Mycoplasmal Pathogens:
New Causes for
Old Diseases
C. R. Hibben
Microorganisms residing
abnormalities harmful
to
in the
phloem of plants induce growth
including lilacs and ashes.
their hosts,
a new era for plant
researchers
pathology, Japanese
reported the
in
of
heretofore
unknown
discovery pathogens
plants. These were called mycoplasmalike
organisms, or MLO. Several diseases thought
to be caused by viruses were subsequently
reexamined, and MLO were identified as the
true causal agents. The significance of this
discovery is shown by the wide host range of
MLO: 700 herbaceous and woody plants in
424 genera within 99 families. These pathogens are responsible for diseases of many eco-
The year 1967
began
as
nomically important vegetable, ornamental,
forest, and plantation crops from cool temperate to tropical regions. The well-known triumvirate of fungal, bacterial, and viral pathogens
of plants has now been joined by a prominent
new member, the MLO.
As an introduction to this unique group of
microorganisms, this report will cover the
nature of MLO, how woody plants react to
them, and recent research on their impact on
two genera in the family Oleaceae, Syringa
(lilacs)
and Fraxinus
(ashes).
Definition and Detection
In simple terms, MLO are single-celled, nonmotile organisms somewhat smaller than bacteria. They are the smallest known living
cells. More technically, they are wall-less
prokaryotes, consisting of an elastic, triplelayered membrane enclosing cytoplasm that
contains ribosomes and strands of DNA.
MLO are variably
shaped, assuming spherical,
beaded, filamentous, or yeast-like appearare smaller than one
millionth
of a meter). Like bac(one
teria, they appear to divide by a cleavage
process, but their reproduction is not well
understood at this point.
As plant pathogens, MLO occur only in the
phloem sieve tube elements of leaves, shoots,
and roots. Their direct study in the phloem
has been by electron microscopy, but a fluorescence test for MLO has simplified the process of infection diagnosis. In this test, the
chemical DAPI (4’6-diamidino-2-phenylindole#2HCl) binds to mycoplasmal DNA and
fluoresces when exposed to ultraviolet radiation. Thin sections of infected plants stained
with DAPI can then be examined with a
fluorescence microscope. In the early work
with plant MLO, the fact that remission of
symptoms could be induced by the injection
of tetracycline antibiotics, but not by penicillin, was considered prima facie evidence for
mycoplasmal infection.
All attempts to achieve sustained growth of
plant pathogenic MLO outside of their hosts
have failed. This inability to grow MLO in
artificial culture continues to be a limiting
factor to characterizing them more completely. One direct consequence of this
difficulty is the use of the cumbersome term
ances.
Most MLO
micron
"mycoplasmalike." Recently,
has been made investigating
some
progress
serological rela-
Figure 1. Mycoplasmalike
orgamsms
(arrow)
in
phloem
tionships among plant MLO utilizing partially purified MLO as immunogens.
Perfection of these serological techniques
would allow rapid advances in research.
can
induce reduc-
tion and stimulation of growth in their hosts
simultaneously. Growth reduction is
as stunted and misshapen leaves
and floral parts, shortened internodes, and
decreased annual ring width. At the same
time, growth is initiated from axillary and terminal buds that normally would have
remained dormant until the following season,
and from adventitious buds in older wood.
Witches’-brooms (dense clusters of stunted
twigs) are the most dramatic manifestation of
this abnormal growth.
expressed
tube cells
trom
a
leaf midnb of mfected lilac.
Infection also causes abnormalities in
flower development, including greening, the
production of leaflike parts, gigantism, sterility, and ultimately, total inhibition. Fruit of
quality is produced, or fruiting ceases
altogether. Yellowing and premature autumn
coloration of foliage occur in some hosts.
Long-term disease expression in woody plants
ranges from yearly chronic symptoms to mortality within months of infection. In some
cases, infected plants can be free of any obvipoor
Plant Reactions
Mycoplasmal pathogens
sieve
symptoms.
The physiological modes of action by which
MLO affect plants are not well understood.
