WEST: MUSHROOM ROOT ROT
185
mind that the plants must continue in a thrifty
roses.
growing condition in order to force a maximum
condition,
bloom production.
moisture supplied so that adequate growth
To do this, it is not abso
lutely necessary to build a specially prepared
With strong vigorous stock in fresh
and the needed plant foods and
will be made, plants of suitable varieties will
bed unless the soil and location are such that
prove satisfactory in giving a heavy bloom
they are entirely unsuited to the growing of
production.
MUSHROOM ROOT ROT IN FLORIDA
to the combined effects of the fungus infection
Erdman West
Florida Agricultural Experiment Station
Gainesville
Mushroom root rot is a disease of woody
plants that causes the death of many kinds of
ornamentals and crops in Florida.
It is espec
ially painful to the home owner because it
frequently causes the loss of key specimens
in
carefully
landscaped
plantings.
Peach,
tung and guava growers have incurred severe
losses in their groves from this cause.
It is
the most serious disease of woody plants in
the state.
Mushroom root rot in Florida is caused by
Clitocybe tabescens (Scop. ex. F.) Bres. a
native fungus belonging to the group generally
known as mushrooms. It has been found in
other states but in the northeastern and
western states, Armillaria mellea Vahl ex Fr.,
a similar organism, is more prevalent.
The
latter is comparatively rare in Florida.
The
two organisms may be distinguished by certain
vegetative and fruiting characters. Armillaria
produces rhizomorphs, the mycelium is phos
phorescent and the mushrooms have an annulus on the stalk.
Clitocybe produces no
rhizomorphs, is not luminescent and bears no
annulus on the stalks of the fruiting bodies.
Mushroom root rot has been found on native
and the injury.
Such factors include lightning,
fire, wind-storms and insect or other animal
damage. Such infected trees thus maintain a
continuous supply of inoculum and are a
constant menace to nearby trees.
Theoretically infection can occur in either
one of two ways but only one has been dem
onstrated experimentally. Roots of a healthy
plant may grow through the soil until they
come in contact with an infected root of a
diseased tree. The fungus can spread from
the diseased root to the healthy root. This
method has been proved with tung in Louis
iana and citrus in Florida. Since the fungus
can persist in several diseased roots for some
time, the infective pieces are not necessarily
still attached to the diseased tree. This fact
explains why newly cleared oak land may show
a relatively high infection of mushroom root
rot when planted to a susceptible crop. The
fragments of diseased oak roots left in the soil
after clearing operations furnish abundant
inoculum to infect any susceptible plant set in
the soil.
By this method infection occurs only
in the vicinity of the original diseased tree
and spread of the disease is relatively slow.
The other method of spread is by spores.
Thousands
under
bodies.
sides
of spores
of
the
are produced
mushrooms
or
on the
fruiting
These spores are minute and may be
Florida plants under natural or uncultivated
carried for long distances by air currents.
conditions.
one of these spores should fall in a favorable
Some of our native oaks, espec
ially turkey oak (Quercus laevis) and laurel
oak (Q. laurifolia) are frequently found in
nature bearing sporophores of the fungus.
These native trees are resistant to the disease
and live long after the root system has been
invaded.
It is only after they have been in
jured by some other agent that they succumb
If
situation on a susceptible tree, it is conceivable
that a new infection could start at that point.
Since the spores are delicate and probabJy
short lived, the chances of all these favorable
factors for infection occurring at the same
time are extremely small. The few infections
that do result in this manner are likely to be
186
FLORIDA STATE HORTICULTURAL SOCIETY, 1949
remote from the original infection, resulting
in the dissemination of the disease over a
wide area.
After infection has occurred by either means,
the progress of the disease and the develop
ment of the fungus proceeds in a similar man
ner.
From the point of infection the fungous
threads penetrate the cambium layer between
the bark and the wood.
If the tree has no
bark, as in the case of a monocot the fungus
grows directly through the pithy tissues.
As
the affected area is killed, the fungus spreads
along the roots to the crown of the plant.
After this has been reached, the fungus grad
ually grows around the stem and outward
along any lateral roots that may be reached.
Soon after the crown or main stem is en
circled, the top of the plant wilts and dies.
Up to this stage, there is little or no evidence
of disease on the plant attacked. After the
plant has been dead sometime, usually just
after the end of the next rainy season, clusters
of fruiting bodies of the fungus appear at the
base of the trunk or stem and sometimes along
the larger roots where these are close to the
surface of the soil. These clusters may con
tain a 100 or more mushrooms but usually
there are not more than 25 to 50. The caps
of these umbrella-shaped mushrooms are 3
inches or less across, pale brown or tan in
color and are borne on crooked stalks 2 to 6
inches long. The underside of the caps are
lined with gills or narrow plates set on edge,
radiating from the central stalk.
Usually the only symptoms of the disease
are the sudden wilting and dying of the foliage
of the affected plant. Some species including
casuarina and citrus show stunting or shorten
ing of the new twigs or growth.
Since fruiting
bodies of the fungus are seldom if ever present
at this stage, we look for other signs by which
to diagnose the cause of the trouble in the
early stages. The most constant of these is
the white layer of fungus growth between the
bark and the wood in the recently killed parts
of the plant.
On thin-barked species like
azalea, the layer is very thin and difficult to
find, On other plants such as guava or loquat,
the white layer is usually conspicuous if the
bark is carefully removed.
