Flegg Investigates: A mushroom mystery – Mummy
article is reprinted from the August edition of The Mushroom
journal of the MGA. Peter Flegg is the Editor of that journal as well
as the newsletter of
the International Society for Mushroom Science.
started with the Mushroom Research Association (MRA) as a chemist in
transferred to the Glasshouse Crops Research Institute (GCRI) in 1954,
continuing his work
on mushrooms. He studied with Dr. Lambert at the USDA in 1962 on a
Kellogg Fellowship and
then returned to GCRI, becoming the head of the mushroom section in
1975 and continued to
work on many aspects of mushroom biology and cultivation until his
retirement in 1983.
Peter published many articles and scientific papers while at GCRI and
after his retirement
remained active in the industry as a consultant and technical writer.
He took over as
Technical Editor of The Mushroom Journal in 1981 and as Editor
this year. He has
been the Editor of the ISMS Newsletter since 1989.
It seems logical. Every
disease has a cause. Mummy disease should therefore be no
exception, but, so far, there appears to be no one single identifiable
Several scientists, between them, have put forward a number of possible
organisms on which
to lay the blame. Surely, one may think, with a number of suspects
lined up, it ought not
to be too difficult to decide which is the culprit. Well, there are
rules which are used
to decide these things. It’s not a matter of setting up a panel of
experts to hear
the evidence and vote.
The rules in an early form
were put forward about 150 years ago by Jacob Henle at
G�ttingen University. It was one of his students, Robert Koch, later
held to be one of
the greatest bacteriologists ever known, who developed these rules into
the form they are
known and followed today. It was he who identified the three separate
species of bacteria
causing anthrax, tuberculosis and cholera. Professor Koch was awarded
the Nobel Prize for
Medicine in 1905.
In essence his four rules for
deciding on which organism is the true cause of a disease
1. In all cases a
specific organism must be identified.
2. It must be obtained in
a pure culture.
3. Organisms taken from
the pure culture must consistently reproduce the disease they are
suspected of causing in experimental animals (or plants).
4. The organism must be
recoverable from the diseased plant or animal.
Clearly, to see what organism
can be isolated from a sick plant or animal and blame
that organism for the disease just won’t do. It has to be shown that
the suspect is
capable of causing the same disease it is suspected of causing - not
just any old disease
but the one causing the initial symptoms.
The rules are quite strict
but they do ensure that disease and causal agent are
Mummy disease - symptoms
One of the troubles with
mummy disease is in trying to fulfil the requirements of these
rules. There are several suspects but the evidence is not absolutely
Another trouble is that we
cannot be sure that all the symptoms ascribed to mummy
disease really are those of genuine mummy.
The disease was first
described in 1942 by CM Tucker and JB Routien in the United
States (Missouri Agricultural Experiment Station Research Bulletin,
358). This is not
available to me, but there is an early account of the disease by C
Fleury in the MGA
Bulletin No. 22 (January 1951). He describes an outbreak which occurred
in Switzerland and
both refers to the earlier American work and reproduces some of the
photographs from it.
Among the main symptoms
described are an early cessation of growth and cropping, fruit
bodies with tilted caps, veil breaking early, the base of the stem
enlarged and the stalk
curved. The tissue of the mushroom becomes spongy, dry and brown giving
A particular feature of this
disease is its rapid rate of spread through the bed, up to
30cm (12 inches) daily has been reported.
Pickers are said to be able
to detect this disease while harvesting the crop because of
the tough and dry texture of the mushroom tissue which has a "gritty"
when cut. Sometimes there is evidence of brown streaks, a water-soaked
cavities in the mushroom tissue. Fred Atkins (Mushroom Growing Today,
1966 Edition) gives
the disease the two alternative and descriptive names "Crook Neck" and
The caps and stem are
reported by some to break with a distinctive "snap".
When pulled from the casing a
large amount of casing is left adhering to the base and
the mycelium has been described as "stringy".
Most writers on the subject
include most of the symptoms already described, but a
little uncertainty must creep in as the cause of the disease has not
yet been definitely
Mummy disease - causes
The authors who first
described the disease plumped for it probably being caused by a
virus. Fleury’s early experience was similar to that of the Americans,
several kinds of bacteria could be isolated from the diseased mushrooms
but when healthy
mushrooms were subsequently infected with cultures of these bacteria,
developed on the mushrooms but with no sign of mummy disease. The
bacteria were probably
acting as a secondary infection on the already diseased and weakened
During the late 1950s
Kneebone and his students at Penn State University made several
studies of mummy disease (Mushroom Science 4, 1959). They described the
similar to many of those already mentioned but were unable to fulfil
Koch’s rules for
determining the cause of the disease. However, they were able quite
easily to transmit the
disease by transferring diseased compost and casing containing mushroom
healthy crops. To transfer mummy disease successfully it was necessary
for the diseased
mushroom strain, to be compatable with the healthy strain, that is, the
strains had to be
able to anastomose to enable the disease agent to pass from one
mushroom strain to
another. This suggested that the disease agent was inside the mycelium.