Phloem degeneration occurs, and a disruption
of phloem function can be inferred by abnormal accumulations of starch in the leaves of
infected plants. Stunting symptoms suggest
inadequate movement of photosynthates to
ous
10
Transmission and Control
difficulty in achieving control of
mycoplasmal pathogens is that they are easily spread. MLO are transmitted from plant
to plant by phloem-feeding insects, such as
leafhoppers, spittle bugs, and psyllids. The
wide host range of these pathogens in woody
and herbaceous plants makes the acquisition
and spread of MLO by insect vectors more
likely. Vegetative propagation materials such
as cuttings, grafting scions and stocks, and
shoot tips for micro-propagation can readily
carry MLO. While plant tissue cultures are
known to sustain MLO, their survival depends
on the rapid differentiation of new phloem by
the explant. Mycoplasmal pathogens are not
One
Figure 2. Hedgerow of white ash in southeastern New
York State, where ash dieback has been severe since
the 1950’s. Most of these trees are now dead.
seed-transmitted.
Practical control measures are essentially
limited to exclusion of the pathogen by the
use of disease-free stock or by roguing infected
plants out of the ground. Injecting trees with
tetracycline antibiotics results in a temporary
remission of symptoms in some hosts, but
these treatments are not curative. Some measure of control of insect vectors is possible with
insecticides, but excluding all vectors from
susceptible plants is unlikely and impractical.
Impact on Two Woody Hosts
Two prominent members of the olive family
growing points. Photographs of phloem sieve
tube elements filled with MLO suggest a
blockage in the downward transport of
starches and sugars, as well as the diversion
of these photosynthates to the growth of the
pathogen. The loss of apical dominance and
the flower abnormalities in infected plants are
obvious indicators of a basic hormonal
imbalance.
In addition to the direct effects of MLO on
their host, there is evidence that MLOinfected woody plants are predisposed to
injury from non-biological stresses, such as
low winter temperatures. We have observed
the death of MLO-infected lilacs following a
severe winter (Hibben et al., 1986), and others
have noted subnormal cold hardiness in ash
trees infected with MLO (Matteoni and Sin-
clair, 1985).
(Oleaceae) appear to be favored hosts of MLO:
Fraxinus and Syringa. From recent research,
some of it conducted in the ash and lilac
plantings at the Arnold Arboretum, we have
learned that ash yellows and lilac witches’broom are mycoplasmal diseases of some
importance.
Ash dieback, characterized by a gradual
dying back of the branches and eventual tree
mortality, has been a major problem in the
northeast since the late 1950’s. White ash
(Fraxinus americana) and green ash (F. pennsylvanica) have been affected in woodlands,
roadsides, and home sites. Drought and
canker fungi were initially implicated as
causal factors, but in 1970, MLO were identified in witches’-brooms associated with ash
in advanced stages of decline (Hibben and
Wolanski, 1971). Cornell researchers, making
11
3. Abnormal branch growth
orgamsms. Healthy ash on the left.
Figure
m
white ash
(nght)
of the DAPI fluorescence test, discovered
that mycoplasmal infection is widespread in
ash, even in trees without witches’-brooms
(Matteoni and Sinclair, 1985). The disease was
named ash yellows. There is now convincing
evidence that ash yellows is the primary cause
of ash dieback in undisturbed sites where ash
should be healthy. Ash yellows has been
reported in several northeastern and midwestern states, and in southeastern Canada.
The host range of MLO in Fraxinus is
greater than first realized. MLO have been
detected in blue ash (F. quadrangulata) and
black ash (F nigra) (Sinclair, 1987). During a
survey of ash in the Arnold Arboretum, we
identified MLO in ten additional species of
Fraxinus susceptible to MLO (Hibben and
Franzen, 1987). These findings were especially
use
is one
consequence
of infection by mycoplasmalike
interesting because the infected ash trees were
adjacent to the lilac collection which contained numerous specimens with the
witches’-broom disease. This provided circumstantial evidence that the mycoplasmal diseases of ash and lilacs may be caused by the
related strains of MLO.
In support of this hypothesis, we were able
to transmit MLO from infected ash to healthy
same or
lilacs, and from infected lilacs to healthy ash
using the parasitic plant, dodder (Cuscuta
subinclusa), as a carrier. The ability of the
same MLO pathogen to infect more than one
host would increase the likelihood of disease
spread by insects, and make control strategies
more difficult.
Lilac witches’-broom was first reported in
1951, and the cause was presumed to be a
12
1986, we identified MLO, rather than
virus. In
the true cause of the disease in
virus,
x
Syrmga josiflexa, S. x prestoniae, S. sweginzowii, S. villosa x sweginzowii, S. josikaea,
and S. x persica. The more widely grown S.
as
a
vulgaris cultivars, including most of the
or French hybrids, never showed
these symptoms, even when interplanted with
infected lilacs jHibben et al., 19861.
