Download Consortium for Educational Communication

Consortium for
Educational
Communication
Module on
Mucor- I mportant Features
and Life Cycle
By
Dr Masood Majaz Ganaie
Asstt. Prof.
Degree collage Pulwama
Cell No: 9419920786
email:[email protected]
Consortium for Educational Communication
Text
Taxonomic Description
Mucor L. (1953), also called as black mould or pinmould, belongs to the subdivision Zygomycotina, class
Zygomycetes, order Mucorales and family Mucoraceae
(Alexopoulos et al., 1996). The fungi included in Zygomycotina
are mostly terrestrial, and reproduce asexually by nonmotile spores. Their thallus is usually mycelial, aseptate and
the cell wall contains chitin and chitosan. They are normally
haploid. The principal characteristic that distinguishes class
Zygomycetes is the production of a thick-walled resting spore
called zygospore (Alexopoulos et al., 1996). The zygospore
develops within a zygosporangium that is formed after
fusion of two gametangia during gametangial copulation.
However, because not all the species placed in this class
have been shown to produce zygopspores, a variety of
morphological, biological and ecological characteristics also
enter into defining the Class. These include usual presence
of a coenocytic mycelium, asexual reproduction usually
by sporangiospores, and absence of flagellate cells and
centrioles. The class Zygomycetes is differentiated from
the related class Trichomycetes, in having their mycelium
not immersed in the host tissue.
The order Mucorales has by far the largest number
of species and morphological diversity within the class
Zygomycetes (Alexopoulos et al., 1996). Mucoralean
species have a well-developed mycelium that is generally
aseptate. When septa are present, they lack pores
with specialized plugs. This character separates them
from Dimargaritales and Kickxellales (Alexopoulos et
al., 1996). Mucorales can also be separated from other
orders of Zygomycetes by their relatively non-specialized
associations with other organisms, besides a combination
of their asexual and sexual reproductive structures. Most
of the genera of Mucorales are saprobes that occur in
dung, soil, humus and other organic debris. Typically they
have a well-developed mycelial thallus. The mycelium
is coenocytic except in family Sigmoideomycetaceae.
The thallus exhibits an active cytoplasmic streaming.
Anastomoses between the highly branched somatic hyphae
Consortium for Educational Communication
are rare in Mucorales and as a result, these fungi do not
form the interconnected network, typical of most of the
fungi (Ingold, 1978). Hyphal septa usually are produced
only to delimit sporangia or gametangia or old or injured
hyphae. Multiporate, gametangium-delimiting septa are
present in a number of species (Benny and Benjamin,
1992) and have been suggested as being typical of
mucoralean species (O’Donnell et al., 1997). The order
Mucorales reproduces asexually by sporangiospores or
rarely by conidia. The outer walls of zygospore are formed
by modification of gametangial walls. Mucoraceae is one
of the largest families of Mucorales. The genera belonging
to Mucoraceae produce non- apophysate sporangia with
either deliquescent or persistent walls. There is often a
slight constriction of the sporangiophore immediately
below the sporangium. Zygospores have opposed, nonappendaged suspensors.
Mucor is a large genus with more than 60 species
and in general has a cosmopolitan distribution. It can
be isolated from almost any organic material that is in
contact with air, so can be found in soil, decaying plant
material, dung, air (M. racemosus and M. mucedo),
and even in certain cases as a parasite on other fungi.
However, M. amphibiorum is currently limited to Tasmania.
Most of the species are saprobes on a variety of materials
including food stuffs. Some are week parasites of fruits
and vegetables. The genus Mucor can be differentiated
from Absidia, Rhizomucor and Rhizopus (related genera)
by the absence of stolons and rhizoids. Structurally Mucor
can be differentiated from Rhizopus in having less mycelial
differentiation and sporangiophores arising from any point
rather than at specific points (as in later one). Unlike
Zygorhynchus (Mucoraceae), most of the species of Mucor
and Absidia (Mucoraceae) are heterothallic. The molecular
phylogeny reveals that the species of Mucor do not form a
monophyletic clade and considerable variation can be seen
even within species. The majority of species can be placed
into one of three groups based on their morphology. These
are the M. hiemalis group, which consists of a number of
morphologically similar species. The other two groups are
the M. circinelloides-group characterized by rather small
Consortium for Educational Communication
species with reddish-brown zygospores (Schipper, 1976),
and the M. mucedo group, characterized by the tall group
species, which generally show optimal development at
temperature below 20° C (Schipper, 1975). Traditionally,
Mucor species were grouped according to morphological
similarities, although these groups do not necessarily reflect
the phylogenetic relationship among species (Schipper,
1973). This was confirmed by O’Donnell et al. (2001) and
White et al. (2006), who found that the monophyletic
groups based on the Elongation 1-α gene 28 sequences
do not reflect family relationships within the Mucorales.