While studying the bacterial
diseases of mushrooms Doreen Gandy (GCRI Annual Report for
1968) isolated a bacterium from mummy-diseased mushrooms which was
similar to the
bacterium Pseudomonas tolaasi (P. fluorescens) which causes
bacterial blotch. This
was also the experience of Schisler, Sinden and Sigel in 1967
944-948). This bacterium from mummy-diseased mushrooms, unlike the
bacterium, was able to develop blue and bright yellow pigments in the
culture media it was
being grown in. Doreen identified this organism as Pseudonomas
while not reproducing mummy disease symptoms, did severely restrict the
growth of mushroom
Here indeed was a mushroom
mystery. In the early days mummy disease in some ways
appeared similar to and was probably confused with another mystery
disease which was,
then, variously called X-disease, Brown disease, Watery-stipe, La
France disease and
Die-back. However, unlike the latter collection of "diseases", which
clearly shown in 1962 to be caused by a number of viruses, mummy
disease retains a
considerable portion of its mystery.
Several years of research
into this mysterious disease done during the 1970s at the
Horst Mushroom Experimental Station were described by Annemarie
Van-Zaayen in, for
example, the Mushroom Journal No. 102 in 1981.
She found the typical
symptoms of mummy and commented on the early confusion with virus
disease. Electron microscopy showed that sections of mummy-diseased
mushroom cells with bacterial cells within them. This rather confirmed
observations in the United States and elsewhere. Van-Zaayen and her
the bacterial cultures obtained by Schisler and colleagues earlier but
were unable to
establish whether the American isolates were really the cause of mummy.
It seemed that the
mummy disease bacteria lost their ability to infect the mushroom.
Infection tests with
their own isolates in the Netherlands did not go smoothly either.
Efforts to produce the
symptoms of mummy disease in mushrooms gave very unreliable results.
More recent work, Betterley
and Olson, 1989 (Mushroom Science 12) and Wuest and
Zarkower, 1991 (Mushroom Science 13) has both clarified and complicated
Betterley and Olson found
several isolates of bacteria in association with mummy
disease and were able to classify most of them as a sub-group of Pseudomonas
fluorescens biotype G (= Biovar V). They were also able to infect
mushrooms, to obtain
mummy symptoms and to recover the bacteria.
On the other hand, several
workers have, over the years found Pseudomonas aeruginosa
to be associated with mummy disease and Wuest and Zarkower decided on a
study of it. Their experiments certainly implicated P. aeruginosa
as a pathogen and
suggested that mummy disease could be caused by at least two different
bacteria, the one named by Betterley and Olson and P. aeruginosa.
They also drew
attention to a report by Fletcher of a disease with similar symptoms to
those of mummy.
Features which distinguish Fletcher’s crypto-mummy, as he calls it,
from mummy are
its lack of rapid spread and that the mushroom crop can recover to
mushrooms. Crypto-mummy, it seems, is often associated with chronic
over-watering of the
By way of explaining the
difficulties and uncertainties of producing mummy disease with
pure-culture bacterial suspensions, Wuest and Zarkower pointed out the
that the bacterium P. aeruginosa can have on the other
microbes in the compost. It
is very active in repressing the growth of many normal inhabitants of
(including the mushroom - remember Doreen Gandy’s work?). They
suggested that the
dominance of this bacterium wherever mummy disease occurred should be
If, by now, you are feeling
that the identity of the causal agent of mummy disease is
surrounded by more than a modicum of uncertainty, then, I cannot
disagree with you.
Mummy disease - control
The most common
recommendation for the control of mummy, and it can only really apply
to shelf beds, is to dig a trench (or trenches if the disease is moving
in both directions
along a shelf). The aim of trenching is to separate the diseased area
from the healthy.
Because of the rapid rate of spread of the disease the trench is
recommended to be dug
around 2m (6-8ft) ahead of the advancing disease and the trench itself
to be several
inches wide. All compost and casing has to be removed from the trench
and the gap
thoroughly disinfected. The whole business of trenching sounds a
potentially messy one to
When the crop is being grown
in separate containers, eg trays or bags, the usual advice
is to isolate or dispose of them. Whichever method of control is used,
good hygiene is
As Fred Atkins wrote of mummy
disease many years ago, "there is no cure",
true then and true now.
It is altogether not a very
comfortable situation. I am not sure what Professor Koch
would have made of it, but the moderately good news is that mummy
disease does not often
seem to be a major cause of crop loss.