Lemoine
After further
inspection of lilac collections
in the eastern United States and Canada, we
found that S. vulgaris cultivars are suscepti-
ble to, but
more tolerant of, infection than
non-vulgaris lilacs. Diagnostic symptoms in
S. vulgaris consisted of premature growth
from current-year buds, growth from adventitious buds in older wood, and sometimes
stunted twigs in bizarre zigzag growth patterns. Occasionally, MLO were detected in
healthy looking lilacs. Witches’-brooms and
dieback were
infected S.vulgaris.
severe
not
associated with
Figure 4. Witches’-brooms are dramatic symptoms of
woody plants infected by mycoplasmal pathogens.
This is the lilac cultivar ’Royalty’.
Figure 5. Stunted, bunchy twiggrowth and scattered
twig dieback are additional symptoms of woody
plants infected by mycoplasmahke organisms. This
is the lilac cultivar
’Royalty’.
A compilation of lilac taxa in which the
witches’-broom disease has been identified
(Table 1) shows that the late-blooming lilacs
are especially susceptible, particularly those
with a josikaea or villosa lineage. Mycoplasmal infection has not been detected in any of
the early-blooming S. x hyacinthiflora cultivars. However, more research is necessary
before lilacs resistant to MLO can be recommended with confidence.
When the diseases of ash and lilac are compared, MLO are more lethal in ash, whereas
reduced aesthetic value is usually the consequence of infection in lilacs. MLO may
become a greater threat to lilacs in the future
as their propagation by tissue culture becomes
increasingly common. Contamination of lilac
explants by MLO will have to be carefully
monitored in order to assure the production
of disease-free plants.
We are convinced by our research on ash
and lilac that these newly discovered pathogens constitute significant threats to horticulture and forestry. Clearly, the primary limiting
factor to learning more about mycoplasmalike
organisms is the inability to isolate and grow
them in pure culture. When this has been
achieved, I predict that they will soon rank in
importance with fungi, bacteria, and viruses
as recognized disease-causing agents in the
environment.
13
References
Hibben,
C. R., and L. M. Franzen. 1987. Coincidence of
lilac witches’-broom and ash yellows m two
arboreta. Phytopathology 77: 118 (Abstract).
Hibben, C. R., C. A. Lewis, and J. D Castello 1986
Mycoplasmalike organisms, cause of lilac
witches’-broom. Plant Disease 70: 342-345.
Hibben C. R., and B. Wolanski. 1971. Dodder transmission of a mycoplasma from ash witches’-broom.
Phytopathology
S. B. Silverborg. 1978. Severity and
of ash dieback Tour of Arboriculture 4:
274-279.
Hildebrandt, V. 1986. Lilac propagation by tissue culture:
academic to commercial. Lilacs 15(1): 25-34.
Lee, I. M., and R. E. Davis. 1986. Prospects for in vitro
culture of plant-pathogenic mycoplasmalike
Hibben, C. R., and
61: 151-156.
Matteoni, J. A., and W. A Sinclair. 1985. Role of the
mycoplasmal disease, ash yellows, in decline of
white ash in New York State. Phytopathology
75: 355-360.
Sinclair, W. A. 1987. Mycoplasmal infection found m four
ash species in midwestern states. Plant Disease
71: 761.
Additional Selected References
Bove, J. M. 1984. Wall-less prokaryotes of plants
of Phytopathology 22: 361-396.
Ann. Rev.
causes
organisms. Ann. Rev. of Phytopathology 24:
339-354.
McCoy, R. E. 1979. Mycoplasmas and yellows diseases,
pp. 229-265. In The Mycoplasmas, vol. 3: Plant
and Insect Mycoplasmas, ed R F. Whitcomb
and J. G. Thlly. New York: Academic Press.
Sehskar, C. E., and C L. Wilson 1981. Yellows diseases
of trees, pp. 35-96. In Mycoplasma Diseases of
Trees and Shrubs, ed. K. Maramorosch and S.
P. Raychaudhun. New York: Academic Press.
Dr. Craig R. Hibben is Research Plant Pathologist at the
Brooklyn Botanic Garden Research Center in Ossmmg,
New York.