However, data from this gene region can effectively be
used to identify undescribed species. The branching of
sporangiophores (branched or unbranched), the shape
of the sporangiospores (round or elongated), maximum
temperature of growth, presence of chlamydospores,
assimilation of ethanol, and molecular analysis aid in
differentiation of Mucor species from each other.
Structure
The thallus is eucarpic and mycelial. Colonies of this
fungus are typically white to beige. Older colonies become
grey to brown in colour due to development of spores.
These are very fast growing and on culture medium may
grow to several centimeters in height. The hyphae are
coarse, coenocytic and richly branched, with branches
usually tapering to fine points. Septa may develop at
later stages or to separate the older or injured parts. The
mycelium growing on substratum can be distinguished into
i) absorption hyphae which penetrate the substratum and
absorb food, and ii) sporangiophores, the bulk of aerial
hyphae. The species produce non-apophysate sporangia
that have a wet or dry wall when mature. The suspensors
are typically enlarged, equal and opposite. Cell walls
are complex with chitin micro-fibrils and abundance of
chitosan. Sporangiophore walls are mainly composed of
polysaccharides with smaller amounts of lipids and proteins.
Four distinct polysaccharides were recognized within the
wall: an acid-soluble polymer of D-glucosamine (chitosan),
an alkali-soluble hetero-polymer of D- glucuronic acid or
fucose or mannose or galactose (mucoran), and two insoluble
Consortium for Educational Communication
polymers, one of N- acetyl-glucosamine (chitin) and the
other of D- glucuronic acid (mucoric acid). Sporangiophore
walls resemble the hyphal walls in qualitative composition
but differ appreciably in quantitative composition. The
cytoplasmic contents contain polysaccharides, proteins,
pyrimidines; Mg and Ca have also been detected. It was
in M. rouxii that chitosomes were first time reported.
The cytoplasm often shows rapid streaming; nuclei are
irregular in shape, which divide by constriction and not by
mitosis and spindle formation. In anaerobic liquid cultures,
especially in presence of CO2, its mycelium breaks down
in to small, spherical and independent cells. This is called
‘torula stage’ or ‘torula condition’. When first discovered,
it was thought to be a different fungus altogether and was
given the name Torula. Then it was found out that it is
just an abnormal condition in Mucor. The condition reverts
back to filamentous state in presence of O2.
Growth Pattern
Colonies of Mucor grow rapidly at 25-30°C and quickly
cover the surface of the agar in culture medium. Its
fluffy appearance, with a height of several centimeters,
resembles cotton candy. From the front, the color is white
initially and becomes grayish brown with time. From the
reverse, it is white. Mucor indicus is an aromatic species
and may grow at temperatures as high as 40°C. Mucor
racemosus and Mucor ramosissimus, on the other hand,
grow poorly or do not grow at all at 37°C or above.
Genetic Constitution and Nuclear Behaviour
Many of the genes and proteins of Mucor have been
sequenced. Sosa et al. (1999) resolved the structural
details of a ribosomal protein gene in Mucor racemosus.
It consists of two exons of 57 and 387 nucleotides. The
protein predicted from the nucleotide sequence contains
148 amino acids and exhibited 61% identity with the
S19 ribosomal protein of Xenopus laevis. Two Mucor
circinelloides structural genes involved in isoprenoid
biosynthesis have been isolated and characterized. The isoA
gene encodes a typical eukaryotic farnesyl diphosphate
synthase (EC 2.5.1.10), whereas the isoB gene deduced
amino acid sequence shows similarity to fungal medium-
Consortium for Educational Communication
chain prenyl diphosphate synthase. Papp et al. (1999)
calculated size of the mitochondrial genome of Mucor
piriformis to be about 33.53 kbp. The deduced primary
translation product of gene encoding the aspartyl protease
of M. miehei contains an N-terminal region of 69 amino
acid residues not present in the mature protein. The M.
