First-strike attack on root diseases

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 article.

These trials confirmed that Pythium is widespread in southern Australia and is reducing grain yields throughout the mid-higher rainfall (>350mm) cropping zone.

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 crops.

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,

GRDC RESEARCH CODE CSO00016, program 4