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