circinelloides gene encoding glyceraldehyde-3-phosphatedehydrogenase (GDP) has also been isolated and analyzed
(Acs, 2003). The complete nucleotide sequence encodes a
putative polypeptide chain of 339 amino acids interrupted
by 3 introns. The predicted amino acid sequence of this
gene shows a high degree of sequence similarity to the
GPD proteins from other filamentous fungi. The predicted
amino acid sequence of this gene shows a high degree of
sequence similarity to the glyceraldehyde-3- phosphate
dehydrogenase proteins from yeast and filamentous fungi
(Papp et al. 2003; Vastag et al. 2003). A 4.4 kb PstI
restriction endonuclease fragment of Mucor circinelloides
DNA has previously been shown to both complement a
leuA− mutation, and to enable the autonomous replication
of plasmids within this organism. The complete nucleotide
sequence of this fragment has been determined and an
open reading frame of 1935 bp with no introns has been
identified, which exhibits significant similarity (75% at
the nucleotide level) with 114 bp of the 5’ coding region
of the Saccharomyces cerevisiae LEU1 gene (Isabel et al.,
1989)
In Torula stage, all the cells were multinucleate, often
containing several dozens of nuclei, while most buds
possessed between 2 and 10 nuclei. Up to a number of
35 nuclei, a linear relationship exists between the nuclei
number and the cell size (radius).
During mitosis in Mucor hiemalis, the chromatin
occupies the periphery of resting as well as dividing
nuclei. It could not be resolved into separately visible
chromosomes and was never seen arranged in the form
of a metaphase plate. Division of the nucleus, and of
the nucleolus within, is achieved by elongation followed
by constriction. The unvarying optical contrast between
nucleoplasm and cytoplasm during division implies that
Consortium for Educational Communication
mitosis takes place within the intact nuclear envelope.
Electron microscopy reveals that the division is accompanied
by the development of a short, straight, slender bundle
of microtubules ( spindle ) in one corner of an elongated
nucleus. The spindle extends between largely intranuclear
spindle pole bodies . It rapidly increases in length, and in
maximally elongated nuclei, runs in a straight line between
the two poles, almost completely embedded in nucleolar
material. It is tentatively proposed that the chromosomes
are attached to the poles of the spindle. The shortness
and peripheral, eccentric position of developing spindles
in already considerably elongated nuclei suggests that
the spindles do not actively initiate nuclear elongation but
follows it passively. Its function is seen as that of a device for
keeping daughter sets of newly formed chromatids rigidly
apart during the course of their distribution to daughter
nuclei.
Reproduction
The fungus reproduces both asexually and sexually.
A. Asexual reproduction
Following are the methods for asexual reproduction:
Fragmentation, chlamydospores and sporangiospores.
1. Fragmentation: Due to mechanical injury, the vegetative
hyphae break up into smaller fragments, and each fragment develops into a new mycelium.
2. Chlamydospores: These are formed under unfavourable
conditions. In anaerobic conditions, tips of the hyphae become septate. Each segment secretes a thick wall and assumes a round shape. These are called chlamydospores.
They can survive in unfavourable conditions, and germinate to form a new mycelium when conditions become favourable. Arthrospore formation in Mucor, the process of
septation and fragmentation of hyphae into individual new
entities, is not very well understood and often referred to
Consortium for Educational Communication
as a mere starvation response (Orlowski 1991). Their development is stimulated by specific conditions, including
high spore inoculum concentrations and low glucose concentrations. The process of arthrospore formation by M.
circinelloides take place more rapidly than in other fungi.
3. Spores (aplanospores or sporangiospores): This is the
most common method of asexual reproduction. The sporangia develop terminally on hyphal branches, known as
sporangiophores. Sporangiophores are hyaline, erect and
simple or branched (e.g. in M. proliferus). In M. renisporus, tall sporangiophores are mostly un- branched, while
short sporangiophores are sympodially branched. They
bear large (60-300 µm in diameter), terminal, multispored
sporangia, which are without apophysis, but have welldeveloped subtending columellae. In case of the species
with branched sporangiophores, terminal sporangium is
larger than the lateral ones. In M. renisporus, collumellae
are cylindrical to pyriform in shape. The smaller columellae are mostly conical. Yellow lipid droplets are present in
most columellae as has been reported in M. renisporus. A
conspicuous collarette (remnants of the sporangial wall)
is usually visible at the base of the columella after sporangiospore dispersal. However, the collarettes are absent
on short sporangiophores in M. renisporus. The sporangiophores of M. grandis only posses a short row of spines in
place of a collarette. Sporangiospores are hyaline, grey or
brownish or dark, globose to ellipsoidal, and smooth-walled
or finely ornamented. However, in Mucor renisporus they
may be kidney-shaped in side view. In Absidia, a related
genus, the sporangia are pear-shaped. There is considerable inter- and intra- species variation in spore size (Taeko,
1974). Large spores are clearly multi-nucleate.
