PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Damping off
Scientific name
Common and serious disease of seedlings and cuttings.
Caused mainly by soilborne fungi, eg
Oomycota
Ascomycota
Basidiomycota
Pythium, Phytophthora
Botrytis, Colletotrichum, Cylindrocladium
Rhizoctonia - sterile (Thanatephorus),
Sclerotium - sterile (Athelia)
Occasionally other fungi, eg Fusarium spp. cause
damping-off. Bacteria, eg Erwinia spp., may also be
involved in pre-emergence damping-off.
Host range
Most damping-off fungi have a wide host range
and most can also grow on plant debris. Almost all
seedlings or cuttings are susceptible.
Symptoms
General. Damping-off, the death of seeds, seedlings
or cuttings when they are attacked by certain fungi,
may take several forms. Affected seedlings may
collapse in circles up to 1 m across (page 372). The
extent of root infection determines the appearance of
symptoms above ground.
Pre-emergence. damping-off. Seeds may rot
before germinating or seedlings may rot before
emerging (Fig.207). It is usually caused by several
different fungi and by bacteria.
Post-emergence. damping-off occurs after
seedlings have appeared and may take various forms
(Fig. 207):
Stem rot. Seedlings develop a stem rot near the
soil surface and fall over. This is the most common
form of damping-off and usually caused by
Phytophthora, Pythium and Rhizoctonia.
Wire-stem or sore-shin. Some seedlings, such
as cabbages, have rather woody stems. The fungus
kills tissues at ground level but the plants remain
standing. Seedlings eventually die. Commonest
cause is Rhizoctonia.
Root and stem rot. Damping-off fungi rot
rootlets, and then travel up in stems, killing plants.
Commonest cause is Phytophthora and Pythium.
Top damping-off. Under damp conditions,
fungi such as Botrytis, Phytophthora and
Rhizoctonia may spread from leaf to leaf or from
stem to stem through the tops of the seedlings or
cuttings. The fungus rots the top of the plant down
to soil level often leaving the crown and roots
uninjured. Depending on the fungus, infection may
be air-borne or originate from the soil, spreading up
the first few plants and then remaining aerial.
Cuttings may rot progressively from cut ends,
from root bases or wounds made by the removal of
buds or leaves, and even from dead leaf bases.
Cuttings are infected through wounds and before
they callus over, a wet rot develops.
Older plants. Rootlets, crown and even fruits of
plants older than seedlings may occasionally be
attacked by some damping-off fungi. Extensive
infection of the root system of older plants by
Pythium may cause slow growth, stunting and
yellowing. Pythium attacks young roots and soft
stems which become water-soaked, darkening with
age. Rhizoctonia causes root and stem rots often
initially at soil level, but under extremely moist
conditions can grow on above ground parts webbing
the seedlings together.
Diagnostics. Causes of damping-off like root rots
generally, are difficult to identify and/or confirm
(page 366).
Confusion. It can be difficult to distinguish one
damping-off fungus from another. It can also be
difficult to distinguish damping-off symptoms from
those caused by waterlogging (anaerobic conditions
in the root zone); excess soluble salts which can
burn roots, etc.
Observe seedlings for wilting and rotting
around the collar region. Different types of
damping off are shown in Fig.207. below.
Carefully remove and wash soil/media from
affected plants and examine under a dissecting
microscope. Compare with the root system of a
healthy plant. Shrunken lower stem tissue may
indicate infection with Pythium. Death of fine roots
from the tip may indicate Pythium but is not
conclusive evidence.
Potting mix can be sown with susceptible
species, eg lettuce. Developing seedlings are
examined for evidence of infection.
Check if a Pythium or other identification kits are
available for your situation. An electronic ‘Pest,
Disease, Beneficial & Weed Identification’ tool
available from NGIA may assist www.ngia.com.au/
Laboratory tests.
– Microscopic examination enables spore
structures to be identified. Taxonomic keys are used
by experts to identify species.
– Lupin baiting in laboratories can identify Pc and
Pythium, but microscopic examination is needed to
tell which species it is. Cultures made from roots into
selective media enable identification.
– If damping off is a problem, laboratory tests
can identify the causal fungi (page xiv).
Pre-emergence
damping-off (fungi
and bacteria).
Root and
stem rot
(Phytophthora,
Pythium,
Thielaviopsis)
Stem rot
(Phytophthora,
Pythium,
Rhizoctonia,
Thielaviopsis)
Top damping-off
(Botrytis,
Phytophthora,
Rhizoctonia)
Wire-stem or
sore-shin
(commonly
Rhizoctonia)
Fig. 207. Symptoms of damping-off.
PhotoNSW Dept. of Industry and Investment.
Basal rot of
cuttings
(various fungi)
Fungal diseases - Examples of fungal diseases 371