By Dr Paul Harvey, CSIRO Land and Water
Pythium root disease has been described as the “common cold”
of cropping systems and its impacts on the productivity of rotations have
often been underestimated or completely overlooked.
Pythium infects germinating seeds and seedlings of all major grain crops
and pastures, continually re-infecting growing roots. Consequently, the
disease has a continuous impact throughout the current season and across
all phases of the crop rotation.
In recent years there have been numerous reports of Pythium root
rots occurring with increasing frequency and severity in southern Australia.
This appears to be related to the increasing adoption of reduced tillage
systems and the switch to less diverse, potentially higher-risk rotations
(eg repetitive canola-wheat rotations).
As both of these trends are here to stay, there is an urgent need to
develop pre-emptive strategies to manage this increasingly important disease.
A collaborative project, funded by the GRDC, involving CSIRO Land and
Water and Landmark AWB aims to develop an integrated management package
for Pythium by using fungicides and crop rotation strategies.
To illustrate the potential disease impacts of Pythium, crop rotation
trials were set up in 2002 at four sites (two in South Australia and one
each in New South Wales and Victoria) using a Pythium-selective fungicide
as a seed dressing.
In 2002, high levels of Pythium were detected at all sites (average 185
spores per gram of soil), with the fungicide decreasing soil-borne levels
by an average of 18 percent and the frequencies of root infection by 22
percent. This resulted in grain yield increases in canola (6 to 38 percent),
cereals (3 to 18 percent) and pulses (8 to 50 percent).
These yield benefits occurred in crops with no obvious “above-ground”
symptoms, with only partial control of the pathogen and during a severe
drought, conditions not generally considered conducive for Pythium diseases.
The 2003 trials had Pythium levels (average 310 spores per gram of soil).
that were almost double those detected during the 2002 drought. Similarly,
the fungicide decreased levels of Pythium root infection and soil-borne
inoculum by averages of 18 percent and 26 percent respectively. Yield
data for the 2003 trials were not available at the time of writing this
These trials confirmed that Pythium is widespread in southern Australia
and is reducing grain yields throughout the mid-higher rainfall (>350mm)
Based on the field-trial results, crops were ranked for their susceptibility
to Pythium. Lupins were the most susceptible to infection, followed by
canola, peas, wheat and barley. Greater levels of Pythium root infections
were also found in crops grown for their second successive year compared
with crops grown in annual rotation.
This effect was particularly evident in continuous wheat.
Glasshouse tests showed evidence of crop preference among different strains
(genotypes) of Pythium, and research using DNA markers has shown that
the genetic make-up of Pythium populations shifts in response to different
Collectively, these results suggest that different crops may be selecting
for crop-adapted genotypes of Pythium and as such, individual genotypes
of Pythium are better adapted to infecting some crops relative to others.
Determining the size and genetic make-up of Pythium populations in cropping
soils, and monitoring how they change over phases of the rotation, will
allow meaningful interpretation of soil tests in order to develop Pythium
disease prediction models.
Sustainable control will require an integrated program of Pythium selective
fungicide applications during highly susceptible phases of the rotation
(eg grain legumes or canola).
For more information:
Dr Paul Harvey, 08 8303 8589, firstname.lastname@example.org
GRDC RESEARCH CODE CSO00016, program 4