During the process of sporogenesis, apex of the
sporangiophore swells and cytoplasmic mass along with
nuclei move in this part. The swollen part enlarges into
a large globose structure. This is the young sporangium.
On maturity, contents of sporangium become differentiated
into a thick, dense layer of cytoplasm, with many nuclei
Consortium for Educational Communication
towards the distal region beneath the sporangial wall, and
a vacuolated subglobose and nuclei free portion towards
the centre. An extension of that sporangiophore, called
columella, protrudes into the sporangium. A dome- shaped
septum is then laid down, cutting off a distal, peripheral
portion (which will contain the spores) from a central
cylindrical or subglobose spore-free core, the columella.
The contents of the distal portion become cleaved into
multinucleate spores. However, spores are uninucleate in
certain species such as M. hiemalis. All the spores are almost
of the same size but in M. proliferus, the spores formed in
the terminal sporangia are larger than those produced in
the lateral sporangia. When the sporangia are mature, the
sporangiophores may be seen as coarser, blunt-tipped aerial
hyphae growing away from the substratum. The sporangial
wall dissolves, except for basal region, where its remnants
can be seen as frill or collar at the base of the columella.
The spores remain adhered to columella and are not easily
disseminated, usually mites bring about dissemination.
In contrast to Mucor, in Rhizopus spores are easily blown
away by wind. Columella after dehiscence of sporangium
does not change shape in Mucor. While columella in most
of the species of Rhizopus become more or less dome- or
umbrella- shaped after dehiscence of sporangium. In M.
plembelis, the sporangial wall breaks into pieces. The wall
of mature sporangiospore consists of an inner, electrontransparent layer and an outer electron dense layer. The
spore wall is covered with two thin layers, each about 10
nm thick, which may correspond to ordinary spore sheath.
The cell membrane of the spores do not have invaginations
like those of higher fungi. Instead, there are numerous
round depressions about 50 nm in diameter. Mitochondria
of the spores are much larger and show wide and deep
invaginations of their membranes. Their cristae are
indistinct. Lipid droplets have multi-layered shells and are
much more highly developed than those found in mycelial
cells, Torula stage or chlamydospores of this fungus. When
they fall on suitable substratum, in presence of proper
moisture and temperature, they germinate by producing g
nerm- tubes, which then establish new mycelia.
Sexual reproduction
Consortium for Educational Communication
Sexual reproduction in Mucor is of isogamous type, i.e.
the two mating gametes are morphologically identical.
Some species are homothallic (e.g. M. genevensis), but
most of them are heterothallic (e.g. M. hiemalis). The
conjugation of two gametangia gives rise to a zygospore.
In heterothallic species, zygospores are formed only when
gametangia of compatible strains contact each other. If
the appropriate strains are inoculated at opposite sides
of a Petri dish, the gametangia grow out and a line of
zygospores develops in the centre where these mycelia
meet each other. In the heterothallic species of Mucor, as
in M. mucedo and M. hiemalis, the sexual act is initiated
when hyphal branches of (+) and (-) strains contact each
other. Initiation of gametangia is believed to be induced
by the trisporic acids. They are produced in appreciable
amounts only when (+) and (-) cultures are in continuous
diffusion contact with each other. Actually the strains first
produce small quantities of pheromones that are converted
to trisporic acids. However, Mesland et al. (1974) reported
that the vegetative mycelium of M. mucedo, even when
not in diffusion contact with vegetative mycelium of
opposite strain, could produce volatile substances which
induce attraction of the compatible gametangia (zygotropic
response). These volatile substances have a dual role- to
enhance trisporic acid synthesis in the opposite mating type,
and to mediate the zygotropic response. In homothallic
species, zygophores of the same thallus can fuse to form
a zygosporangium. Each branch (zygophore) swells at
the tip to develop a progametangium. Dense cytoplasm
and numerous nuclei flow to the contacting tips which
enlarge further. A septum then separates the terminal
part called gametangium from the remaining part of the
progametangium, the suspensor. When the compatible
and mature gametangia contact one another, their tips
swell, and fuse apically to form a fusion septum. The fusion
septum (separating wall) dissolves and the protoplasts of
both mix with each other. The structure formed by the
fusion of two gametangia, enlarges, develops a thick wall,
and is called zygosporangium. The plasmogamy is followed
by nuclear fusion (karyogamy) of the two compatible
strains to give numerous diploid nuclei. Secondary wall
Consortium for Educational Communication
material is deposited, causing the zygosporangium wall
to thicken and become pigmented. Localized thickening
results in the formation of the ornamentation characteristic
of each species. The mature zygosporangium is more or
less globose. The plasmogamy and karyogamy within
the zygosporangium gives rise to a resting spore, called
zygospore. However, in certain conditions zygospores may
also be formed parthenogenetically, and is called azygospore
as has been reported in M. bainieri (Ginman and Young,
1993). The zygospore is hyaline and characteristically
with a single, eccentric globule and many diploid nuclei.
