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Februarie 2019
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head and stem rot
S
clerotinia sclerotiorum
is a hemi-
biotrophic fungal plant pathogen,
therefore a biotrophic and necro-
trophic pathogenic lifestyle is fol-
lowed. This fungus has an extensive host
range of more than 500 plant species, in-
cluding sunflower, soybean and canola.
These crops play an important role in the
South African economy as oil seed crops.
However, some vegetables may serve as
alternative hosts which include (but are not
limited to) cabbage, potatoes, squash, car-
rots and tomatoes. Many common South
African weeds are also susceptible to infec-
tion by this pathogen and are associated
with these crop production systems.
Common names
As a result of the many host plants associ-
ated with this pathogen there are more than
60 common names associated with
S. scle-
rotiorum
infections. The most common are
Sclerotinia head rot, Sclerotinia stem rot,
white (may in some cases be cream in col-
our) mould and cottony soft rot.
These common names are associated with
the distinct symptoms and signs associated
with Sclerotinia infections. The symptoms
associated with a disease are the host's re-
sponse in a conducive environment, where-
as the signs are a physical reaction of the
pathogen. These signs may also be seen
morphologically in the laboratory.
In the case of Sclerotinia diseases, myce-
lium (mass of fungal hyphae) and sclero-
tia (
Photo 1
) are the primary signs of the
pathogen’s presence. Sclerotia, melanised
masses of hyphae, are key to the life cycle
of this fungus as they are the survival struc-
ture produced.
Symptoms
The visible symptoms are dependent on the
host growth stage. Seedling wilting may
occur, although Sclerotinia stem and head
rot frequently develop at flowering and pod
or seed filling stages. Brown water-soaked
lesions (
Photo 2a
) are initial symptoms
observed that become covered with white
cotton-like mycelium on sunflower heads
(
Photo 2b
) and soybean pods as well as in
and on the stems of both sunflower and
soybean (
Photo 2c
and
Photo 2d
). The white
mycelium on the face of sunflowers even-
tually develops into a net of black sclerotia
(
Photo 2e
). As the disease matures, a shred-
ded appearance, with sclerotia between
plant fibres can be observed in sunflower
specifically (
Photo 2f
). This fungus can also
infect the subterranean crown and form
sclerotia within the lower stem of sunflower
and soybean.
The initiation of the infection cycle of Scle-
rotinia diseases is dependent on the inter-
action between the sclerotia present, i.e.
primary inoculum source, the presence of
the host and an environment conducive to
disease development. Sclerotia are highly
resistant to dry heat of up to 70˚C and pro-
longed periods of freezing.
Sclerotia can stay viable in the soil for up to
eight years, although this is dependent on
the strain and locality, as well as soil char-
acteristics. Sclerotia found in the first 5 cm
of soil are considered important for disease
initiation. Infection by
S. sclerotiorum
can
occur through two means of germination to
produce primary inoculum, namely myce-
liogenic and carpogenic germination.
Each method of germination is dependent
on the environmental conditions present.
Myceliogenic germination occurs when
sclerotia produce hyphae under tempera-
tures ranging from 20˚C to 25˚C in the pres-
ence of high relative humidity. The latter is
assumed to be greater than 80% in litera-
ture.
Carpogenic germination results in the for-
mation of apothecia (
Photo 3a
on page 48),
or mushroom-like structures. This occurs at
lower soil temperatures ranging between
16˚C and 20˚C at a higher relative humidity
as for myceliogenic germination. Apothecia
are commonly misidentified as the common
bird’s nest fungus (
Photo 3b
on page 48).
Apothecia release ascospores into the air
under high relative humidity and changes in
air pressure, favouring long distance disper-
sal and infection. Literature states the move-
ment of ascospores can be within-fields
and between fields. However, it is accepted
that most ascospores are dispersed within
150 cm of their apothecial source. Further-
more,
S. sclerotiorum
has the ability to re-
main dormant in the form of mycelium in
infected plant residues when environmental
conditions are unfavourable for germination
and infection.
ON FARM LEVEL
Sunflower / Soybean / Fungus
Integrated pest control
MARLESE BESTER
and
LISA ROTHMANN,
Department of Plant Sciences:
Plant Pathology Division, University of the Free State
“
“
...there are more than
60 common names
associated with
S. sclerotiorum
infections.
1: Morphology of
Sclerotinia sclerotiorum
sclerotia (C), mycelium (A) and the apothecia
initials, known as stipes (B).
Photo: Marlese Bester
1