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Know your microbe
Pythium emerges as a significant pathogen
Pythium root disease has been described as the ‘common cold’ of cropping systems and its impact on crop productivity
often has been underestimated or completely overlooked. In this second article of the series ‘know your microbe’
Pythium is put under the microscope to show how the soil pathogen operates.
Disease symptoms
Pythium rapidly infects germinating seeds
and seedlings of crops and pastures causing
roots to develop brown lesions.
Infected roots produce fewer laterals and
become stripped of their outer layers,
exposing internal vascular tissue.
The disease eventually progresses up the
roots to the stem. Low to moderate Pythium
infections cause poor early growth and
affected plants can then become more
susceptible to other diseases.
Two-pronged attack
Pythium produces two types of infectious
agents — oospores and zoospores — which
Gupta Vadakattu,
and Janet Paterson,
Photos: CSIRO Land and Water
ythium’s success as a soil pathogen lies
in the ability of the fungus to survive long
periods of hot, dry conditions.
The fungus’ thick-walled spores can lie
dormant in the soil until moist conditions
enable them to germinate and infect
plant roots.
Pythium species cause seedling root rot
diseases in many plant species including
agricultural crops such as wheat, barley,
legumes and canola.
The pathogen often kills crop seedlings and
damages the roots of surviving plants,
causing reduced crop vigour and significant
reductions in grain yield.
Pythium oospore colonising canola stubble.
Oospores are thick-walled and capable of surviving
long periods of dry, hot conditions within soil, old
roots and stubble. During moist conditions,
oospores germinate and infect crop roots, causing
root rot.
can continually re-infect growing roots
throughout the season. Oospores are thickwalled structures capable of surviving long
periods of dry, hot conditions within soil, old
roots and stubble.
When the soil becomes moist, the oospores
germinate and infect susceptible plant roots.
At a glance
• Severe Pythium infections cause
root rot while low levels of the
pathogen can increase a crop’s
susceptibility to other diseases.
Zoospores are produced by Pythium when
the soil becomes saturated after rainfall.
Under these conditions, reproductive
organs known as sporangia release hundreds
of zoospores which then ‘swim’ toward the
roots of susceptible host plants, causing
infection or becoming dormant if conditions
are not ideal.
These dormant zoospores then germinate
rapidly in response to specific compounds
produced by the plant’s roots, following the
return of moist conditions.
Managing Pythium
CSIRO scientist, Dr Paul Harvey, has
shown certain Pythium strains are better
adapted at infecting some crops over others,
resulting in shifts in the balance of Pythium
strains between cropping rotations.
While Pythium can infect all crops and
pastures, some crops are more susceptible
than others. Lupin is the most susceptible,
supporting high levels of the disease, followed
by canola, peas, wheat and barley.
This finding offers the possibility
of determining which Pythium strains
predominate in a paddock and then making an
appropriate crop and management selection to
reduce the risk of Pythium damage.
For more information contact Gupta
Vadakattu at [email protected], phone (08) 8303 8579 or fax
(08) 8303 8550.
• Thick-walled Pythium spores
survive long periods of hot, dry
conditions, germinating to infect
roots when the soil becomes moist.
• Crop residues vary in the
populations of Pythium supported.
• Grain legumes and canola are
particularly sensitive to
Pythium infection.
Next issue
Know your microbe features soil protozoans
Oospores, sporangia and zoospores are pictured colonising canola stubble. Sporangia are reproductive
organs of Pythium which release hundreds of zoospores when the soil becomes saturated. Zoospores
‘swim’ toward susceptible roots, colonising and causing disease. If soil conditions are dry, the zoospores
become dormant until sufficient moisture triggers their germination. Higher populations of Pythium are
found near canola stubble and in soils after canola than on wheat and vetch residues.
No. 151
August 2004