Meanwhile the zygospore enlarges and secretes a thick
wall, which is differentiated into exospore and endospore.
The zygospores are usually brown to black; with rough
blunt projections or warts. The wall of mature zygospore
is probably five-layered. The endospore is made up of
three thin inner elastic layers, whereas the exospore
consists of two brown or black outer fragile layers. As the
zygospore matures, the wall of the zygosporangium breaks
up into fragments, which fall apart. All the diploid nuclei,
formed in the zygospore undergo meiosis usually before
the germination. The zygospore germinates usually after
a long period of rest. During germination, the outer wall
cracks and the inner wall comes out in the form of germsporangiophore. All the nuclei in the zygospore migrate
to the tip of the germ-sporangiophore. Consequently, it
swells to form a sporangium called germ- zygosporangium.
Rarely more than one germ-sporangiophores arise from
a single zygospore. However, occasionally the germsporangiophores may be branched and in such cases
more than one sporangia are present on each germsporangiophore. Each of which bears a single terminal
germ-sporangium containing many spores.
Life Cycle
In asexual phase, Mucor reproduces either by means
of vegetative propagation or by means of spores. Many
a times due to mechanical injury, the vegetative hyphae
may break into smaller fragments, and each fragment
regenerates to form a new mycelium. Additionally, under
unfavourable conditions, the tips of the hyphae become
Consortium for Educational Communication
septate. Each septate region secretes a thick wall around
itself to form a chlamydospore. On the arrival of favourable
conditions each such spore germinates to form a new
mycelium. However, the most common method of asexual
reproduction is by sporangiospores. They develop within
sporangia, arising on the tips of sporangiophores. Contents
of the mature sporangium become differentiated into dark,
elliptical or ovoid spores. The dehisced spores, after falling
on suitable substratum, in presence of proper moisture
and temperature, germinate to form new mycelia. In the
asexual phase, all the stages are haploid.
In sexual phase, Mucor reproduces by means of
isogamy and the most of species are heterothallic. The
compatible gametophores contact with each other and
their distal segments fuse to form zygosporangium.
The plasmogamy is followed by karyogamy and a brief
transitory diploid stage is established. The contents of the
zygosporangium develop into a spore called zygospore.
The zygospore germinates after a long period of rest.
Before the germination diploid nuclei formed in the
zygospore undergo meiosis. Thus haploid stage is reverted
back. All the nuclei in the zygospore migrate to the tip
of the germinated zygosporangium. The contents of the
zygosporangium differentiate into spores, which in turn,
germinate to establish a new haploid mycelium. Thus the
life cycle of Mucor is haplontic.
Dimorphic Mucor species are capable of growth as
either aseptate filamentous mycelial form, or as yeastlike form. The gaseous atmosphere constitutes a pivotal
factor in determining the type of the form. The hyphal
form predominates when conditions are aerobic, whereas
strict anaerobic condition is required for the development
and maintenance of the yeast- like form. Furthermore,
the nature of the carbon source is crucial: while the yeast
like form can only grow on fermentable hexoses, hyphal
form has been described on a wide range of substrates
including complex carbon sources. The exact requirements
of each Mucor species to grow in a particular mode may
vary. Mucor circinelloides is able to grow in a filamentous
mode on a wealth of carbon sources. However, not only
Consortium for Educational Communication
anaerobic condition but also a concentration of 30% CO2
in the sparging gas are necessary to facilitate its yeast-like
growth. Experiments have shown that new buds emerge
at random locations all over the cell surface.
During germination more than one germ tubes may
emerge from a spore. This is more likely to be the case with
xylose than with glucose as a carbon source. The formation
of new branches occurs randomly. Comparatively larger
number of tips per hyphal length is formed in presence of
xylose than on glucose.