The host range of this fungus has proved
very wide in Florida and new species are fre
quently being added to the host list. Over
200 native and introduced plants are now
reported to be attacked by this disease. Such
crop plants as peach, tung, and guava are
known to be quite susceptible.
The list of
ornamentals is long and I am not going to
mention but a few as Dr. Rhoads, a plant
pathologist formerly with the Experiment Sta
tion is preparing a complete list for publication.
Among the known susceptible plants are
azaleas, roses, jasminums, phoenix palms, dog
woods, royal poincianas, bananas, wistarias
and aborvitaes. Many casuarinas have been
killed by this fungus and even native sand
pines succumb occasionally.
So many differ
ent kinds of plants have become infected that
we have almost reached the point of wonder
ing whether any woody plant is immune.
However, some species appear more or less
tolerant of the disease, that is they may con
tract the disease but not succumb to it for a
considerable length of time. Citrus comes in
this category for although they do not thrive
or produce good crops of fruit, neither do they
die. The native oaks mentioned earlier also
show the same tolerance.
Control measures for mushroom root rot
have not proved satisfactory.
By the time
unmistakable symptoms have developed, the
plant is dead or nearly so. Some measure of
success has been obtained by drastic surgery
in the case of plants that grow in clumps. For
instance parts of infected azaleas can be saved
if the diseased portion is isolated at one side
of a crown bearing several stems. If all of
I he infected portion can be pruned out and
the remaining healthy portion set in clean soil,
success may
be
expected.
Plants
growing
from a single stem can not be treated in this
manner. No fungicides as sprays or soil treat
ments are likely to be effective.
Replacing plants killed by mushroom root
rot is attended by some risk unless certain
precautions are observed.
If it is wished to
set another woody plant of the same or differ-
WILMOT: ORNAMENTAL RESEARCH
187
ent species in the same place as one killed by
useful.
this disease, all roots should be removed care
soil in the affected area may be removed and
fully from the soil before replanting.
One other method may be used.
The
If there
replaced with fresh soil obtained in an area
are no other plants adjoining the spot, the soil
in which no woody plants have been growing
may be sterilized by drenching it with for
for at least 5 years.
malin solution, 1 part of commercial formalin
safest procedure in home plantings.
This is the easiest and
The fumes will diffuse
Considering all of the characteristics of this
for several feet in all directions in the soil
disease and its causal agent, mushroom root
and other plants nearby may be injured. The
formalin treatment is seldom feasible in land
rot is the most serious disease affecting woody
scaped
combat.
to 50 parts of water.
plantings
but
in
groves
it
is
very
plants in Florida and the most difficult to
RESEARCH ON ORNAMENTALS AT THE FLORIDA
AGRICULTURAL EXPERIMENT STATION
year.
R. J. WlLMOT
Florida Agricultural Experiment Station
Gainesville
This discussion will be limited to the work
the Main Station at Gainesville but it
should be remembered that there is consid
erable work being carried on at Branch Sta
tions and Laboratories located at Leesburg,
Sanford, Belle Glade, Bradenton, Homestead
and Lake Alfred that has to do with the
growing of ornamentals.
at
Dr. A. N. Tissot has published on the
aphids that attack ornamentals.
The Department of Plant Pathology, of
which Dr. W. B. Tisdale is head, has projects
on diseases of ornamentals that have either
been completed or are still in progress. Many
years ago J. L. Seal worked on Phytophora bud
"Plants Affected by Root Knot," published in
rot of coconut palms jointly for the State Plant
Board and the Department and Mr. Erdman
West on Rhizoctonia damp off of Boston Ferns.
Dr. Tisdale proved that Pythium root rot was
responsible for the large losses of Chinese
Evergreen by some growers and made recom
mendations for its control. Mr. West has
spoken to you on Witches broom of oleander
and has worked on Powdery Mildew of crape
myrtle, Cercospora leaf spot of pittosporum,
Sclerotium Rolfsii attacking annuals, and has a
project on die-back of camellias. Dr. H. N.
1901, is_probably the first publication on orna
mental plants by the Experiment Station. Al
though Dr. Hume served in an administrative
capacity for the past 18 years he has continued
to publish works on various ornamentals such
Miller is working on diseases of pot plants.
Some of this work dates back 25 years. The
book Native Trees of Florida was written
jointly by Miss Arnold and Mr. West. Plants
sent in for identification are handled in this
as azaleas, camellias, holly and zephyranthes.
department.
It should be pointed out that the work at
Gainesville is not limited to the Department of
Horticulture but various aspects are carried
on by other Departments.
Press Bulletin 10 by H. H. Hume entitled
The late Prof. J. R. Watson of the Depart
ment of Entomology probably published more
popular
articles
on
insects
of
ornamentals
than any other worker in the field.
This De
Samples taken from soil in which orna
mentals are grown is given to the Soils Depart
ment for analysis and they make recommenda
tions as to treatment necessary to modify it
partment has research work in progress on
for the best growth of the plant involved.
root-knot and the control of insects on orna
Nathan Gammon is a cooperator in the ca
mentals.
mellia project of the Department of Horticul
Dr. L. C. Kuitert gave a paper last
year to this Section and has another for this
ture.
Dr.
The late Dr. R. M. Barnett collaborated