M. piriformis is a soil-borne fungus that is found mainly in
the surface layers of orchard soil. It infects fallen fruit during
and after harvest, and as a result orchard populations are
highest in the months immediately following harvest. In the
orchard, the fungus may be spread by rain and irrigation
splash, by bird and insect feeding, and by practices such
as mowing which can spread infected pieces of fruit.
Economic Importance
The genus Mucor is of considerable economic importance
(Domsch et al. 1980). Many species of Mucor are used in
industrial fermentation at the initial stages, for converting starch into sugar. Mucor circinelloides is proving to be
a promising cost-effective biofuel (Vicente et al., 2010).
Amylase, cellulase and oxido- reductase enzymes are being obtained from Mucor. Mucor circinelloides is a multifaceted organism, capable of growing on a diverse range
of substrates, possessing the ability of multiple modes of
growth and having the potential of industrial application
for the production of both degradative enzymes and primary metabolites. The growth capabilities of the organism
facilitate its attractiveness as an industrial workhorse. Mucor circinelloides produces an extracellular enzyme polygalacturonase, which can be of tremendous economic importance (Thakur et al., 2010). Certain species have been
used for lupeol transformation (Carvalho et al., 2010) and
production of enzymes like glucose isornerase. Mucor pusillus is being used for production of chymosin (Wei-Dong
et al., 2010). M. racemosus has been used for the production of rennin.
Consortium for Educational Communication
Most of the species are unable to infect humans. Most
infections reported list M. circinelloides and similar species
such as M. indicus (M. rouxii), M. ramosissimus and M. amphibiorum as the causative agents. However, M. hiemalis
and M. racemosus have also been reported as infectious
agents, although their inability to grow at temperatures
above 32 ₒC raises doubt as to their validity as human
pathogens and their pathogenic role may be limited to
cutaneous infections (de Hoog et al. 2000). Some thermo-tolerant species sometimes induce rapidly spreading
necrotizing infections, known as zygomycosis. Deja et al.
(2006) found M. indicus involved in gastro-intestinal zygomycosis. While M. circinelloides was identified as causative agent of primary cutaneous zygomycosis (Chandra
and Woodgyer, 2002 and Irwin et al., 2008) and invasive
maxilla-facial zygomycosis (Khan et al., 2009). Similarly
epidermal necrosis was found to be complicated by Mucor infection by Jin et al. (2008). Some species of Mucor
attack stored grains. M. amphibiorum is the only pathogen known to cause significant morbidity and mortality in
free living platypus in Tasmania (Connolly et al., 2010). It
causes ulcers in it, which can be secondarily infected and
potentially fatal. It can also reduce the ability to regulate
body temperature.
Mucor species are mostly known as spoilage organisms,
and only one species (M. piriformis) has been reported
as a pathogen on stone- and pome- fruit (Michailides and
Spotts, 1988). Mucor rot is a post- harvest disease of apples, pears and other fruits caused by the fungus Mucor
piriformis and less often by other Mucor species. The disease is found in Australia, North America, South Africa and
Europe, where it can cause serious losses, especially after
long- term storage of fruit. Mucor rot causes a light brown,
soft, watery rot. It commonly occurs where the fruit skin
has been damaged and also at the stem and calyx end of
fruit. Where breaks in the fruit skin occur, white, whiskery
fungal growth may appear which is soon covered by black
spore masses. The rot progresses rapidly at room temperature and even at storage temperature (0°C). The disease
Consortium for Educational Communication
can cause extensive rotting over several months. Disease
symptoms are superficially similar to those of rhizopus
rot caused by Rhizopus stolonifer; however, mucor rot
can develop at 0°C whereas rhizopus rot is restricted by
temperatures below 4°C. In Australia, mucor rot has only
relatively recently been recognized as a serious problem.
In some seasons considerable losses have occurred, especially in CA-stored fruit for both local and export markets.
Spread within a packing shed can be rapid, as healthy
fruit can be contaminated and infected during post- harvest dipping, and during sorting and grading operations.
M. piriformis can also be a major cause of core rot in Red
Delicious apples. M. piriformis enters the packing shed on
soil adhering to fruit bins and machinery, and on fruit that
was picked off the ground during harvest. It may also survive between seasons on fruit storage bins. Inoculum can
be spread during post- harvest handling operations, for
example, through drench and flotation solutions. During
storage, rotted fruit break down and release juice which,
in the case of pears, assists in secondary spread. Secondary spread during storage is less common in apples.